Hi, I have the circuit below.The transformer is one center-tap transformer 380/2*12. I want to have two dc voltages, one 24 volt for some relays and 12 volt that feeds 7805 regulator. But the problem is when one or more relays are energized, in this case the output voltage, 24V (and also the voltage across two end of secondary of transformer) drops by few volts . and also the voltage across 12V supply drops. But I don't want this. I want to 12V be constant regardless of changing 24V, because I want to sample the input voltage of transformer (380V) by lower circuit. Is it possible to have constant voltage between center and lower terminal of transformer while voltage between high and low terminal changes?!

 

Your center tapped secondary has no connection to ground.

If you want 0-12-24 you need to ground the end point of the secondary and do not use the bridge type rectifier.

 

Your center tapped secondary has no connection to ground.

If you want 0-12-24 you need to ground the end point of the secondary and do not use the bridge type rectifier.

In this circuit I have 12 and 24 volt but they are not independent of each other, what should I do if I need to don't use bridge rectifier?!

 

Single diode rectifier off the top and center.
Bottom tap grounded.
24 volt from top tap. 12 volt from center tap.
You will need to increase the size of the filter cap, or add another in parallel to get the same ripple voltage levels.

 

Single diode rectifier off the top and center.
Bottom tap grounded.
24 volt from top tap. 12 volt from center tap.
You will need to increase the size of the filter cap, or add another in parallel to get the same ripple voltage levels.

Thanks, I did it but still they are not independent! by changing the output current the 24V output and also the 12V output change!

 

Transformers will not regulate CURRENT. If you increase the load the output "sags".

You must select a transformer that has acceptable voltage levels at the highest current output you could use in your project.

For 24 volt, you might have 29 volts unloaded, 25 volts at half power and 23.8 volts at your max power level.

ANY change in load on the transformer WILL change the voltage you are monitoring.

To keep voltage steady requires another circuit called a regulator. Like the 7805 in you schematic

 

Second.

Of course the 24 volt and 12 volt change together and are not independent.

THEY COME FROM THE SAME PIECE OF TRANSFORMER WIRE.

Independent would require TWO seperate (independent) transformers

 

Hi,

In theory i think you can get 24v and 12v with load current independence, but you'd have to settle for half wave rectification for both. You can easily get +24v and -12v, but you might be able to find a diode combination connection that allows this too. The trick is to load the 24 volt (two windings) on one half cycle, and load the 12v (one winding) on the other half cycle. You'll get half wave, but they should be independent except for the wire heating effect but that should be much less than the loading effect.

If you cant find a diode connection combination that allows both +24 and +12, then you will have to move to active rectification. That's assuming you still want to use one transformer. That will require at least one transistor and a little control circuit.

 

...........
If you want 0-12-24 you need to ground the end point of the secondary and do not use the bridge type rectifier.

It may seem a little strange but the circuit he has does generate both a rectified 12v and a rectified 24V output.
The 24V is generated by the bridge and the 12V is generated from the center tap where the two bridge diodes going to ground act as a full-wave rectifier.
See this for an explanation.

 

Ya know. If I had just looked closer I still would have missed it.

 

Can you replace the relays with lower current relays?