Tranny question

[Brit01]

I have great difficulty finding power trannies here in this country.... I have one builder but does them in his spare time.

Now I can buy a voltage converter 220-110V for very little.

200W 220V-110V

If I use this in a voltage doubler circuit I could get 260 Volts at 900mA I believe.

200/110=1.8 Amps
1.8/2 = 900mA. (loads of power)

This could be an option no?

[leadbelly]

Tranny means something slightly different over here

If the voltage converter you are referring to is one of those cheap ones that has vents and seems light: forget it, there's no transformer inside; won't work.

If you mean one that is transformer based, yes it could work, but I wouldn't count on either 260V or 900mA, both seem optimistic. Remember a voltage doubler has increased ripple and you will need to do something about this.

Your best option is to look for a 1:1 isolation transformer.

[ChrisA]

Originally posted by Brit01
[B]I have great difficulty finding power trannies here in this country.... I have one builder but does them in his spare time.

Now I can buy a voltage converter 220-110V for very little.

You can use these "backwards" and you can wire two of them back to back. Just watch the current draw and put fusses in place (in each stage) in case you don't.

I'm working on a project now and I'm running a small transformers salvaged from a power cube "wall wort" backwards, The transformar was designed for 120V-->9V but I put 12VAC into the "secondary" and get about 200V out, enough to run a preamp tube.

I have a 120V-->48V CT transformer from a wheelchair battery charger. If I put 120V in the the secondary I expect to get 300V and about 2 amps.

Look for things like these and remember you can two or more in combination.

The cost is that you end up with a physically large, heavy and ugly looking power supply with loads "loss". but overall very low cost/watt ratio.

[HollowState]

Originally posted by Brit01
If I use this in a voltage doubler circuit I could get 260 Volts at 900mA I believe.
200/110=1.8 Amps
1.8/2 = 900mA. (loads of power)
This could be an option no?

In theory yes, in practice no. Voltage doublers don't work very well as the current increases. Much above 200/250 mA and they have problems with efficiency and ripple. If all you need is 200 mA or less, then go ahead.

[Brit01]

If the voltage converter you are referring to is one of those cheap ones that has vents and seems light: forget it, there's no transformer inside; won't work.

How do these work if they don't have a step down transformer?

If you mean one that is transformer based, yes it could work, but I wouldn't count on either 260V or 900mA, both seem optimistic. Remember a voltage doubler has increased ripple and you will need to do something about this.

Well I only need about 40 mA out of it. Simulated with PSU2 to see the ripple. Could give it a go.

[leadbelly]

Originally posted by Brit01
How do these work if they don't have a step down transformer?

They are solid state and they work by chopping off the peaks of the AC waveform.

[Brit01]

They are solid state and they work by chopping off the peaks of the AC waveform.

Doesn't sound pleasant.

So these would have a disastrous effect on a psu for a small load of 40mA or could they be used if enough filtering is added?


I remember reading that someone used a laptop transformer with good success for the heaters.

[leadbelly]

Originally posted by Brit01
So these would have a disastrous effect on a psu for a small load of 40mA or could they be used if enough filtering is added?

These devices offer no isolation. You NEED a transformer for electrical isolation.

[Brit01]

These devices offer no isolation. You NEED a transformer for electrical isolation.

Yes very true leadbelly.

I had no idea the equipment I use everyday, eg laptop, had no isolation.

That's a pity these cheap step downs are no good then. I got my hopes up before.

[leadbelly]

Originally posted by Brit01
I had no idea the equipment I use everyday, eg laptop, had no isolation.

No no, that's not a good assumption. The laptops do have isolation, I believe. When you use a laptop with its brick, the device has received electrical certification as a whole unit. The problem with those stepdowns is they don't need to provide isolation, because they are only to be used with end devices that have received electrical certification on their own.