Jumping current on outdoor 12v lighting circuit

[AdamB]

Hello,

I have two almost identical circuits of outdoor 12v lighting setup in my garden, each running off a separate 20W 240v-12v transformers (12v Waterproof Transformer (Various Wattages) | 240v - 12v | 10W - 150W - https://www.lumenalights.com/shop/product/12v-waterproof-transformer-various-wattages/). Each circuit has 5 x 12v LED lamp fittings, connected using IP68 3-way connectors (
https://www.amazon.co.uk/ATPWONZ-Ju...19233&sr=8-7&keywords=ip68+junction+box+3+way
) and 0.75mm 2 core 12v cable (2 Core Rubber Cable Reel (12v) - 0.75mm² - 25m / 50m / 100m - https://www.lumenalights.com/shop/product/0-75mm²-2-core-rubber-cable-12v/). There is approx. 1-2m between each connection. One circuit uses recessed light fittings (Decimax - 12v Recessed Light – Stainless Steel (304) - 12v Plug & Play - https://www.lumenalights.com/shop/product/decimax-12v-recessed-light-stainless-steel-304-12v-plug-play/) buried in chippings, the other uses hanging light fittings (Pergolux 12v Garden Hanging Light - 'Earth' Brown Aluminium - Lumena - https://www.lumenalights.com/shop/product/pergolux-12v-hanging-light-earth-brown/). Each light fitting holds a 3W LED bulb, so the total wattage on each circuit is 5 x 3W = 15W.

These were all initially working fine; however, recently both circuits have started to cut out, as the transformers are overheating after about 15 minutes. I bought a clamp meter to measure the current on each circuit and am seeing some very odd behaviour. Sometimes when I switch the lights on, the reading is around 1.4A, whereas at other times it shows as around 6A - this is when the transformers start to heat up. Strangely, by switching off and on again the 6A reading will sometimes drop down to 1.4A and vice versa; however, once on, the current stays around the same level from then on - so if I switch on at 6A it will stay around 6A and the transformer will overheat, whereas if I switch on at 1.4A it will stay around that level and will not overheat.

Can anyone offer any suggestions as to what might be causing this behaviour? I there likely to be something wrong with the configuration of the circuit or could a fault with any of the components which may be to blame?

Any help or advice extremely gratefully received. I can't figure this one out.

 

[Shoei]

When drawing 6A do all the lamps illuminate normally? Just a guess but it sounds like one or more of them isn't starting properly when power is applied. 12V LEDs contain a switch mode inverter to drive current through the led, you may have a faulty one or perhaps they're not all getting a full 12V if the wire is too long/too thin. I'm guessing the inverters sometimes start ok and sometimes not.

 

[AdamB]

Hi Shoei,

Thanks for your quick reply. You seem to have hit the nail on the head. Yes, all the lamps appear to come on slightly dimmer, which correlates with the higher current reading. When they come on full brightness the current seems to be okay.

Can you recommend anything that would remedy this, other than just switching off and on until they come on at full brightness?

Many thanks

 

[Shoei]

Hi Adam,
This is guesswork so don't be too surprised if it's wrong but I'd suspect that the LEDs are getting confused by voltage drop when switched on simultaneously. Although the rating of the transformer (20W) is sufficient for the continuous draw of the LEDs (15W) , LEDs typically have in inrush current that can be many times the continuous current. The inrush lasts only a few tens of milliseconds but if all five LED's demand a large gulp of current, the transformer voltage will drop briefly which may cause one or more LEDs to get into an unstable state.

If the clamp meter is correct and the transformer is supplying 6A then the voltage is likely to be much reduced. A 20W transformer won't supply 72W (6A x 12V) so the voltage is probably quite low. Do you have a meter to check the voltage?

To confirm that is the problem you could start by disconnecting (say) three of the LED's and check that the remaining two switch on consistently. If so, try another pair and so on. If all the lamps work correctly in ones, two or threes then you could try either different LED's, (others may be less demanding during switch-on) or you could try a higher power transformer. If you have a bigger transformer to hand you could try that to confirm reliable operation before sourcing another water-resistant one. If you changed to the 50W or even 100W version it would take no more power in normal running but it would possibly hold the voltage high enough for a clean switch-on.

All a bit vague but your setup seems basically correct, it may just need a transformer with a bit more power.

 

[Richard Burns]

LED bulbs normally require 12 V dc and the transformer you have supplies 12 V ac, this usually reduces the life of the LED bulbs as they try to cope with the ac and so one or more bulbs are probably starting to fail. If you can get a 20W 12Vdc LED driver / switch mode power supply it may help.

 

[Shoei]

LED bulbs normally require 12 V dc and the transformer you have supplies 12 V ac, this usually reduces the life of the LED bulbs as they try to cope with the ac and so one or more bulbs are probably starting to fail. If you can get a 20W 12Vdc LED driver / switch mode power supply it may help.

I've never seen an MR16 LED specified for 12V DC only, they rectify the supply internally and run off 17V or can use 12V DC. The inverter is a current source designed to work on a wide range of supply voltages, to ensure compatibility with the many transformer-based installations still in use.

 

[AdamB]

Thank you for the information. This is really helpful. I do have a multi meter for measuring voltage, so will give that a go and also try out your other suggestions, when I have time, to hopefully get to the root cause.

Many thanks again.