I'm not an electrician, just a hobbyist trying to build a small sculpture which incorporates LEDs, but they keep burning out. I am using a 8w/24v Meanwell power supply, a single 20mA Dynaohm resistor, and 3 strings of four 5mm warm white LEDs each, in parallel (see attached diagram). They all lit briefly, but very quickly one string after the next went dark. What am I doing wrong, and how to fix it? THANKS!
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Discuss Why are my LEDs burning out? in the Lighting Forum area at ElectriciansForums.net
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What are the specs of the LEDs?
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The OP's "resistor"is a 20mA constant current device.
Surely you shouldn't put three strings of led's in parallel like that?
Any slight difference in the forward voltage drops of the diodes will mean the current is not equally shared between each of the strings. I'd think all the current will go through the diodes with the lowest drop, and the others won't light?
Broughaha - I think you need a Dynaohm in each of the three strings of LED's for your scheme to work.
Can you give details of the led's - recommended running current, forward voltage when running?
Surely you shouldn't put three strings of led's in parallel like that?
Any slight difference in the forward voltage drops of the diodes will mean the current is not equally shared between each of the strings. I'd think all the current will go through the diodes with the lowest drop, and the others won't light?
Broughaha - I think you need a Dynaohm in each of the three strings of LED's for your scheme to work.
Can you give details of the led's - recommended running current, forward voltage when running?
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Thank you all for the very prompt replies. I see that I did not provide enough detail in my original post. I should also mention that I had previously tried using a 12W/15V Meanwell and with that 11 of the 12 bulbs illuminated but the 12th did not. Folks at LEDsupply.com thought I needed more voltage, but then they all burnt out. Below are the product details for the parts I am using:
DynaOhm (mine is the 20mA)
LEDs:
Meanwell Power source (I am using 8w/24v)
DynaOhm (mine is the 20mA)
Product Details
| Output Current: 20mA, 25mA or 30mA | Input Voltage Range: (2.6VDC + Vf LED) to 50VDC |
| Connection: 2 Wire | Output Protection: Open & Short Circuit |
| Maximum Voltage Drop: 30V | Size: 0.25" (Diameter) X 0.5" (L) |
| Control: Pulse & Strobe Capable | Power Dissipation: 0.6W |
LEDs:
Product Details
| Luminous Intensity: 4,700mcd typ. @ 20mA | Power Dissipation: 120mW |
| Max Forward Current: 30mA | Operating Temp: -30 to +85 C |
| Pulse Current: 100mA for <= 10ms, duty <= 1/10 | Soldering Temp: 265 C for 10 secs |
| Forward Voltage: 3.2V typ. 3.5V max @ 20mA | Max Reverse Current: 50uA @ 5V |
| Max Reverse Voltage: 5V | 50° Degree Half Angle |
Meanwell Power source (I am using 8w/24v)
Product Details
| INPUT POWER: 90 ~ 264Vac | CASING: Fully Isolated Plastic Case, IP42 NON PFC |
| OUTPUT VOLTAGE: 5-36VDC | WATTS: 8W up to 35W |
| CONNECTIONS: UL Rated 18-20 AWG Wire | Low Cost, High Reliability |
| WARRANTY: 2-Years | SAFETY: Class 2 Power Unit, Short-Circuit, Over-Load & Over-Voltage |
| Super Small & Compact Size! | APPLICATIONS: LED Lighting & Moving Sign Applications |
Hi Avo I just posted a general reply with the details of what equipment I am using. I'd be very grateful if you take a look at that and then let me know if you think putting a separate dynaohm on each string would be the best approach, or if you think some other arrangement would be more suitable. Thanks!!
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Hi Brouhaha
Thanks for the data. Everything we need is there.
Are your LED's embodied in the sculpture already, or are you still at the experimental stage?
If you still have freedom to play with the arrangement, I would suggest splitting the LED's into two strings of six, each with a Dynaohm in series.
Taking the 'typical' figures, with 3.2V across each lamp, thats 19.2V across six, plus the drop across the Dynaohm of 2.6V = 21.8V so with a 24V supply it should work - BUT if we take 'worst case' of 3.5V across each lamp, that's 21V across all, plus 2.6V = 23.6V, rather close to your power supply output. That's why I think it's best to try it!
