I have a number of small lamps that use 12V halogen bulbs, either 10W or 20W G4 types. Each lamp has a 12V step down, wall-wart style transformer with te on/off switch in the low power side of the circuit. I want to change to LED bulbs instead of the G4 halogens but must I also change the transformers for LED driver modules? The LED bulbs I have are AC/DC and seem to work OK with the transformer output. How much power is the transformer taking when the bulb is switched off and will I save very much by switching to an LED driver that is only turned on from the mains side of the circuit. My "smart" electricity meter tells me that a 12V transformer plugged in, but with the lamp switched off, hardly takes any power at all - is this true?

 

I have a number of small lamps that use 12V halogen bulbs, either 10W or 20W G4 types. Each lamp has a 12V step down, wall-wart style transformer with te on/off switch in the low power side of the circuit. I want to change to LED bulbs instead of the G4 halogens but must I also change the transformers for LED driver modules? The LED bulbs I have are AC/DC and seem to work OK with the transformer output. How much power is the transformer taking when the bulb is switched off and will I save very much by switching to an LED driver that is only turned on from the mains side of the circuit. My "smart" electricity meter tells me that a 12V transformer plugged in, but with the lamp switched off, hardly takes any power at all - is this true?

What reason do you have to disbelieve your meter?

 

What reason do you have to disbelieve your meter?

It is a basic piece of kit, giving a non-instantaneous reading of the whole house consumption to the nearest Watt. It fluctuates when other electrical items switch state, which is quite often, so it is difficult to track small changes in amongst the noise.

 

The transformer will draw some current when the bulb circuit is not complete though not a huge amount, If you are asking is it green, no it's not. You'd need to measure the current draw of the transformer to know just how "not green" it is. You got a big win switching to LED, fixing the "vampire" draw will be a small win.

 

Current does flow in the primary of a transformer that has no load connected. It is called "magnetizing current" because it creates the magnetic field in the core of the transformer. Essentially what you have is just a simple inductor, so the phase relationship between the applied voltage and the current is such that, ideally, the actual true power would be zero. This is called "reactive" current or reactive power. In actuality, some "real" power is wasted due the the current flowing through the resistance of the winding of the transformer, and there is a small amount of "hysteresis loss" and "eddy current loss" in the iron core of the transformer. In a conservatively designed transformer, the magnetizing current is quite small and the other losses are small. Magnetizing current does not change when you load the transformer. Depending on where you live, you might pay for reactive power, but in many places you don't. Reactive current still causes losses is power transmission systems, so power companies don't like it much. (You can compensate for inductive reactance by adding capacitive reactance. This is done with magnetic ballasts for fluorescent lamps.)

Simple LED drivers rectify the AC mains input and filter the DC with a capacitor before switching it down to drive the LED. The capacitive filter draws current in spikes only at the peaks of the AC voltage, so it is also nasty from a power distribution perspective. Good drivers use "active power factor correction" (called other things, like "active harmonic filtering" in some places). With active power factor correction the driver looks like a resistor to the AC line, with the current in phase with and proportional to the voltage. Power companies like this because you take only useful current - and you pay for all of it. Many of the new drivers use active power factor correction.

Some of the low-cost power meters aren't very accurate at very low powers.

 

Thank you, that is very informative.

It sounds like the only reason I would want to change the transformer for a dedicated driver would be for DC only LEDs - and, on further testing, my initial "these LEDs seem to work with AC" claim should include "as long as you don't mind an annoying flicker in the corner of your eye". So the transformers will not work as they stand. I could add a few diodes and a smoothing cap, but my next avenue to pursue will be to try one of the many 12V DC supplies that I have courtesy of the ISPs that provide modems & routers but don't care to want them back.

 

Thank you, that is very informative.

It sounds like the only reason I would want to change the transformer for a dedicated driver would be for DC only LEDs - and, on further testing, my initial "these LEDs seem to work with AC" claim should include "as long as you don't mind an annoying flicker in the corner of your eye". So the transformers will not work as they stand. I could add a few diodes and a smoothing cap, but my next avenue to pursue will be to try one of the many 12V DC supplies that I have courtesy of the ISPs that provide modems & routers but don't care to want them back.

The simple way to avoid any wasted power in those separate transformers when the lights are off is to switch the mains power feeding the transformers. Slightly less convenient but far more efficient, with the added advantage that when a transformer feed is switched off there is no chance of a failure in the transformer starting a fire.