current control (tricky one)

[bmachining]

Hi,

Im trying to build a DC power supply, but cannot find a circuit to suit.

I need variable current limiting, via a pot, voltage control isnt necessary.
Sounds simple, but i need it to handle up to 40V and as much current as reasonable, if i can draw 30 amps i be happy.

I have built a voltage controlled PS, with the above specs, using Lm317 and 4 mj2955, it works ok, but does produce alot of heat. But have been confused a few times on how to make this work with current limiting. PWM?

I can build, but not real flash on designing.

 

[dknguyen]

Have a comparator drive a pass transistor, and feed a current sensor output into the comparator. Adjust the comparator threshold using a pot or something. Whenever the current gets too high, the comparator's output changes such that the pass transistor turns off disconnecting all the current flow.

 

[bmachining]

I should mention this is for electroplating, the process needs constant current, voltage will adjust to maintain the selected current flow.

A circuit diagram would be much appreciated, as i cant design this stuff myself. But i do like building these things hehe

 

[dknguyen]

Wait, are you asking about how to add variable current limiting to a voltage source? Or how to produce a current source?

 

[bmachining]

dknguyen, Which ever is more suitable/easier

I have the transformer, bridge rectifier and caps, a handy supplier of components and an unhealthy appetite for punishment!

the voltage regulator is on a small proto board so if it goes into the parts box im not worried.

 

[dknguyen]

THe one I described is really current limiting. It will try and stop the current from getting too high, but doesn't stop the current from getting too low (though it will allow current to flow again once it falls below the threshold so technically it kind of does). It's not so much "variable current" by modifying the voltage or duty cycle as much as it is "bang-bang on-off". Since there is no duty cycle and there is only on or off. It's far from ideal however and there will be overshoot and undershoot but it's simple.

A time delay might need to be added somehow to the pass transistor isn't switching on and off insanely and burn out (switching also causes losses).

 

[bmachining]

Ahh ok. i do have a circuit diagram for a current limiting PS, using a UA 741, which senses voltage drop across a small resistance, which goes into a BD 149 that drives 4 of 2n3055. this works great!! But the Op amp is limited to 18V..... and i have 40v.

 

[dknguyen]

SO change the opamp?

 

[bmachining]

That sounds too simple!!! hahahah

Any ideas on one that can handle the voltage?

Ps, im an engineer, not an electronics person.

 

[dknguyen]

Any op-amp would do since this isn't a signal application. www.digikey.com. Search around.

The only 40V capable op-amp I have in my directories is the OP547-549 but those are high current op amps.

 

[bmachining]

Not the easiest device to find, but i have found one thats available
Texas Inst, TLE2021CPE4 Would an expert here mind checking the specs compared to a Ua741 and see if it willl work?

 

[bmachining]

Heres a link,

**broken link removed**

 

[Ubergeek63]

Hi,

Im trying to build a DC power supply, but cannot find a circuit to suit.

I need variable current limiting, via a pot, voltage control isnt necessary.
Sounds simple, but i need it to handle up to 40V and as much current as reasonable, if i can draw 30 amps i be happy.

I have built a voltage controlled PS, with the above specs, using Lm317 and 4 mj2955, it works ok, but does produce alot of heat. But have been confused a few times on how to make this work with current limiting. PWM?

I can build, but not real flash on designing.

Oh yuch! Wrap appropriate components around a https://www.mouser.com/Search/ProductDetail.aspx?qs=ZYAvNeFW%2b8tcMQn/eLUu7A==
a .006 ohm 10W resistor and a 0.25V max LD signal (linear dimming) with the proper IGBT and inductor running off a rectified mains will do what you are looking for.

 

[bmachining]

Ubergeek63,

a PWM power supply? I like the sound of it! More info please!

 

[bmachining]

No other ideas from anyone? Maybe a schematic?

 

[dknguyen]

A PWM Power supply is a called a switching DC converter. Boost (Step-up), Buck (step-down), Buck-boost (does both while being less efficient), or flyback (uses a transformer instead of an inductor for isolation of the output), are the 4 kinds.

It is a big topic. With proper sensors for things like current feedback and with a microcontroller, you can adjust the duty cycle (the PWM) and thus do current limiting. Google it. It is a big...big complicated topic that I cannot cover on here unless you know the basics first.

 

[bmachining]

Thankyou! Yes is very involved but you have explained a few things already that i need to knoe to refine my search.

Needs to be Buck, (stepdown), using a transformer for step down from 240 to 40V.

 

[blueroomelectronics]

You want 40V at up to 30A, that's a pretty beefy 1200W, not likely a simple design.

 

[dknguyen]

THe inductor that would be pretty big. The transistors aren't too bad though since you can find transistors that just barely pull that off, and you only need one for a switching converter so you can parallel them vairly easily.

 

[bmachining]

Yep, its a serious power supply. I imagine the current control could be much like some of the new switchmode arc welders, but im not wanting switchmode. Just the old transformer for stepdown. Half the battle seems to the be terminology im not used to. Can inductors be paralled? it would save winding one big one and the difficuilty finding a suitable core?