You can of course keep to your original scheme of having three strings, but with a smaller number of lamps in each string, I think it's a good idea to have a Dynaohm for each. I was just trying to save you having to buy more Dynaohm's than you need!
If you have a multimeter it would be useful to check you do get these voltages and currents in practice.
Finally I'm not clear why the bulbs should "blow up" unless you connect one the wrong way round, or directly across the supply!
I can see why some may not light when you have parallel strings of a small number of lamps - that's what you need to avoid.
Unless your Dynaohm has blown up while experimenting. It's vital you have a current limiting device like that in each circuit, otherwise you will destroy the bulbs connecting them straight to the psu.
It would be really useful to have a multimeter to check you have 20mA flowing through the Dynaohm(s).
Good luck with the project
Thanks for the data. Everything we need is there.
Are your LED's embodied in the sculpture already, or are you still at the experimental stage?
If you still have freedom to play with the arrangement, I would suggest splitting the LED's into two strings of six, each with a Dynaohm in series.
Taking the 'typical' figures, with 3.2V across each lamp, thats 19.2V across six, plus the drop across the Dynaohm of 2.6V = 21.8V so with a 24V supply it should work - BUT if we take 'worst case' of 3.5V across each lamp, that's 21V across all, plus 2.6V = 23.6V, rather close to your power supply output. That's why I think it's best to try it!
You can of course keep to your original scheme of having three strings, but with a smaller number of lamps in each string, I think it's a good idea to have a Dynaohm for each. I was just trying to save you having to buy more Dynaohm's than you need!
If you have a multimeter it would be useful to check you do get these voltages and currents in practice.
Finally I'm not clear why the bulbs should "blow up" unless you connect one the wrong way round, or directly across the supply!
I can see why some may not light when you have parallel strings of a small number of lamps - that's what you need to avoid.
Unless your Dynaohm has blown up while experimenting. It's vital you have a current limiting device like that in each circuit, otherwise you will destroy the bulbs connecting them straight to the psu.
It would be really useful to have a multimeter to check you have 20mA flowing through the Dynaohm(s).
Good luck with the project
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I'm attaching a photo of the sculpture, taken when it was running on the 15V power supply, and a 30mA dynaohm, and one of the bulbs was not lighting up. I then tried changing to the current power supply, which is 8W/24V, and all the lights illuminated but then one string went dark (they did not blow up or even look different in any way, just stopped lighting. I then rebuilt it, replacing the four dead bulbs, keeping the 8W/24V power supply but changing to a 20mA Dynaohm (at the suggestion of someone at LEDSupply.com) but once again a string of bulbs went dark, followed in short order by the remaining strings. I wish that the bulbs were ok and that I just need to fix something about the power supply and resistor, because replacing the bulbs means a time-consuming rebuilding process, but I am assuming that in fact the bulbs are permanently dysfunctional now and will need to be replaced. So, in answer to your question, I guess I am back to the experimental stage!Hi Brouhaha
Thanks for the data. Everything we need is there.
Are your LED's embodied in the sculpture already, or are you still at the experimental stage?
If you still have freedom to play with the arrangement, I would suggest splitting the LED's into two strings of six, each with a Dynaohm in series.
Taking the 'typical' figures, with 3.2V across each lamp, thats 19.2V across six, plus the drop across the Dynaohm of 2.6V = 21.8V so with a 24V supply it should work - BUT if we take 'worst case' of 3.5V across each lamp, that's 21V across all, plus 2.6V = 23.6V, rather close to your power supply output. That's why I think it's best to try it!
You can of course keep to your original scheme of having three strings, but with a smaller number of lamps in each string, I think it's a good idea to have a Dynaohm for each. I was just trying to save you having to buy more Dynaohm's than you need!
If you have a multimeter it would be useful to check you do get these voltages and currents in practice.
Finally I'm not clear why the bulbs should "blow up" unless you connect one the wrong way round, or directly across the supply!
I can see why some may not light when you have parallel strings of a small number of lamps - that's what you need to avoid.
