Why does voltage drop slowly on the output of my adjustable PSU?

Thread Starter

LewisMF

Joined Nov 15, 2014
100
I've decided to build a simple, adjustable bench PSU unit to power my little projects at home.
To keep things rather simple, I decided to build it around the LM2576-ADJ step down switching voltage regulator from TI.

I have breadboarded the circuit and it works fine, the only issue I'm having is that when I turn the potentiometer to lower the voltage on the output, the voltage is taking some time to drop down...

Here is my complete circuit:
circuit.png

I think the issue must be coming from one of the caps on the output (C2 or C3). They must be holding energy and taking some time to discharge and that is what's causing the voltage to drop slowly on the output.

Is there any technique o circuit I can use so that the caps discharge quicker when I want to lower the voltage?
 

ericgibbs

Joined Jan 29, 2010
10,171
Hi Lewis.
Depending on the current draw by the load, that 2200uF on the feedback smoothed supply could be causing the delay.
E
 

Dodgydave

Joined Jun 22, 2012
9,197
I've decided to build a simple, adjustable bench PSU unit to power my little projects at home.
To keep things rather simple, I decided to build it around the LM2576-ADJ step down switching voltage regulator from TI.

I have breadboarded the circuit and it works fine, the only issue I'm having is that when I turn the potentiometer to lower the voltage on the output, the voltage is taking some time to drop down...

Here is my complete circuit:
View attachment 168445

I think the issue must be coming from one of the caps on the output (C2 or C3). They must be holding energy and taking some time to discharge and that is what's causing the voltage to drop slowly on the output.

Is there any technique o circuit I can use so that the caps discharge quicker when I want to lower the voltage?

Try this circuit..


1.2V-35V-Adjustable-Power-Supply-Using-LM2576-ADJ-Step-Down-DC-DC-Converter-6.png
 

crutschow

Joined Mar 14, 2008
25,103
If there is no output load, the only place for the charge on C2 and C3 to go is through the control pot.
Depending upon the pot value, that could take the decay time you are noting.
Adding a dummy resistive load on the output should help.

Why is C2 so large?
You should go with the recommended value from the data sheet.
 

Thread Starter

LewisMF

Joined Nov 15, 2014
100
Hi Lewis.
Depending on the current draw by the load, that 2200uF on the feedback smoothed supply could be causing the delay.
E
Hi Eric,
Thanks for your reply.

When measuring this, I had no load, so just the multimeter across the output (+) and (-), I don't know if this could have anything to do with it?

Regarding the 2200uF cap, I already tried removing it and even changing it for a smaller one and I get the same issue...
 

Thread Starter

LewisMF

Joined Nov 15, 2014
100
If there is no output load, the only place for the charge on C2 and C3 to go is through the control pot.
Depending upon the pot value, that could take the decay time you are noting.
Adding a dummy resistive load on the output should help.

Why is C2 so large?
You should go with the recommended value from the data sheet.
Hi crustschow,
What type of dummy load would you recommend? A small value resistor with high wattage?

I actually got the value of C2 from the datasheet:
circuit.png
 

Thread Starter

LewisMF

Joined Nov 15, 2014
100
It's probably the capacitors C2, C3 being too large and no load,, try making C2 220uF, and put a bulb or 100R resistor as a load.
I just switched the 2200uF cap to a 220uF cap and I'm still having the same issue.

To place a dummy load I'm gonna have to get some power resistors as if I place a standard 1/2W resistor it' gonna burn as soon as I have, for example, 24V on the output...

Is there no auxiliary circuit I could place to discharge the caps while I lower the voltage on the output?
 

crutschow

Joined Mar 14, 2008
25,103
Is there no auxiliary circuit I could place to discharge the caps while I lower the voltage on the output?
For 55V maximum output, a 120V incandescent lamp would likely be the simplest, such as a small halogen type.
You would have to experiment to get the proper wattage that gives a reasonable discharge time.
 
Last edited:

Thread Starter

LewisMF

Joined Nov 15, 2014
100
For 55V maximum output, a 120V incandescent lamp would likely be the simplest, such as a small halogen type.
You would have to experiment to get the proper wattage that gives a reasonable discharge time.
I know this would be an easy solution, but I would actually like to learn something from this...why is this actually happening?

A professional bench PSU doesn't do this, so there must be a way to overcome this issue without a bulb or a power resistor on the output...

Any suggestions?
 

crutschow

Joined Mar 14, 2008
25,103
A professional bench PSU doesn't do this, so there must be a way to overcome this issue without a bulb or a power resistor on the output.
A professional supply either has a dummy load, or some sort of active circuit to pull the output down when a voltage change is commanded.
That would involve significantly extra circuitry for your simple design.
 
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