Dumbproofing a lab power supply? (Input/Output protection)

Thread Starter

TheLaw

Joined Sep 2, 2010
228
I'm building a power supply based off this schematic: http://www.extremecircuits.net/2010/06/fully-adjustable-power-supply_17.html

Overall a nice schematic, but I'm not sure how it would hold up if I hooked something up the wrong way or applied a voltage to the output by accident.

What do you guys think would be some good provisions to make? What type of protections and where should I put them?

Thanks.
 
Last edited:

mcgyvr

Joined Oct 15, 2009
5,394
Do you require output short circuit protection too? (i.e. crowbar circuit)

Normally the output of a benchtop power supply allows either - or + referenced grounds.
Basically hook the output up one way and you've got positive voltage, hook it the other and you've got a negative voltage.
 

#12

Joined Nov 30, 2010
18,224
My first impression is: just put a 10 amp, 100 volt Shottky on the output and adjust the minimum out voltage to correct for the voltage difference. Here's one for 60 cents.
 

Attachments

Thread Starter

TheLaw

Joined Sep 2, 2010
228
Yes, I would like to be able to generate either positive or negative voltgages based on how you hook up the test leads. So in that case, do I just use one Schottky in series with the + output? Perhaps a PTCC also? Or a real fuse. I've seen people use TVS diodes, but I'm not sure the purpose of those.

Thank you.
 
Last edited:

#12

Joined Nov 30, 2010
18,224
Yes, I would like to be able to generate either positive or negative voltgages based on how you hook up the test leads.
Simply reversing the way you connect the output leads reverses the voltage, so, what's the problem? You would like to be able to do that but you aren't able to do that?

One diode will control current direction, no matter which way you connect the leads. A fuse is quicker than a PTCC.
 

Thread Starter

TheLaw

Joined Sep 2, 2010
228
Simply reversing the way you connect the output leads reverses the voltage, so, what's the problem? You would like to be able to do that but you aren't able to do that?

One diode will control current direction, no matter which way you connect the leads. A fuse is quicker than a PTCC.
Sorry. I'm having some theory breakdown. There's no issue in me reversing the leads. I was just agreeing with what was said.

Anyway, I think I will install a Schottky.

Is there anyway to protect the LM317/pass transistor. Sometimes in regulator datasheets you see a diode reverse biased across pins 1 and 3 of the regulator?

Or would I be covered with a diode at the output?

Sorry! Thanks
 

#12

Joined Nov 30, 2010
18,224
The diode across the 317 protects it from the output capacitor discharging backwards through the chip if the input suddenly changes from a positive voltage to grounded. You can add that diode to this circuit and do no harm.
 

crutschow

Joined Mar 14, 2008
34,285
My first impression is: just put a 10 amp, 100 volt Shottky on the output and adjust the minimum out voltage to correct for the voltage difference. Here's one for 60 cents.
To avoid the voltage drop you can place the diode across the output terminals (cathode to plus side). That way it will short any reverse voltage you might accidentally apply to the outputs. You might also fuse the output if you want to limit the current from this reverse voltage.
 

Thread Starter

TheLaw

Joined Sep 2, 2010
228
To avoid the voltage drop you can place the diode across the output terminals (cathode to plus side). That way it will short any reverse voltage you might accidentally apply to the outputs. You might also fuse the output if you want to limit the current from this reverse voltage.
Right that makes sense. That doesn't necessarily have to be Schottky then..or?

Fuse the power supplies output? Or fuse in series with that diode?

Not sure if I got you on that second part.

I appreciate all the help. Thank you.
 

crutschow

Joined Mar 14, 2008
34,285
Right that makes sense. That doesn't necessarily have to be Schottky then..or?

Fuse the power supplies output? Or fuse in series with that diode?

Not sure if I got you on that second part.
It could be a standard silicon diode.

You want to fuse the power supply output after the diode. If you fuse the diode then you would loss the protection when the fuse blew.
 

THE_RB

Joined Feb 11, 2008
5,438
If you have current limiting (I have often used another LM317 as a current limiter before the voltage regulator) then you don't have to worry about reverse connecting a battery or something to the output.

A reverse connected voltage source will act just like a short circuit and all it will do is activate the current limiting.

Your only issue is connecting a voltage higher than the PSU voltage (not reversed). If you use the diodes as shown on the LM317 datasheet they will protect from that situation.

I still think there should be a sticky or tutorial on the forum about how to make a simple LM317 adjustable power supply. :)
 

Thread Starter

TheLaw

Joined Sep 2, 2010
228
If you have current limiting (I have often used another LM317 as a current limiter before the voltage regulator) then you don't have to worry about reverse connecting a battery or something to the output.

A reverse connected voltage source will act just like a short circuit and all it will do is activate the current limiting.

Your only issue is connecting a voltage higher than the PSU voltage (not reversed). If you use the diodes as shown on the LM317 datasheet they will protect from that situation.

I still think there should be a sticky or tutorial on the forum about how to make a simple LM317 adjustable power supply. :)
Well a regular old LM317 is pretty simple. But some theory could help silly people like me to understand how they work and not have to ask silly questions. ;)

Thanks.
 

THE_RB

Joined Feb 11, 2008
5,438
There's nothing silly about questions. :) It seems the forum has lots of people with LM317 questions of late, things like the diodes, dropout voltage, how to wire it up etc etc.
 
Top