Unless your Dynaohm has blown up while experimenting. It's vital you have a current limiting device like that in each circuit, otherwise you will destroy the bulbs connecting them straight to the psu.
It would be really useful to have a multimeter to check you have 20mA flowing through the Dynaohm(s).
Good luck with the project
Do you think that there is any chance that the existing bulbs might still be functional and that they would light up again if I fixed some other problem (e.g. replace the Dynaohm or power supply with something different)?
If not, I will try to redesign as you suggest with two strings of six, each with it's own dynaohm (do you think 20maA is the correct size for those?)
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Given your bad luck with this, and the work you've put in, I feel it would be worth getting a cheap multimeter (if you don't already have one), get familiar with it, and every time you connect things up, put the meter in circuit on a suitable milliamp range to check the current is what it should be.
Also you would be able to check the resistance values of the lamps you have, to establish if they still might work before you connect them to power.
My concern is that once things have started to go wrong, you can never be sure what might have been damaged. Are the power supply and dynaohm doing what they should? And if a bulb is out, is it because the wires are shorted together, or the bulb is faulty etc. etc.
Your lamps have an absolute max forward current of 30mA, so I would be inclined to run them at 20mA, unless you need the extra brightness. And if you do, I suspect they won't last as long!
If you can get at the wiring to each lamp, you could check each in turn using a working Dynaohm and supply. Be aware that putting the power on the wrong way round, or supplying excessive current because the Dynaohm is faulty, will instantly destroy the lamp (you won't be able to tell by looking at it).
Your power supplies have protection devices in them, but activated at much higher current/ voltage than your lamps will survive. A safer way of proceeding would be to find a constant current 20mA supply that could give up to 48V, and connect all your lamps in series to that. But I confess I haven't found one yet!
I hope the above makes sense. I would be cautious connecting things up now, as I don't really understand why so many bulbs have failed unless your Dynaohm has too, or the power supply is doing something strange.
Regards.
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Hi Brouhaha
One thing occurred to me - have you connected the Dynaohm the right way round? There's a little "+" on the positive end, and, as supplied, the leads each end are different lengths to also indicate the polarity (don't remember which way round - sorry)
People call it a resistor, but it is a semiconductor device, and only does its job when wired the right way round.
Putting it the wrong way round could destroy the lamps.
And as far as testing/re-using lamps - is the wiring you've done marked with the polarity of the lamps? ie red and black wires, or a tracer mark on one wire, so within the sculpture you know which is anode and which cathode for each lamp?
One thing occurred to me - have you connected the Dynaohm the right way round? There's a little "+" on the positive end, and, as supplied, the leads each end are different lengths to also indicate the polarity (don't remember which way round - sorry)
People call it a resistor, but it is a semiconductor device, and only does its job when wired the right way round.
Putting it the wrong way round could destroy the lamps.
And as far as testing/re-using lamps - is the wiring you've done marked with the polarity of the lamps? ie red and black wires, or a tracer mark on one wire, so within the sculpture you know which is anode and which cathode for each lamp?
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Hi Avo, I have the positive wire coming off of my power supply going to the negative terminal of the Dynaohm--is that correct?
I did color code the wires to the bulbs.
I do have a cheap, old analog multimeter, but I've wondered about it's reliability, so maybe I should get a new one...
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I think this is incorrect. The Dynaohm positive should got to battery positive according to the data sheet.
OH!! That would explain a lot! I am confused however, because I have similarly been wiring the positive power wire to the negative terminal of my LEDs and they do light up when I connect them that way (and not the opposite way). So is there a different convention regarding how the terminals on a resistor are labeled versus those on a bulb?
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As DPG replied earlier, no I'm afraid not!
Brilliant - that means you can check (with multimeter) if any are at least still diodes, rather than open or short circuit!
nothing wrong with old analogue multimeters. mine's 50+ years old and still good enough to use. If yours does resistance measurements, great! (Might need a new battery?)
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You are connecting these the correct way.
The Dynaohm is NOT A RESISTOR
. Sorry, not your fault, it's theirs for marketing it like they do, and everyone calling it a resistor! With an ordinary resistor it doesn't matter which way round it's connected, it does the same thing either way.Whereas the Dynaohm must have some semiconductor devices in it, to alter the voltage across it so that the current through it stays the same.
