Dear All hello,
I have a question about a power supply transformer with center tap.
I would like to use it with a bridge rectifier but I would like to have the option to drop half-voltage with a switch.
I am thinking to use one end of secondary to one end of the bridge and to select between the other end for full voltage or the center tap for half voltage.
Is that correct?
Does this if correct create any other problem to bridge or filter capacitors?
The transformer is 225-0-225 at 250mA. The circuit I want to supply consumes about 150 mA .
Thank you in advance for your help.
Regards.
Dimi

 

That sounds like a working plan to me.

 

That sounds like a working plan to me.

Hi AlbertHall, thanks for reading,
Any suggestions as I am trying to figure this out?
Thanks.
Dimi

 

What are you trying to figure out?

 

What are you trying to figure out?

Just to be sure that if I connect like that the transformer with the bridge I would have half the voltage and not a burned transformer at the end.
What do you think from your experience, is it possible?
Thanks in advance.
Dimi

 

Below is a way to get both voltages using a bridge rectifier, discussed here.

 

Use a full-wave bridge rectifier with the ends of the transformer windings to the normal AC inputs of the bridge and the positive output of the bridge to positive of the cap.
If you connect the negative of the bridge to the capacitor negative, you have a full-wave bridge rectifier that will charge a filter capacitor with no load to about 630 VDC. If you connect the centre tap of the transformer (i.e. instead of the negative of the bridge) to the capacitor negative you have a full-wave rectifier that will charge the cap to about 315 V with no load and still uses the full secondary winding of the transformer (each side of the secondary supplies current for half of each AC cycle).

I recommend against switching while primary power is connected to the transformer. Be sure the switch has adequate voltage rating. Circuits like this always carry a risk of accidentally switching to the higher voltage and possibly damaging something, so you might want to consider some means of preventing accidental switch operation or using a movable connection inside an enclosure. I used to use a sort of similar arrangement with a transformer. The configuration required opening the box and I had a label on the outside, held in place with a screw, that I flipped to the appropriate side up so I'd be much less likely to connect something thinking the configuration was 60 V output when it was actually 240 V.

 

Below is a way to get both voltages using a bridge rectifier, discussed here.

View attachment 160625

That's cunning. I haven't seen that arrangement before.

 

Dear ebp that sounds exactly what I was looking for, as in this way I have to use instead a two position selector a normal on-off switch to center tap. I have a toggle one with spring guard above.
As concern the capacitors I will use six 100+100/500v.
I have to admit that is the first time I see such arrangement.
I will try it next days.
Thanks for help to all.
Best regards to forum.
Dimi

 

As concern the capacitors I will use six 100+100/500v.

The high voltage output of this arrangement using a 225-0-225 winding will be 630V so 500V capacitors will not be adequate.

 

The high voltage output of this arrangement using a 225-0-225 winding will be 630V so 500V capacitors will not be adequate.

Thanks Alberthall, I will use six of them in pairs of two in series.
Also I will use 220k resistors in parallel with the capacitors in series.
Again thanks for the advice.
Brgds.
Dimi

 

Yep, that'll work.