-DIY- Powering a 3 volt LED from a 4.5V supply

[T800mc]

Hello,

I'm wanting to power 1 x 3v led from a 4.5v power supply and 2 x 1.5v leds from another 4.5v supply.
Can anyone advise what resistor I have to install on each one. Using an online calculator I'm getting 82 Ohm on a calculator. is that correct? Is there anything else I need to take into account?

Thanks

 

[Lucien Nunes]

The actual chip in an LED requires the current to be set by the resistor, not the voltage. You feed in a known current, and the voltage drop that occurs across it is whatever it is, usually close to the stated value. This is due to the exponential relationship between current and voltage in a semiconductor.

To calculate the resistor value, you first need to decide what current is to flow, which depends on the desired brightness, whether power saving is important e.g. in a battery application, the efficiency of the LED, and of course must be within the LED's rated maximum. A typical current for an indicator LED is 20mA, a high-efficiency one might work fine on 1mA.

Then, subtract the forward voltage (Vf) from the supply voltage (Vs) and divide by the current (If).

If Vs=4.5V, Vf=3V, If=20mA
R = (Vs - Vf) / If
= (4.5 - 3) / 0.02
=75Ω

 

[T800mc]

The actual chip in an LED requires the current to be set by the resistor, not the voltage. You feed in a known current, and the voltage drop that occurs across it is whatever it is, usually close to the stated value. This is due to the exponential relationship between current and voltage in a semiconductor.

To calculate the resistor value, you first need to decide what current is to flow, which depends on the desired brightness, whether power saving is important e.g. in a battery application, the efficiency of the LED, and of course must be within the LED's rated maximum. A typical current for an indicator LED is 20mA, a high-efficiency one might work fine on 1mA.

Then, subtract the forward voltage (Vf) from the supply voltage (Vs) and divide by the current (If).

If Vs=4.5V, Vf=3V, If=20mA
R = (Vs - Vf) / If
= (4.5 - 3) / 0.02
=75Ω

Thanks for the input. That lines up with the calculator I used. It also said "Nearest higher rated resistor" as 82 Ohms. Is there a recommendation for having a higher rated one?

 

[marconi]

Re #1: Is there anything else I need to take into account?

Once you have calculated the required resistance value, the next thing to work out is the power rating of the resistor. Because a current is flowing through the resistance there will be Ohmic heating inside it - so it will warm up and this heat must be lost to the surroundings. If the resistor warmed up too much its value would increase and may be damaged too.

The power dissipated P is calculated by P = Isquared x R where I is the current through the resistor and R is its value.

In your case P = 0.02 x 0.02 x 75 = 0.03Watts

You should select a 0.25Watt rated resistor because it is greater than 0.03W and one of the standard power ratings for axial resistors.

 

[Lucien Nunes]

By 'higher rating' we usually mean power rating, not resistance, which is normally referred to as the 'value.' The higher the resistance, the lower the current. You'll hardly notice the difference here, but in any case you will probably want to assess the brightness and adjust accordingly. 20mA is just a starting point for vanilla-flavour LEDs. If you choose a high efficiency type you might be OK on 2mA, in which case 750 ohms is the value you need.

 

[T800mc]

Re #1: Is there anything else I need to take into account?

Once you have calculated the required resistance value, the next thing to work out is the power rating of the resistor. Because a current is flowing through the resistance there will be Ohmic heating inside it - so it will warm up and this heat must be lost to the surroundings. If the resistor warmed up too much its value would increase and may be damaged too.

The power dissipated P is calculated by P = Isquared x R where I is the current through the resistor and R is its value.

In your case P = 0.02 x 0.02 x 75 = 0.03Watts

You should select a 0.25Watt rated resistor because it is greater than 0.03W and one of the standard power ratings for axial resistors.

Thank you for that.
[automerge]1597131115[/automerge]

By 'higher rating' we usually mean power rating, not resistance, which is normally referred to as the 'value.' The higher the resistance, the lower the current. You'll hardly notice the difference here, but in any case you will probably want to assess the brightness and adjust accordingly. 20mA is just a starting point for vanilla-flavour LEDs. If you choose a high efficiency type you might be OK on 2mA, in which case 750 ohms is the value you need.