We should assume the Dynaohm could be damaged, though I think it does have some overload features - I don't know if they save it from reverse connection. If not too costly I would suggest getting another one or two, and putting yours aside for the moment. UPDATE: just looked at the data sheet which says maximum reverse voltage 1V so I suspect it's broke!
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I'd take Avo's advice. At the minute you have a Dynaohm of unknown condition, and LEDs which may have been damaged. Get a new Dynaohm, and some known good LEDs. Connect them up correctly, maybe in a string of 4, and test them.
Use your multimeter to measure LED current if it will do DC amps. Monitor it at switch on and be ready to switch off if the current is too high.
Use your multimeter to measure LED current if it will do DC amps. Monitor it at switch on and be ready to switch off if the current is too high.
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Thank you Avo and DPG for your very helpful replies! It will probably be a few weeks before I can get the replacement Dynaohm and have an opportunity to try testing/rebuilding, but I will come back here either with a success story--or more questions! Thanks so much!
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DPG gives good suggestions in his last post.
Just to reiterate, don't wire it up as your original drawing back in post #1. You need 20mA through each string of lamps, so however many you end up with in series, four or six, have a Dynaohm for each string. Perhaps they'll give you a loyalty discount
Best wishes for getting it all working!
ok, I got new 20mA dynaohms, new LEDs and started over; I made three parallel strings of 4 LEDs and tested each string using a couple of 9v batteries (and a Dynaohm, although it's possible that I might have left out the Dynaohm at some point while testing...) maybe that caused my subsequent problem?). I then wired everything up and tried powering it with my Meanwell power source and found that one of the bulbs in one of the strings is not lighting up--but the other three bulbs in that string are lighting up. I don't even understand how that is possible, I thought if one bulb was bad the whole string would go dark. Any idea how that could happen? And any ideas why the strings were working before I wired up to power source but not afterward?DPG gives good suggestions in his last post.
Just to reiterate, don't wire it up as your original drawing back in post #1. You need 20mA through each string of lamps, so however many you end up with in series, four or six, have a Dynaohm for each string. Perhaps they'll give you a loyalty discount
Best wishes for getting it all working!
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Sorry you still have problems.
regarding the LED not lighting up, if a diode fails 'short circuit', the others will carry on working, and the Dynaohm will adjust the current to still be correct for the others.
It could be the diode legs, or the wires to that diode, are shorted together.
It could be that you've inadvertently wired it the wrong way round, and as a result its gone short circuit (but in that case it would never have lit up when you first tried it).
It could be that the diode has just failed short circuit.
Suspect you need to replace that one diode unfortunately, if that is practicable.
ah--I hadn't realized that current could still pass through a dead bulb. I will replace that one (again!) and hopefully eventually get back here to report success!
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Yes unlike a traditional filament lamp which fails open-circuit like a fuse, an LED typically fails short-circuit especially if due to overcurrent, as the diode semiconducting junction breaks down and becomes fully conductive. Unless it has been blasted with such a high current that the bond wires have ruptured.
Each string must have its own constant current device.
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I think that's what the OP has done now. He refers to more than one Dynaohm in post #27.
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Sort of covered that issue in post #10 onwards!
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Yeah we've covered that I think.
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I have ordered some Dynaohms from a US supplier and similar leds from a UK one. I suspect that the assumption that the dynaohm regulates the current through it to 20mA whatever the potential difference across it is false. I would be surprised if such a simple device was a perfect constant current regulator.
Regarding the LEDs the current/Vforward characteristic will be similar to the plot I have drawn as A in the attachment. Above the minimum space charge voltage (about 2V) the current through it rises rapidly with any change in potential difference across the LED. This is the reason the LED current must be regulated to prevent the LED burning out due to escalating Ohmic heating. A typical LED has a forward resistance once conducting of about 20 Ohms (the slope of the upward rising part of the plot dV/dI).
To make matters worse, the forward voltage for a given forward current reduces with LED junction temperature which means as it gets hotter the forward current will increase which in turn causes more heating which in turn results a lower forward voltage and thence higher forward current - thermal runaway. This is the mechanism which leads to burnout.
