Hi people
I can use some help here - I've assembled a mig welding power supply (constant voltage) from a transformer with 30Vac 140A secondary winding that is connected to a bridge diode rectifier and an inductor.
the problem is that I can not regulate the output voltage - I must have it variable between the max output down to about 15Vdc. please let me know if you have some idea how to do it ( I think with scr's?)
Thanks.

Hello Amron

Welcome to the forum.

There are a number of approaches that you can take to this.
Is your transformer a single phase input, or a three phase input?
What is the input voltage?

You can control the AC input to the transformer by a number of techniques depending on the voltage and no of phases, and you can also control the output by using a controlled bridge rather than a standard bridge rectifier.

Best regards,

Hi Thanks for the welcome greetings.
From all the options I think that the controlled bridge that you have mentioned might be the best.
The transformer is a single phase 230Vac input.
Thanks for any help.

Hello amron

OK, yes you can use a controlled bridge on the output.
This requires two SCRs and two rectifiers all rated to carry the current you require.
The SCRs would normally supply the positive leg, and the rectifiers the negative leg.

There are many circuits around to enable you to do this, and a quick reference to the SCR manufacturers application notes will probably give you the information.

The other way to do this, is to use a non controlled bridge on the output, and a pair of reverse parallel connected SCRs on the input to vary the voltage into the transformer.
The advantage of this is that the SCRs are much smaller and it may be a cheaper option.
There are also single phase AC controllers available designed for inductive loads.

Best regards,