current control (tricky one) - Electronic Circuits Projects Diagrams Free

bmachining · 24th April 2008, 01:43 AM

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 · 24th April 2008, 01:46 AM

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 · 24th April 2008, 01:51 AM

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 · 24th April 2008, 01:52 AM

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

bmachining · 24th April 2008, 02:00 AM

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 · 24th April 2008, 02:12 AM

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 · 24th April 2008, 02:32 AM

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 · 24th April 2008, 02:36 AM

SO change the opamp?

bmachining · 24th April 2008, 02:51 AM

That sounds too simple!!! hahahah

Any ideas on one that can handle the voltage?

Ps, im an engineer, not an electronics person.

dknguyen · 24th April 2008, 02:54 AM

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 · 24th April 2008, 07:16 AM

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 · 24th April 2008, 07:18 AM

Heres a link,

http://au.farnell.com/1234767/semico...TS-TLE2021CPE4

Ubergeek63 · 24th April 2008, 12:26 PM

Quote:

Originally Posted by 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.

Oh yuch! Wrap appropriate components around a http://www.mouser.com/Search/Product...eLUu7A %3d%3d
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 · 24th April 2008, 03:46 PM

Ubergeek63,

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

bmachining · 27th April 2008, 05:45 AM

No other ideas from anyone? Maybe a schematic?

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Latest
current control by microcontroller	anandpatel27	Micro Controllers	3	18th January 2008 08:42 PM
Current source	GagoX	Electronic Projects Design/Ideas/Reviews	13	12th January 2008 06:32 PM
Voltage	ElectroMaster	Electronic Theory	44	8th August 2007 05:35 PM
Current and Conductivity	ElectroMaster	Electronic Theory	1	2nd February 2007 12:50 PM
Problems with Dual Brightness (Current) LED Tail Lights? Eagle Schematics!	Ocelaris	Electronic Projects Design/Ideas/Reviews	12	31st December 2006 06:18 PM

24th April 2008, 01:43 AM
bmachining Experienced Member	current control (tricky one) 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.

24th April 2008, 01:46 AM
dknguyen Experienced Member	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. __________________ Disclaimer: Avatar for entertainment purposes only.

24th April 2008, 01:51 AM
bmachining Experienced Member	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

24th April 2008, 01:52 AM
dknguyen Experienced Member	Wait, are you asking about how to add variable current limiting to a voltage source? Or how to produce a current source? __________________ Disclaimer: Avatar for entertainment purposes only.

24th April 2008, 02:00 AM
bmachining Experienced Member	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.

24th April 2008, 02:12 AM
dknguyen Experienced Member	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). __________________ Disclaimer: Avatar for entertainment purposes only.

24th April 2008, 02:32 AM
bmachining Experienced Member	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.

24th April 2008, 02:36 AM
dknguyen Experienced Member	SO change the opamp? __________________ Disclaimer: Avatar for entertainment purposes only.

24th April 2008, 02:51 AM
bmachining Experienced Member	That sounds too simple!!! hahahah Any ideas on one that can handle the voltage? Ps, im an engineer, not an electronics person.

24th April 2008, 02:54 AM
dknguyen Experienced Member	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. __________________ Disclaimer: Avatar for entertainment purposes only.

24th April 2008, 07:16 AM
bmachining Experienced Member	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?

24th April 2008, 07:18 AM
bmachining Experienced Member	Heres a link, http://au.farnell.com/1234767/semico...TS-TLE2021CPE4

24th April 2008, 03:46 PM
bmachining Experienced Member	Ubergeek63, a PWM power supply? I like the sound of it! More info please!

27th April 2008, 05:45 AM
bmachining Experienced Member	No other ideas from anyone? Maybe a schematic?