If the forward voltage reduces with temperature, and the circuit is supplied by a low impedance regulated voltage source such as the Meanwell PSU, the stabilisation of LED current requires a series device which has a very small increase in current when the voltage across it increases and over quite a range of dynaohm voltage drop. Ideally, the dV/dI is such that the current decreases a little each time the the voltage across it increases a little ie: it slopes downwards.
My suspicion is that the dynaohm's voltage current characteristic slopes upwards and that there is a large increase in current for each increase in voltage across it as I have attempted to show in plot C.
The combination of the dynaohm and the LED string does not perform to regulate the current to a 'safe' LED current which is leading to LEDs burning out. To make matters worse, it is not the reduction in forward voltage of one LED - it is a reduction of 3 to 4 times the voltage of on LED since they are in series. This exacerbates the increase in current through the dynaohm as the potential difference across it rises not by one LED's reduction but by the sum of all the reductions of how many LEDs are wired in series.
Anyway a theory at the moment on why your LEDs are failing. When the dynaohm and LEDs arrive I will does some tests and let you know what I find.
Regarding the LEDs the current/Vforward characteristic will be similar to the plot I have drawn as A in the attachment. Above the minimum space charge voltage (about 2V) the current through it rises rapidly with any change in potential difference across the LED. This is the reason the LED current must be regulated to prevent the LED burning out due to escalating Ohmic heating. A typical LED has a forward resistance once conducting of about 20 Ohms (the slope of the upward rising part of the plot dV/dI).
To make matters worse, the forward voltage for a given forward current reduces with LED junction temperature which means as it gets hotter the forward current will increase which in turn causes more heating which in turn results a lower forward voltage and thence higher forward current - thermal runaway. This is the mechanism which leads to burnout.
If the forward voltage reduces with temperature, and the circuit is supplied by a low impedance regulated voltage source such as the Meanwell PSU, the stabilisation of LED current requires a series device which has a very small increase in current when the voltage across it increases and over quite a range of dynaohm voltage drop. Ideally, the dV/dI is such that the current decreases a little each time the the voltage across it increases a little ie: it slopes downwards.
My suspicion is that the dynaohm's voltage current characteristic slopes upwards and that there is a large increase in current for each increase in voltage across it as I have attempted to show in plot C.
The combination of the dynaohm and the LED string does not perform to regulate the current to a 'safe' LED current which is leading to LEDs burning out. To make matters worse, it is not the reduction in forward voltage of one LED - it is a reduction of 3 to 4 times the voltage of on LED since they are in series. This exacerbates the increase in current through the dynaohm as the potential difference across it rises not by one LED's reduction but by the sum of all the reductions of how many LEDs are wired in series.
Anyway a theory at the moment on why your LEDs are failing. When the dynaohm and LEDs arrive I will does some tests and let you know what I find.
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Thank you Marconi for your lucid narrative.
Being a commercial product claiming to do the job (!), I had naively assumed it's performance would be adequate for the job in hand, and concentrated on why the OP was experiencing problems.
There were a number of unfortunate incidents during his experiments, incluging connecting diode strings directly across the Meanwell psu, putting 3 diode strings in parallel through one Dynaohm, and having the Dynaohm the wrong way round!
I'm hoping your experiments show that the Dynaohm product does in fact work adequately to prevent a 'runaway' situation.
The data sheet does say it reduces current as it gets hotter (!), but doesn't give any graphic detail on its actual performance:
Thank you for your interest
Being a commercial product claiming to do the job (!), I had naively assumed it's performance would be adequate for the job in hand, and concentrated on why the OP was experiencing problems.
There were a number of unfortunate incidents during his experiments, incluging connecting diode strings directly across the Meanwell psu, putting 3 diode strings in parallel through one Dynaohm, and having the Dynaohm the wrong way round!
I'm hoping your experiments show that the Dynaohm product does in fact work adequately to prevent a 'runaway' situation.
The data sheet does say it reduces current as it gets hotter (!), but doesn't give any graphic detail on its actual performance:
Thank you for your interest
Thanks for taking an interest in my little problem! As Avo MK8 mentioned, I made a number of bone-headed errors--so it will probably come as no surprise that your post goes way beyond my comprehension of electronics! But I will be curious to see the results of your experiments. My equipment came from a US company called LEDSupply.com. I have had a good impression of them so I am hoping it was just my own errors that caused the problems...