[automerge]1597131289[/automerge]

By 'higher rating' we usually mean power rating, not resistance, which is normally referred to as the 'value.' The higher the resistance, the lower the current. You'll hardly notice the difference here, but in any case you will probably want to assess the brightness and adjust accordingly. 20mA is just a starting point for vanilla-flavour LEDs. If you choose a high efficiency type you might be OK on 2mA, in which case 750 ohms is the value you need.

By 'higher rating' we usually mean power rating, not resistance, which is normally referred to as the 'value.' The higher the resistance, the lower the current. You'll hardly notice the difference here, but in any case you will probably want to assess the brightness and adjust accordingly. 20mA is just a starting point for vanilla-flavour LEDs. If you choose a high efficiency type you might be OK on 2mA, in which case 750 ohms is the value you need.

I honestly don't know what to do with this information. It's beyond me.

This is the LED:

Vishay3 V Green LED 3mm Through Hole, TLHG4900 | RS

uk.rs-online.com

This is the power supply:

http://www.soapstudio.com/index.php?route=product/product&path=477&product_id=645

It has 4.5v 0.03a output.

 

[marconi]

resistors | Rapid Online - https://www.rapidonline.com/Catalogue/Search?Query=resistors&Size=20&Attributes={%22Case%20Type%22:[%22Axial%22]}

Here is a link to a component supplier from which you can buy suitable resistors. You can use 'the select attributes' to search for your desired resistor.

You can buy leds too and obtain their datasheets to find out forward voltage (-changes with colour - why? - look up Fermi levels) and forward current - eg:

Kingbright | Rapid Online - https://www.rapidonline.com/brands/kingbright?ra_source=tier-page

 

[marconi]

https://www.vishay.com/docs/83009/tlhg4900.pdf

Studying page 2 of the reference at link, the tlhg4900 green led has a typical forward voltage of 2.4V and a forward current of typically 10mA.

From a 4.5V supply, the required series voltage dropper resistance to drop 2.1 V at 10mA is (4.5 - 2.4)/0.01 =

2.1/0.01 = 210 Ohms. Nearest value is 220 Ohms and at least 250mW

Buy some of these:

Royal Ohm CFR01SJ0221A10 220R 5% 1W Axial Carbon Film Resistor - https://www.rapidonline.com/royal-ohm-cfr01sj0221a10-220r-5-1w-axial-carbon-film-resistor-73-0065

and wire in series with the led and then connect to power supply. Your power supply delivers up to 30mA per output so in theory you could power three of these LEDs.from one output; the device has eight 4.5V 30mA outputs so better to spread your LED supplies over these outputs. In theory 24 such LEDs could be illuminated.

Do it like this noting that the longer lead of the LED connects to the + side of the battery. (In your case the 9V battery is one of the SOAPBOX PSU outputs and you will use a 220 Ohms resistor instead of the 470 one shown below):

 

[T800mc]

https://www.vishay.com/docs/83009/tlhg4900.pdf

Studying page 2 of the reference at link, the tlhg4900 green led has a typical forward voltage of 2.4V and a forward current of typically 10mA.

From a 4.5V supply, the required series voltage dropper resistance to drop 2.1 V at 10mA is (4.5 - 2.4)/0.01 =

2.1/0.01 = 210 Ohms. Nearest value is 220 Ohms and at least 250mW

Buy some of these:

Royal Ohm CFR01SJ0221A10 220R 5% 1W Axial Carbon Film Resistor - https://www.rapidonline.com/royal-ohm-cfr01sj0221a10-220r-5-1w-axial-carbon-film-resistor-73-0065

and wire in series with the led and then connect to power supply. Your power supply delivers up to 30mA per output so in theory you could power three of these LEDs.from one output; the device has eight 4.5V 30mA outputs so better to spread your LED supplies over these outputs. In theory 24 such LEDs could be illuminated.

Do it like this noting that the longer lead of the LED connects to the + side of the battery. (In your case the 9V battery is one of the SOAPBOX PSU outputs and you will use a 220 Ohms resistor instead of the 470 one shown below):

View attachment 60053

Okay great, thanks. I will power that LED from one output and use another output for the other two.
For the two other LEDs, I don't have much information other than they are normally powered by 2 x LR44 batteries for which I work out as 3v total so one battery per LED, can you advise what resistors to fit that cable?

