Transformer centre tap.

[pre65]

I've got a small (25VA) transformer with a 230v secondary.

I'd like to use an EZ80 rectifier, but there is no centre tap on the secondary.

Can I create a pseudo centre tap with two resistors, and if so what value of R is optimal ?

[Nick]

No.

[Nick]

What final voltage are you after? You can create a hybrid bridge using two SS rectifiers, that will give you 320v or so

[Cressy Snr]

Deleted,
Nick beat me to it.

[pre65]

What final voltage are you after? You can create a hybrid bridge using two SS rectifiers, that will give you 320v or so

Thanks Nick.

It all seemed so simple in my head.

I need to do some calculations.

[pre65]

2nd thought.

I need 135v at about 7ma so I could use a voltage divider ?

[Nick]

Well, if we are considering going down the bad power supply route, may be better to consider a voltage double, or some back to back transformers.

[pre65]

Well, if we are considering going down the bad power supply route, may be better to consider a voltage double, or some back to back transformers.

Maybe. Thanks for the suggestions.

[pre65]

Because my brain has a serious malfunction, and my maths skills have declined to that of a 2 year old marsupial, I've been doing some empirical measurements with back to back transformers and a 47K load.

Not got to where I think I need to be yet but it's keeping me occupied.

[Nick]

Just remember the transformer secondaries don't have to be equal.

[pre65]

Just remember the transformer secondaries don't have to be equal.

Yes, I was aware of that, but thanks for the reminder Nick.

It didn't help when the 47K resistor turned out to be 4.7K, time for a trip to Specsavers.

Next experiment is bridge with choke input.

[pre65]

Right, this mornings experiment has got me to 140v, and the target was 135v.

Using a 250v secondary, SS bridge, choke, bleeder resistor (25ma draw), cap , 2nd choke and interstage.

The load was 20K to simulate the 112A.

So now I'm in the right ballpark to start making the real thing.

I've just realised I've connected the interstage before choke 2 so I'll correct that and see if it makes a difference to the voltage.

The bleeder resistor is to draw enough current to bring the transformer volts down to the rated figure.

[pre65]

Changing things around has got me to 136V, and the only thing of concern is the 4K7 bleeder resistor is too hot, but that just needs a higher wattage.

All in all, I'm pleased with the way things turned out.

I'll need to forgo the EZ80 option (in a hybrid bridge), but that's no big deal, just makes the circuit a bit more simple.

[Nick]

Did you end up using back to back transformers?

[pre65]

Did you end up using back to back transformers?

No.

I did try several combinations but I could not get what I wanted with what I had. The main thing was trying to end up with a centre tap so I could use the EZ80.

One encapsulated 9v + 9v secondary transformed made horrible noises whether the secs were connected in series or parallel.

The transformer I used was a 240v - 240v, but the primary has 4 connections.

0v, 240v, 240v - 5% and 240v + 5% - so it gave me more options. I ended up using 240v + 5% as it gave me a slightly lower end voltage.

I'll add further details as they come on the build thread.