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For clarity this is how you should connect up your circuit - see attachment.
Are you insulating the bare soldered connections and leads to the components? Heat shrink tubing is a good way of doing this. You can buy it from many electronic component suppliers and on Amazon. Just gently hold the barrel of your soldering iron against the tube and slide up and down the length of the tube until it contracts. Have a few practices to develop the technique. 3mm should work well if you are using thin connecting wire. See:
315-13002 TA37 6-2-PO-X-BK | HellermannTyton Adhesive Lined Heat Shrink Tubing, Black 6mm Sleeve Dia. x 1.2m Length 3:1 Ratio, TA37 Series | RS Components - https://uk.rs-online.com/web/p/heat-shrink-tubing/8296079?cm_mmc=UK-PLA-DS3A-_-google-_-CSS_UK_EN_Cables_%26_Wires_Whoop-_-Heat+Shrink+Tubing_Whoop-_-8296079&matchtype=&pla-524231416997&gclid=Cj0KCQiAu62QBhC7ARIsALXijXSs9qVR5NI9ztEVmarqlds-0N_2t69wFRx7WIAyFduRNwxJx5kN0o4aAtl1EALw_wcB&gclsrc=aw.ds
Are you insulating the bare soldered connections and leads to the components? Heat shrink tubing is a good way of doing this. You can buy it from many electronic component suppliers and on Amazon. Just gently hold the barrel of your soldering iron against the tube and slide up and down the length of the tube until it contracts. Have a few practices to develop the technique. 3mm should work well if you are using thin connecting wire. See:
315-13002 TA37 6-2-PO-X-BK | HellermannTyton Adhesive Lined Heat Shrink Tubing, Black 6mm Sleeve Dia. x 1.2m Length 3:1 Ratio, TA37 Series | RS Components - https://uk.rs-online.com/web/p/heat-shrink-tubing/8296079?cm_mmc=UK-PLA-DS3A-_-google-_-CSS_UK_EN_Cables_%26_Wires_Whoop-_-Heat+Shrink+Tubing_Whoop-_-8296079&matchtype=&pla-524231416997&gclid=Cj0KCQiAu62QBhC7ARIsALXijXSs9qVR5NI9ztEVmarqlds-0N_2t69wFRx7WIAyFduRNwxJx5kN0o4aAtl1EALw_wcB&gclsrc=aw.ds
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For Brouhaha: The LEDs have arrived. I have just connected 4 LEDs with a 500 Ohm resistor ( made from two 1kOhm 1/4 W resistors in parallel) across a 24V dc regulated power supply to do a 24 hour soak test. Current 20 to 30mA. Turned on at 1115am.
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For Brouhaha:
TruOhm CR-50 560R 0.5W Carbon Film Resistor - Pack of 100 - https://www.rapidonline.com/truohm-cr-50-560r-0-5w-carbon-film-resistor-pack-of-100-62-0550
The forward voltage drop Vf I measured for each white LED were: 2.85V. 2.99V, 2.89V and 2.87V (or about 3V). These were measured after 22 hours being on. See attachment.
At link are the details of the required series voltage dropper resistor R which one calculates as follows:
PSU voltage - (number of series LEDs x Vf) =
24- (4 x 3) = 12V
R = 12/Led current
R = 12/0.02 = 600 Ohms.
Nearest value is 560 Ohms.
Power dissipation is (24 - 12) squared/560 = 144/560 = 0.26W Next higher power rating is 0.5W.
LED current is 12/560 = 0.021mA
You could use a much cheaper single 560 Ohms 0.5 Watt resistor instead of the Dynaohm to run 4 white LEDs using a 24V power supply. Other colour LEDs have different nominal forward voltages Vf.
See:
How to calculate the series resistor for an LED – Stompville - https://stompville.co.uk/?p=37
I will still do the test on the dynaohm + LED circuit when it arrives from the USA.