 

[marconi]

It looks to me that the plug you need to insert into one of the 30mA sockets on the soapbox reactor is of this type:

dc plugs | Rapid Online - https://www.rapidonline.com/Catalogue/Search?Query=dc%20plugs&Size=20&Tier=Low%20Power

If you measure the external diameter and internal diameter you can then order them.

You can even buy pre-wired connectors:

wired low voltage connectors | Rapid Online - https://www.rapidonline.com/Catalogue/Search?Query=wired%20low%20voltage%20connectors&Size=20
[automerge]1597146035[/automerge]
What colour are these two leds and what size roughly?
[automerge]1597146726[/automerge]
If you send me a private message with your address I will post you some resistors gratis.

 

[T800mc]

It looks to me that the plug you need to insert into one of the 30mA sockets on the soapbox reactor is of this type:

dc plugs | Rapid Online - https://www.rapidonline.com/Catalogue/Search?Query=dc%20plugs&Size=20&Tier=Low%20Power

If you measure the external diameter and internal diameter you can then order them.

You can even buy pre-wired connectors:

wired low voltage connectors | Rapid Online - https://www.rapidonline.com/Catalogue/Search?Query=wired%20low%20voltage%20connectors&Size=20
[automerge]1597146035[/automerge]
What colour are these two leds and what size roughly?
[automerge]1597146726[/automerge]
If you send me a private message with your address I will post you some resistors gratis.

I got quite a few leads with it of various type so I can adapt them but handy to know if I want more. And thank you very much for the offer of resistors, I can get them through work luckily.

The two LED's are in the head of this so other than knowing they are powered by 2x LR44 I know nothing else about them:

DC Comics Armored Batman Premium Format(TM) Figure by Sidesh

The Armored Batman Premium Format Figure is now available at Sideshow.com for fans of Batman v Superman: Dawn of Justice and Ben Affleck.

www.sideshow.com

 

[marconi]

Ok. The two leds for Batman's eyes look white to me so forward voltage drop is similar to the green led ie 2.4V with forward current of 10mA. They light up when connected to a 3V battery which suggests the two LEDs are connected in parallel across it - 2.4V < 3V. If the two LEDs were in series the required battery voltage would need to be greater than 2 x 2.4 = 4.8V.

The required voltage drop resistor to illuminate a paralleled pair of these LEDs using the 4.5V supply is (4.5-2.4)/ (2 x 0.01) = 2.1/0.02 = 105 Ohms (nearest value is 100 Ohm) nb (2 x 0.01) because there are two leds in parallel).

You could obtain 100 Ohm by wiring two 220 Ohm resistors in parallel or by buying a 100 Ohm component.

Power rating is 0.02 x 0.02 x 100 = 0.04 W so a 1/4Watt or higher rated resistor.

Please tell us how you fare with this project.

 

[T800mc]

Ok. The two leds for Batman's eyes look white to me so forward voltage drop is similar to the green led ie 2.4V with forward current of 10mA. They light up when connected to a 3V battery which suggests the two LEDs are connected in parallel across it - 2.4V < 3V. If the two LEDs were in series the required battery voltage would need to be greater than 2 x 2.4 = 4.8V.

The required voltage drop resistor to illuminate a paralleled pair of these LEDs using the 4.5V supply is (4.5-2.4)/ (2 x 0.01) = 2.1/0.02 = 105 Ohms (nearest value is 100 Ohm) nb (2 x 0.01) because there are two leds in parallel).

You could obtain 100 Ohm by wiring two 220 Ohm resistors in parallel or by buying a 100 Ohm component.

Power rating is 0.02 x 0.02 x 100 = 0.04 W so a 1/4Watt or higher rated resistor.

Please tell us how you fare with this project.

Okay great! Will do, thanks!

 

[T800mc]

Okay great! Will do, thanks!

I fitted a 100 Ohm resistor today. Voltage is still 4.5 when I test it. What am I doing wrong?

 

[marconi]

Hello. four things first:

1. please draw me circuit diagram of what you have connected together.

2. Show me where you connect your voltmeter to measure 4.5V.

3. Do the LEDs glow?

4. Read out the colours on the resistor from one end to the other.

 

[DPG]

Marconi, are you thinking the same as me, ie he has the resistor connected to the power supply, and is expecting the voltage to drop even without a load connected?

