Hi Guys,

Firstly, I am an absolute beginner.

I designed a very simple power supply of which I have attached the circuit diagram.

It only has voltage control and no current control or current sensing.

When nothing is connected to it, I can set the voltage to a specific voltage. As soon as I attach a load, the voltage drops a couple of volts depending on the load. Then I have to adjust it.

So my questions are:

1) Is this what off-the-shelf power supplies do as well?
2) If not, why is it happening
3) How do I prevent it?

Thanks a lot guys.

 

Welcome,

Why all the diodes between the regulator and the load?

 

I was trying to drop the voltage as close to zero as possible to compensate for the min. Voltage output of the lm317.

 

I was trying to drop the voltage as close to zero as possible to compensate for the min. Voltage output of the lm317.

The diodes are the reason for your poor voltage regulation.
Many ways to get the output down to zero Volts. Have you looked at the applications sections of the LM317 or LM338 data sheets. I think they show how to do it?

Here it is:

 

Thanks Mike. I'll have a look at the datasheets again.

Is there anything else that you can recommend to improve voltage regulation?

 

To get a power supply adjustable down to zero V, you will likely need a negative power supply.

 

True. Since the only current into the negative supply is the regulator static current, this can be done with a charge pump off of the transformer before the bridge. This went by last fall when Bill (?) was fiddling with a power supply design. Cap - diode - diode - cap.

Also, the 7805 is built to dissipate heat, but a typical zener diode is not. If the peak voltage across C1 is less than 24 V, you can eliminate D15.

What are your AC input voltage, desired output adjustment range, and desired current from the adjustable output?

ak

 

The AC input is 24V before rectification (from 220V transformer). After rectification and filter cap coms to about 30V.

My design is intended to be adjustable from 0V to 20V with a 1A max output. Using a 1.2A fuse at the moment.

 

You can eliminate D7 and D15, if you use half wave rectifier for the 7805 and its own smoothing cap, thats about 16 V input,