Electronic transformers

[Crackle]

Hi
Does anyone happen to know if these particular electronic transformers have a very high frequency 12 volt output. https://www.varilight.co.uk/configurator...ode=YT210Z
I have seen the spec. on other similar units that the output is 12 volt at 22kHz, but I cant find any such information on this particular unit.

The reason for asking is someone I know has been using one of these transformers in a demonstration of the wiring of low voltage lighting. They are using 5 x 12 volt 50 watt halogen bulbs all fed from the transformer with individual switches controlling each bulb.
The problem found is that the current meter, wired in series with the transformer, one of these, http://uk.rs-online.com/web/p/analogue-p...s/8861881/ is not reading correctly, it is giving a reading of about half the expected value.

I suspect it is because the moving iron meter is not working well if the electronic transformer is pushing out 22kHz.

Am I correct.

Thanks
Mike

[Mark Hennessy]

Without a look inside, it's hard to be sure, but it appears to be a switched mode power supply minus output rectification. So the AC is converted to DC with a bridge and capacitor, then chopped up ready to be pushed through the internal transformer. The chosen frequency will be a design decision - the higher the frequency, the smaller the transformer, but too high a frequency and other problems occur. I'm not aware of 22kHz being a standard, and if anything, that does seem to be a bit on the low side for the physical size of the unit.

In theory, the switching frequency shouldn't affect the current draw from the mains because of the initial rectification. In practice, there will be an amount of the switching frequency leaking out, but I don't think it should significantly affect a moving iron current meter.

Digging deeper, there are two more interesting clues: a near-unity power factor, and it's dimmable.

Both these features imply the presence of power-factor correction. This is an additional boost converter that sits between the bridge rectifier and the main switching converter, and its job is to ensure that the current draw from the mains is as sinusoidal as possible (as opposed to the "gulps" of current that are drawn when you have a simple rectifier and capacitor). To do this, the IC controlling the boost converter monitors the input voltage, and it's easy to use this as an input for the purposes of dimming.

If this is the case, then there's even less reason to suspect that a moving iron ammeter will be grossly inaccurate.

Further investigation is required. I'd start by measuring the output voltage and current delivered to the load. Then, remembering that these units aren't 100% efficient (perhaps 90%?), that gives me a better idea of what the input current should be (rather than making assumptions based on the bulbs used). But, measuring the output is non-trivial, given the high-frequency nature of the output waveform. I'd want to look at the AC input current on a 'scope that is also showing the applied input voltage to see how sinusoidal and how in-phase it is.

Nothing is simple

[Refugee]

A moving iron meter made to work on 50Hz will not give a useful reading at any other frequency.
You will need something else to measure it.

[ppppenguin]

It's worth remembering that these electronic transformers for halogen bulbs have a MINIMUM output as well as a maximum. It's often 20W or so which means you can rarely put 12V MR16 LED bulbs in as a direct replacement for halogens. If you have too little loading the result is usually really bad flicker. With a wirewound transformer this won't be a problem but the voltage will be higher that usual due to the low load. i don't know how tolerant LEDs are but halogens will have a short, if bright life.

[Crackle]

In theory, the switching frequency shouldn't affect the current draw from the mains because of the initial rectification. In practice, there will be an amount of the switching frequency leaking out, but I don't think it should significantly affect a moving iron current meter.

Thanks for your replies, not sure if I made it clear but in this case the demonstration was setup with the current meter on the output of the electronic transformer.
I cant find the one where I saw the frequency as 22kHz but here is one which states the output as 42kHz.
http://www.electricsandlighting.co.uk/pr...ransformer
Mike

[ppppenguin]

It's worth remembering that these electronic transformers for halogen bulbs have a MINIMUM output as well as a maximum.

Just looked at the data sheet for the transformer you're using and it is says 0-210VA. In other words no minimum output. not sure I quite believe this. Also the max rating is 210VA and I think you said you were using up to 5x 50W bulbs which will overload it.

[Keithstv]

I had an electronic transformer for three MR16 halogen bulbs where the switching frequency was around 50kHz. As has been said there is a minimum load as it didn't perform very well when I fitted three LED equivalents. Fitting one 20W halogen bulb cured the problem but eventually I replaced the electronic transformer with a conventional transformer.

Keith

[Crackle]

So I guess my original question and assumption about the moving iron meter not reading correctly, was because of the high frequency.

thanks
Mike

[Murphyv310]

Hi.
I have a couple of moving iron ammeters and they are wildly inaccurate above a few hundred hertz. One clearly states 40-100cps.
Normally I would check the input current from the mains and allow 10% error for efficiency.

[Mark Hennessy]

No, because as my reply explained, the meter is seeing 50Hz. How can it see anything else - the mains is 50Hz!

There might well be an amount of high-frequency interference riding on the mains as a result of the transformer, but that will be very small compared to the 230V 50Hz input. The transformer should have filtering at the input, designed to keep the switching noise inside the unit.

As explained, there might be PFC (power factor correction), which will make the current draw more sinusoidal, which - if anything - will improve the accuracy of the current readings. Either way, the only unknown is the exact shape of the current waveform - but it will be predominately 50Hz.

PFC is increasingly important today, as without it, power supplies only draw current at the peaks of the AC voltage, and these peaks are many times the average current delivered to the load - this causes all sorts of problems for supply companies, etc, so PFC - which aims to make the current draw more sinusoidal so the device looks like a simple resistive load - is seen increasingly often in really quite small power supplies.