 

[T800mc]

Hello. four things first:

1. please draw me circuit diagram of what you have connected together.

2. Show me where you connect your voltmeter to measure 4.5V.

3. Do the LEDs glow?

4. Read out the colours on the resistor from one end to the other.

1. Drawing attached.

2. Shown on drawing.

3. No they don't.

4. There are no colours on the resistor, it's one of these:

Vishay 100mΩ Wire Wound Resistor 1W ±1% RS01AR1000FE12 | RS

uk.rs-online.com

 

[T800mc]

Marconi, are you thinking the same as me, ie he has the resistor connected to the power supply, and is expecting the voltage to drop even without a load connected?

That is what I did first. I tested it without having the lights connected. Connecting the lights too I get 4.5 volts.

 

[DPG]

Thanks. In that case, your answers to Marconi's questions will solve the mystery.

My bet, assuming the LEDs are lit, is that you are measuring across the wrong points.

 

[marconi]

Sir,

The resistor you have bought and wired in is 0.1 Ohms ie: 100milli Ohms/100mOhms.

I think you have clicked on the wrong value as you require 100 Ohms - check what I said earlier - is this for the Batman eyes?

So step by step you need to obtain some 100 Ohms resistors. Let me know when they arrive and I will talk you through it.

Meanwhile are you using a digital multimeter? Can you see the 'Ohms range'? Just switch to the lowest Ohms range and short the leads together - does the display read zero or near sero Ohms? Now measure the resistance of the LED with the leads connected one way round and then reversed. With one of these tests does the LED glow faintly? - you may need to reduce the lighting to tell.

Offer stands - do you want me to post you some 220 Ohm and 100 Ohm resistors? - they are only gathering dust in my man-shed.

The good news is that the SoapBox seems to work!

 

[T800mc]

Sir,

The resistor you have bought and wired in is 0.1 Ohms ie: 100milli Ohms/100mOhms.

I think you have clicked on the wrong value as you require 100 Ohms - check what I said earlier - is this for the Batman eyes?

So step by step you need to obtain some 100 Ohms resistors. Let me know when they arrive and I will talk you through it.

Meanwhile are you using a digital multimeter? Can you see the 'Ohms range'? Just switch to the lowest Ohms range and short the leads together - does the display read zero or near sero Ohms? Now measure the resistance of the LED with the leads connected one way round and then reversed. With one of these tests does the LED glow faintly? - you may need to reduce the lighting to tell.

Offer stands - do you want me to post you some 220 Ohm and 100 Ohm resistors? - they are only gathering dust in my man-shed.

The good news is that the SoapBox seems to work!

Oops, makes sense why it hasn't worked then.

I do have some 220 Ohm resistors too (he thinks lol). I could wire them in parallel like you suggested but I'm not sure how to do that.

If you really don't mind sending them to me I'll pm you my address, thank you.

Yes a digital multimeter. I will try that tomorrow.

 

[DPG]

Connect the ends of each of the two 220 ohm resistors together and you can treat them as a 110 ohm device.

 

[T800mc]

Connect the ends of each of the two 220 ohm resistors together and you can treat them as a 110 ohm device.

So they are next to each other? Both ends soldered together with a wire for each end of the double?

 

[DPG]

So they are next to each other? Both ends soldered together with a wire for each end of the double?

Yes, place the resistors together. Solder both wires together at each end. Google it and it will become obvious

 

[marconi]

Resistors posted today so should be with you Wed 19th.

 

[T800mc]

Resistors posted today so should be with you Wed 19th.

Resistors posted today so should be with you Wed 19th.

Thanks again, I shall have a play and report back.

 

[T800mc]

Resistors posted today so should be with you Wed 19th.

They all turned up great, thanks again!

I've put it together and the voltage still isn't dropping. See pic. I tried it anyway and the eyes don't light up.
Before I did anything I checked that the lead that goes into Batman head worked, put it in his head and attached a battery to the other end they lit up. Is it possible the soap reactor is somehow compensating?

 

[DPG]

So the LED lights when powered via the resistor from a battery?

But not when powered from the soap unit?

 

[T800mc]

So the LED lights when powered via the resistor from a battery?

But not when powered from the soap unit?

I meant the battery adapter with wires on it that goes in the head to replace the batteries worked just via the battery attached.
I just tried batteries through the resistor too and they lit up.

