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For my project I plan on controlling an off-the-shelve LED lamp using a relay board which is connected to my Raspberry Pi.

As can be seen from the Lamp image below, there is a power converter which changes the 230V/50Hz to something the LED's can work with. (There are 9x 1W LEDs, the lamp says 10 Watts in total).

My question is whether I should place the relay between the converter and the lamp or between the converter and the power socket.

(http://www.ikea.com/gb/en/products/indoor-gardening/indoor-growing-cultivators/v%C3%A4xer-led-cultivation-light-silver-colour-art-40333455/)

  • Here is a link to the lamp – stklik Oct 17 '17 at 7:15
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It's not possible to predict what will happen if you break the circuit between the converter and the lamp. The converter may fail catastrophically if it is switched on without the load it was designed for.

If you could be confident that the converter is nothing more than a step-down transformer, perhaps with a load limiting resistor, then you could insert the relay between the converter and the lamp. But since this product is touted as a superior energy saving device, it is likely to use a TCBH1 power technique that requires the lamp to be permanently connected.

Therefore I recommend inserting the relay between the power socket and the converter.


1 Too Clever By Half

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The instructions say the lamp and power block are a "matched set". The cord between them does not have a plug, and mustn't be cut, altered or extended.

Considering how little IKEA says generally, and how much they say on this particular topic, it's clearly important.

That makes sense: It's preferable to drive LEDs in "constant current" mode, and as a practical design, all 9 LEDs would be connected in series, and fed from a single constant-current driver module. Those (by nature) are variable voltage, and will increase the voltage as necessary to get the current to flow! The old transformer-based industrial types were scary as they had no upper voltage limit and would arc across anything.

So tampering with the connecting cable is out of the question. You have to switch the mains supply on these devices.

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A couple points to consider:

A relay on the DC side could be smaller and less protected since it is low voltage.

However, if the converter uses any power at all in standby mode, then it would be more efficient to control the AC side and prevent the converter from drawing power while the light is off. This would require a high voltage relay that is mounted in a junction box.

Good luck!

  • When you say "high voltage", you mean "the same voltage as the main power grid" (in my case 230V), right? – stklik Oct 17 '17 at 8:06
  • @S.K. Yes, that's correct. – Harper Oct 17 '17 at 14:31

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