Power Supply Challenge

[dondon]

My friend dares me to build a power supply from his multi-output transformer. He wants me to build it by following these instructions:

1. Build without any use of IC
2. Use rotary switch as a selector
3. Have an additional output of 5V

As far as I remember, these are the things he ordered me. So the circuit I designed was base from those orders.

I don't know if his dare is feasible bu I found it to be interesting so I am determined to make it.

This is the circuit diagram I made:

Base on this circuit, the 5V output was not added but instead replaced the 6V output of transformer. If this circuit is right, I will let the 6V output to be gone.

Please look at it if it this circuit is correct. If there is a mistake, please correct me. If you have better circuit, please share it.

I hope this thread can help future readers. Thank you very much.

 

[Harald Kapp]

Your variable output is unregulated. and the 5V output will drop under load due to R1.

If you want to addd regulation, a simple transistor circuit can be used. Note that in this case you'll need a double pole rotary switch to chnage zener-diode aps, too.

 

[dondon]

Sir does the circuit drawn above is correct connection-wise? I mean, suppose that it operates well, is there any short circuit that will be going to happen? I am afraid there might be.. Please kindly verify it for me.

Your variable output is unregulated

Does a certain value of C2 isn't enough to regulate the the output? or there should be specific capacitor in every output?

and the 5V output will drop under load due to R1.

so the 6V winding of transformer can't do the job to produce the 5V output on zener.. does the 7.5V winding can do the job?

Thank You.

 

[BobK]

I think there is a problem with that circuit. You cannot have the negatives of the two bridges connected together. This creates a short of two windings through a diode.

Bob

 

[dondon]

I think there is a problem with that circuit. You cannot have the negatives of the two bridges connected together. This creates a short of two windings through a diode.

Bob

Ouch. that really hurts Sir. Would you mind if you show me proper circuit regarding this problem?

 

[BobK]

If you disconnect the negative sides of the two bridges you will be okay, you just cannot have a common between the two supplies.

I am not absolutely sure I am right about this. I hope someone else will weigh in.

Bob

 

[Harald Kapp]

Well seen, Bob. Right you are. I missed that point. (Checked it with a quick simulation on top of my thought experiment - confirmed.)

show me proper circuit regarding this problem?

You need to use a half wave rectifier. Two of the, to be exact. The point marked "0" in your schematic will be the common ground for 5V and for the variable output.
You will need larger capacitors to compensate the missing half wave.

Note that also the transformer will see a DC component and is more likely to saturate, become hot and generally not perform very well.

so the 6V winding of transformer can't do the job to produce the 5V output on zener.. does the 7.5V winding can do the job?

This is not the point. As soon as you draw an output current from the 5V output, this current will develop a voltage drop V=R*I across the series resistor. The output voltage will be lower by the amount of this voltage drop.

Does a certain value of C2 isn't enough to regulate the the output? or there should be specific capacitor in every output?

The capacitors don't "regulate" the output voltage. They only help to bridge the time where the (sinusoidal) input voltage is lower than the output voltage. See the paragraph "rectifier output smoothing" in the Wikipedia article I linked above. For a stabilized output you will need at least a simple circuit as the one in my post #2.
Note that this is still not a regulated output, just stabilized (enough for many purposes). A truly regulated output requires that the output voltage is sensed, compared to a refenrence voltage and the pass transistor be controlled accordingly such that the output voltage is stable regardless of current (within the limits of the power supply, as should be obvious).