 

[marconi]

I reckon the Soap reactor contains circuitry such that each 4.5V 30mA output acts just like a battery without the need for the extra resistor. The no-load voltage is 4.5V but more importantly it will provide a current up to 30mA to illuminate the LEDs; as it provides this current the output voltage at the soap box socket will fall a little but not so much that it becomes less than the forward voltage requirement of the LED.

I will send you some LEDs to experiment with to confirm my thinking. If they are damaged then it does not matter but we don't want to damage those in the figurines.

I will draw out and explain the experiments in a later email - may not be today though.

When I look at the youtube explanations for the soap reactor the lead from the reactor turns into two thin wires at the end of which is a stubby cylindrical shape which substitutes for one or more 'hearing aid batteries' - this is inserted in lieu of the battery/batteries. Is this what you have pictured with the meter prongs at either end?

You will receive a 4 LED indicator (green, yellow, yellow, red) with wires already soldered on. Should arrive tomorrow 20 Aug.

 

[T800mc]

I reckon the Soap reactor contains circuitry such that each 4.5V 30mA output acts just like a battery without the need for the extra resistor. The no-load voltage is 4.5V but more importantly it will provide a current up to 30mA to illuminate the LEDs; as it provides this current the output voltage at the soap box socket will fall a little but not so much that it becomes less than the forward voltage requirement of the LED.

I will send you some LEDs to experiment with to confirm my thinking. If they are damaged then it does not matter but we don't want to damage those in the figurines.

I will draw out and explain the experiments in a later email - may not be today though.

When I look at the youtube explanations for the soap reactor the lead from the reactor turns into two thin wires at the end of which is a stubby cylindrical shape which substitutes for one or more 'hearing aid batteries' - this is inserted in lieu of the battery/batteries. Is this what you have pictured with the meter prongs at either end?

Yes that is what I have pictured with the prongs at either end.

I do have some spare LEDs I can play with here. I have some these if they will work:

Broadcom3.6 V Green LED 5mm Through Hole, HLMP-CM2H-130DD | RS

uk.rs-online.com

Vishay3 V Green LED 3mm Through Hole, TLHG4401 | RS

uk.rs-online.com

Vishay3 V Green LED 3mm Through Hole, TLHG4900 | RS

uk.rs-online.com

 

[marconi]

Right. Take your multimeter(MM) and select dc volts and identify which of the ends of the stubby item is +ve and which is -ve. Confirm 4.5V.

Now select dc amps on MM on a scale which will read 30mA.

Take an LED. Put long lead on the +ve of stubby item. Then put one MM lead on short lead of LED. Finally put the other MM lead on -ve of stubby item. Current should be 30mA or lower and LED lights up.

 

[T800mc]

Right. Take your multimeter(MM) and select dc volts and identify which of the ends of the stubby item is +ve and which is -ve. Confirm 4.5V.

Now select dc amps on MM on a scale which will read 30mA.

Take an LED. Put long lead on the +ve of stubby item. Then put one MM lead on short lead of LED. Finally put the other MM lead on -ve of stubby item. Current should be 30mA or lower and LED lights up.

The stubby item has + and - on it. When I connect red to + and black to - I get 4.5v.
If I connect the other way round I get -4.5v.

My MM doesn't do milliamps, I'll try and find another one.

Am I supposed to be doing this test on the lead with the resistor in it or normal one?

 

[marconi]

Your MM does do mA. Turn the knob so that its dot is at 6 o'clock - the last position going clockwise. the 'A' position without Hz beside it.

Use the lead without the resistor.

 

[marconi]

I did not have any single LEDs so I sent you a four LED panel indicator. Somes wires have been soldered to it from a previous project. Please study the attachment which shows you how the wires have been connected.

These LEDs only pass 10mA to light up so what I'd like you to do is connected all the wires on side together and all the wires on the other side together as I showed in my first attachment.

Then connect it up as I described in my #32 - all four LEDs should light up. Leave it on for five minutes. Note down the current you measure on your MM

Now connect up as shown in my second drawing and then connect to the Soap box as in my #32. Again all the LEDs should light up. Leave it on for 5 minutes. Note down the current.

Then tell me the results.

