Current control add on for a nice power supply

[be80be]

Here the deal. I have a SAMLEX PSA-305 It's nice heavy power supply 5 amp 30 volts I modded the supply to get rid of the over shoot that worked
great, It did on power down if you didn't unhook it the voltage would shoot up from like 5 volts to 10. I fixed this. It Now drops down from set voltage to 0.

What i want now is to add current control 0 to 5 amps would be nice But not much luck coming up with anything.
I tried this

I tested it but for some reason it doesn't seem to change the output of pin 1 stays at the same voltage no matter where the pot is set.
any ideal how to make this work I want it to work with 0 to 30 volts so I can keep the output from supplying a larger 5 amps on
some of the things I'm playing with.
The supply has current limit set at 5 amps I just want add something I can use from the supply output for 0 to 5 Amps.

 

[audioguru]

The circuit will not work. Its output voltage is determined by the load resistance.
With the 12V supply, how to you expect the output to go to 30V?? You need about +38V, not +12V.
Then the 2N3055 output transistor heats with 190W (!) when the output voltage is set low.
With the 12V supply, the opamp and 3 emitter-followers produce a maximum output voltage of only 7.5V when the output current is 5A.
The reference voltage from the pot needs a regulated supply voltage so that the circuit will have a regulated output voltage.

If you use a 38V supply so that the output can be 30V then all the transistors will melt when the output voltage is fairly low at 5A.
On another forum there is a 30V/5A power supply that uses two 2N3055 transistors in parallel to share the heat plus a fan blowing cool air on their huge heatsink. The transistor driving them is a medium power transistor with a fairly large heatsink bolted to its metal tab.
It uses a second opamp as a current-limiting comparator and uses a third opamp as a reference voltage regulator.

I made notes on your schematic:

 

[crutschow]

Here's a simple current-limit circuit (discussed here) that should do what you want.
Of course the MOSFET M1 needs to be on the appropriate heat sink.
Depending upon the design of the regulator circuit, you may be able to use one of the regulator transistors already there.

 

[be80be]

I know the circuit wouldn't work I posted. I posted it to start with something LOl
I have a big heat sink going to try post 3 out so i need something like 1.2 ohm for RLim
I have a 1 ohm 5 watt would that be ok for RLim

This is a really nice supply just by fixing the over shoot
But I want add current control so i don't blow up a Pic chip using it at 5 volts and it gives the full 5 amps before I catch a wiring mistake LOL

 

[Nigel Goodwin]

I know the circuit wouldn't work I posted. I posted it to start with something LOl
I have a big heat sink going to try post 3 out so i need something like 1.2 ohm for RLim
I have a 1 ohm 5 watt would that be ok for RLim
View attachment 125523

This is a really nice supply just by fixing the over shoot
But I want add current control so i don't blow up a Pic chip using it at 5 volts and it gives the full 5 amps before I catch a wiring mistake LOL

Don't set your supply to 5V - set it to 9V, and make up a little interface board with a 7805 or 78L05 to feed the PIC - problem solved!.

 

[be80be]

Its good for 5 volts now just be worried about 5 amps when testing guess I could be careful
And always wear my glasses lol

 

[Nigel Goodwin]

Its good for 5 volts now just be worried about 5 amps when testing guess I could be careful

The cheap external 5V regulator will limit the current nicely, and a 78L05 (100mA) should be enough for most applications.

And I wouldn't say it's 'good' for 5V now, as it's far too easy to catch the knob and increase the voltage destroying the ciruit, the 78L05 will stop that as well.

 

[be80be]

Im use a 5 volt regulater
Your right about hiting things
I hit my 16f1825 with 12 volts it lived
But boy did it get hot

 

[JimB]

This is a really nice supply just by fixing the over shoot

Yes, a nice looking PSU.
But for use on an electronic experimenters bench, its original designers should have incorporated:
1 A current limit control (the subject of this thread)
2 An output On/Off switch, so that the output voltage may be set, before switching the output to the circuit under test.

In terms of providing a separate current limiting module, Nigels idea of using a separate 78xx regulator is good, but removes the facility of being able to trim the voltage (say +/- 10%) to investigate possible effects of supply sensitivity in the circuit under test.

My own opinion is that the best way to incorporate an adjustable current limit facility is to integrate it with the existing voltage control circuitry within the PSU, rather that build a completely separate current limiting circuit.

At the moment we do not know what the existing voltage control circuit looks like, without that information, discussion of a current limiting circuit could be a little bit erratic.

Maybe the best solution is to completely replace the existing circuit.

JimB

 

[be80be]

I can post it i have it in pdf thats how I fixed the over shoot.
It would be set say 5 volts and I'd you powered it off it would shoot up to 10 volts before dropping down to 0 volts

 

[Nigel Goodwin]

For the original PSU, to add adjustable current limiting it would be best to do it internally, modifying the original design.

 

[be80be]

I was just wanting to make it better for my shop. A friend gave it to me it's really not a bad supply.
I posted the circuits used in it in post 10

 

[Nigel Goodwin]

Im use a 5 volt regulater
Your right about hiting things
I hit my 16f1825 with 12 volts it lived
But boy did it get hot

Wow! - I'm amazed it survived that - I've still got a 16C84 with my finger print burnt in the top of it, and that was just putting the PIC in it's socket the wrong way round, and the supply was only a PP3 battery with a 5V regulator. It didn't work when I connected the battery, and I thought "something smells warm", so touched it (rather stupidly) without licking my finger first - on the assumption that a PP3 couldn't possibly make a PIC that hot

Normally I would ALWAYS lock my finger first, if it's really hot the saliva provides a layer of insulation, and you hear the 'hiss' as it boils (old service engineer trick).

 

[Nigel Goodwin]

I was just wanting to make it better for my shop. A friend gave it to me it's really not a bad supply.
I posted the circuits used in it in post 10

I can't see it there?.

 

[be80be]

Here all I think

 

[be80be]

The pic melted the bread board before I could unplug it Like in less then a second LOL
But it cooled off and still worked.
I hooked some 28 pin chips up backward they wasn't so lucky.
every time it's been cause I wasn't wearing my Glasses I hate them cant see without them and with them I can't see the computer without moving them down to see over them.

 

[Nigel Goodwin]

The pic melted the bread board before I could unplug it Like in less then a second LOL
But it cooled off and still worked.
I hooked some 28 pin chips up backward they wasn't so lucky.
every time it's been cause I wasn't wearing my Glasses I hate them cant see without them and with them I can't see the computer without moving them down to see over them.

You should do what I do - have three different pairs

I have an expensive pair of varifocals, which I wear most of the time, these 'tweak' my vision, and allow me to read prices, use a computer etc. - but with the obvious varifocal limitations.

So I ordered cheap prescription glasses on-line, specifically for close up work (reading, soldering etc.) and another pair for computer work (mid-range glasses), this avoids all the 'up down' of varifocals. In fact I have two pairs of each of the cheap ones, one pair for home, one pair for work - they cost about £10 each.