At a charity jumble sale I picked up a handful of Osram ET-Parrot 70 transformers. They are typically used for low-voltage Halogen lighting in the home, but I want to use them for other things. Here is the spec.

The input is 220/240v output 11.5v AC. The specs promise full isolation between input and output. The one I have opened looks very nicely engineered, but I have not drawn a schematic or anything yet. (When used for lighting they can be dimmed on the mains side by leading edge (inductive load) and trailing edge phase control. (I'm getting a bit out of my depth here...)

So I have two questions that perhaps someone with some experience can answer:
a) If I run two of these side-by-side, should I be able to tie the outputs in series to get 23v AC? I obviously need to get the phase correct so that they add rather than cancel each other.
b) They are marked 20-70W. I saw a post somewhere that suggested that these devices won't function unless your load draws at least 20 watts. Can anyone confirm or explain why this might be. I have yet to start the "plug-it-in-and-try" phase, so any insights or links to what I should read might help me.

And if you do have or stumble across a schematic, please let me know where I can find it.

Thanks
Peter

 

a) If I run two of these side-by-side, should I be able to tie the outputs in series to get 23v AC? I obviously need to get the phase correct so that they add rather than cancel each other.

Yes, you should be able to connect them in series.
Try the connection.
If the voltage is not 23Vac, then just reverse one of the connections.

b) They are marked 20-70W. I saw a post somewhere that suggested that these devices won't function unless your load draws at least 20 watts.

I have no idea why that might be.
Just try it and see. It's very unlikely to be a serious problem if it doesn't have a load.

 

The fact that they have a lower power limit makes me think that these are what are called electronic transformers. The mains input is rectified but not smoothed. It is then chopped at a high frequency. (Tens or hundreds of Khz.) and fed through a small high frequency transformer. (This type of transformer is cheaper and lighter than a transformer working at 50 or 60 Hz.) The output looks like a high frequency square wave which is modulated with twice the mains frequuency. The outputs of this type of "transformer" can not be connected in series or parallel as the chopping signal will not be synchronised between "transformers" This is a link to some more information on "electronic transformers".

Les.

 

I think if I was going to use a transformer like that for anything I might come into electrical contact with I would want to check the secondary for leakage from the line because they were not necessarily designed and tested to have low enough leakage to be safe for human contact.

One way to measure leakage is using a high impedance voltmeter (Such as a modern DVM) measure the voltage drop across a 1K resistor that is shunted by a 0.015 uf capacitor that is placed between the thing you want to test (like the secondary and the transformer frame) and earth. If you see more than a few volts, you might not want it in some applications. A few volts you can feel, a couple tens of volts can be very painful.

 

Osram ET-Parrot 70 transformers

After looking at them I believe Les is correct, thus connecting them in series is probably not advisable.

 

What you have is an electronic circuit that works like a transformer for the halogen lamp.

The halogen 'ballast" that I played with had a very complicated ac waveform. I was never sure why.

 

The halogen 'ballast" that I played with had a very complicated ac waveform. I was never sure why.

Possibly a combination of the AC input voltage and the chopping introduced by the ballast circuit.

 

After looking at them I believe Les is correct, thus connecting them in series is probably not advisable.

However, if what you ultimately want is DC, you could rectify and smooth each separately, then connect in series.

Bob

 

they are not designed for consumer use other than the lamp bulbs

They will burn out from the faintest short or overload instantly

The voltage will be noisy and messy

Many of them dont work without the bulb as load

Bcz...

Theyre just extremly cheap

 

The halogen "ballast" that I played with had a very complicated ac waveform. I was never sure why.

Possibly a combination of the AC input voltage and the chopping introduced by the ballast circuit.

Here is the thread where I discussed an electronic transformer:
https://forum.allaboutcircuits.com/...transformer-for-halogen-and-led-lamps.126528/

The last entry in the thread has pictures of the waveform.

 

Thanks Richard. The thread you referenced together with the reference that Les Jones gave explains much of what I needed to know. They are indeed quite a lot more complex than a simple step-down transformer.