 

[T800mc]

Right. Take your multimeter(MM) and select dc volts and identify which of the ends of the stubby item is +ve and which is -ve. Confirm 4.5V.

Now select dc amps on MM on a scale which will read 30mA.

Take an LED. Put long lead on the +ve of stubby item. Then put one MM lead on short lead of LED. Finally put the other MM lead on -ve of stubby item. Current should be 30mA or lower and LED lights up.

I had to swap the red lead to the red port for amps, sorry. The LED

Your MM does do mA. Turn the knob so that its dot is at 6 o'clock - the last position going clockwise. the 'A' position without Hz beside it.

Use the lead without the resistor.

I had to move the red lead to left red amp plug too. The LED lit up and it red 0.015A
[automerge]1597907082[/automerge]

I did not have any single LEDs so I sent you a four LED panel indicator. Somes wires have been soldered to it from a previous project. Please study the attachment which shows you how the wires have been connected.

These LEDs only pass 10mA to light up so what I'd like you to do is connected all the wires on side together and all the wires on the other side together as I showed in my first attachment.

Then connect it up as I described in my #32 - all four LEDs should light up. Leave it on for five minutes. Note down the current you measure on your MM

Now connect up as shown in my second drawing and then connect to the Soap box as in my #32. Again all the LEDs should light up. Leave it on for 5 minutes. Note down the current.

Then tell me the results.

Okay great I will try that when it arrives. Got to admit I wasn't expecting so much great help from a forum!
Does what I told you about the 0.015A mean anything?

 

[marconi]

Well done Sir. Yes the current measurement does. An LED like that will pass a current of about 15mA or 0.015A.

(I am only one of very many who like to help out when and where we can. Generally though we do not do 'step by step' for any electrical work which has safety or legal consequences but your question is not one of these).

 

[T800mc]

Well done Sir. Yes the current measurement does. An LED like that will pass a current of about 15mA or 0.015A.

(I am only one of very many who like to help out when and where we can. Generally though we do not do 'step by step' for any electrical work which has safety or legal consequences but your question is not one of these).

So if I needed instructions on replacing a contactor in a star delta starter it would be tumbleweeds? Lol

So does the information that is it passes 0.015A tell you something in partciular about the soap reactor?

 

[marconi]

Yes it does. Assuming you used a 3.6V green led and the open circuit voltage of the soap reactor is 4.5V then the internal resistance of the 4.5V supply is:

(4.5-3.6)/0.015 = 0.9/0.015 = 60 Ohms

For a 3V green led:

(4.5-3)/0.015 = 1.5/0.015 = 100 Ohms.

The soap reactor 4.5V 0.03A outputs are equivalent to a battery of 4.5V with an internal resistance circa 60-100 Ohms - see my attached diagram - the Thevenin equivalent circuit for the output.

You could work out the internal resistance more accurately if you measured the voltage across the led (the forward voltage) and used it instead of 3.6 or 3 in my sums above.

Do the checks using the 4 leds I sent you and report back and then I will confirm you can simply connect a Soap reactor output (4.5V 30mA) without an extra series resistor.

 

[T800mc]

Your delivery times are better than amazon!

0.026
0.008

Yes it does. Assuming you used a 3.6V green led and the open circuit voltage of the soap reactor is 4.5V then the internal resistance of the 4.5V supply is:

(4.5-3.6)/0.015 = 0.9/0.015 = 60 Ohms

For a 3V green led:

(4.5-3)/0.015 = 1.5/0.015 = 100 Ohms.

The soap reactor 4.5V 0.03A outputs are equivalent to a battery of 4.5V with an internal resistance circa 60-100 Ohms - see my attached diagram - the Thevenin equivalent circuit for the output.

You could work out the internal resistance more accurately if you measured the voltage across the led (the forward voltage) and used it instead of 3.6 or 3 in my sums above.

Do the checks using the 4 leds I sent you and report back and then I will confirm you can simply connect a Soap reactor output (4.5V 30mA) without an extra series resistor.

Your delivery times are better than amazon!
Leds turned up and I made an attempt.

Connecting it via the first diagram I got 0.026A
Connecting it via the 2nd diagram (hopefully I did it correct) I got 0.008A

The MM went off after 4.5 minutes but thats what the readings were. The 0.008 did dip to 0.007 fractionally at the end but that could have my grip slipping.