TruOhm CR-50 560R 0.5W Carbon Film Resistor - Pack of 100 - https://www.rapidonline.com/truohm-cr-50-560r-0-5w-carbon-film-resistor-pack-of-100-62-0550
The forward voltage drop Vf I measured for each white LED were: 2.85V. 2.99V, 2.89V and 2.87V (or about 3V). These were measured after 22 hours being on. See attachment.
At link are the details of the required series voltage dropper resistor R which one calculates as follows:
PSU voltage - (number of series LEDs x Vf) =
24- (4 x 3) = 12V
R = 12/Led current
R = 12/0.02 = 600 Ohms.
Nearest value is 560 Ohms.
Power dissipation is (24 - 12) squared/560 = 144/560 = 0.26W Next higher power rating is 0.5W.
LED current is 12/560 = 0.021mA
You could use a much cheaper single 560 Ohms 0.5 Watt resistor instead of the Dynaohm to run 4 white LEDs using a 24V power supply. Other colour LEDs have different nominal forward voltages Vf.
See:
How to calculate the series resistor for an LED – Stompville - https://stompville.co.uk/?p=37
I will still do the test on the dynaohm + LED circuit when it arrives from the USA.
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The traditional methods are often the best.
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The dynaohms I ordered from digikey Minnesota USA on Monday arrived at lunch today Friday. Great postal service!
Now in circuit instead of the conventional resistor I used before. See attachment for dynaohm voltage drop and led current. I will leave it on for 24 hours and then do the tests I mentioned a few posts ago.
Now in circuit instead of the conventional resistor I used before. See attachment for dynaohm voltage drop and led current. I will leave it on for 24 hours and then do the tests I mentioned a few posts ago.
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For Brouhaha: After 24 hours all 4 LEDs lit and current steady at 23mA using a dynaohm.
I then varied the applied voltage from the psu and filmed the two meters measuring current flowing and voltage drop across the dynaohm. Current remains nice and constant at 23mA until dynaohm’s minimum voltage drop criteria is not met. See attachment.
I cannot find anything wrong with using a series circuit of 4 Leds in series with a 20mA dynaohm fed from a 24 V dc power supply.
I hope this gives you confidence in your project‘s lighting design.
(You could wire more LEDs in series and then in series with one dynaohm provided that you make sure 24V - (number of LEDs x Vforward) is a voltage greater than the minimum dynaohm voltage drop of 2.6V - assuming a 24V psu. Off the top of my head I would only design a circuit which ensured the dynaohm voltage drop is no less than say 6V to take into account variations in individual LED forward voltages and PSU output voltage. Always check first though before you assemble in your project. A 6V dynaohm volt drop would drive 6 LEDs each dropping 3V fed by a 24V psu. If the same LEDs each dropped 3.5V, then 6 would in total drop 21V leaving 3 volts across the dynaohm - greater than 2.6V but not by much margin. You get the idea. Probably best to stick at 4 or 5 LEDs in series maximum to ensure at least 6V across dynaohm if your LEDs all drop typical 3.5V.)
I then varied the applied voltage from the psu and filmed the two meters measuring current flowing and voltage drop across the dynaohm. Current remains nice and constant at 23mA until dynaohm’s minimum voltage drop criteria is not met. See attachment.
I cannot find anything wrong with using a series circuit of 4 Leds in series with a 20mA dynaohm fed from a 24 V dc power supply.
I hope this gives you confidence in your project‘s lighting design.
(You could wire more LEDs in series and then in series with one dynaohm provided that you make sure 24V - (number of LEDs x Vforward) is a voltage greater than the minimum dynaohm voltage drop of 2.6V - assuming a 24V psu. Off the top of my head I would only design a circuit which ensured the dynaohm voltage drop is no less than say 6V to take into account variations in individual LED forward voltages and PSU output voltage. Always check first though before you assemble in your project. A 6V dynaohm volt drop would drive 6 LEDs each dropping 3V fed by a 24V psu. If the same LEDs each dropped 3.5V, then 6 would in total drop 21V leaving 3 volts across the dynaohm - greater than 2.6V but not by much margin. You get the idea. Probably best to stick at 4 or 5 LEDs in series maximum to ensure at least 6V across dynaohm if your LEDs all drop typical 3.5V.)
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