Current Reguation - Electronic Circuits Projects Diagrams Free

andreww · 26th September 2005, 11:55 AM

HI all,

I'm after some help /advice if anyone can help please.

What i want to do is keep a constant current output of 500 milli amps on a load that is changing is resistance / conductivity. At the moment when the resistance changes, the current runs away. Supply voltage is between 12 - 40 volts, and the current draw needs to stay around 500ma. In this situation the resistance starts off high - more voltage needed to keep current at 500ma but as the resistance starts to drop i need to be able to regulate the current at 500ma by reducing voltage. I've read about current sensing, current regulators, but i'm not sure what i need.

Any help would be greatly appreciated.

thanks in advance

Andrew

Optikon · 26th September 2005, 01:17 PM

Quote:

Originally Posted by andreww

HI all,

I'm after some help /advice if anyone can help please.

What i want to do is keep a constant current output of 500 milli amps on a load that is changing is resistance / conductivity. At the moment when the resistance changes, the current runs away. Supply voltage is between 12 - 40 volts, and the current draw needs to stay around 500ma. In this situation the resistance starts off high - more voltage needed to keep current at 500ma but as the resistance starts to drop i need to be able to regulate the current at 500ma by reducing voltage. I've read about current sensing, current regulators, but i'm not sure what i need.

Any help would be greatly appreciated.

thanks in advance

Andrew

Use a transistor current source/sink.
Couple of low cost parts. look at this.
http://www.4qdtec.com/csm.html

Do you need values & a whole shematic drawn or can you take it from here?

eblc1388 · 26th September 2005, 09:09 PM

Quote:

Originally Posted by andreww

What i want to do is keep a constant current output of 500 milli amps on a load that is changing is resistance / conductivity. At the moment when the resistance changes, the current runs away.

The following circuit would suit your requirement. The LM317 is a cheap three terminal voltage regulator which can be bought in different packaging. Remember the LM317 is giving out 15W heat when the load is taking 12V at 0.5A so a good heatsink and mounting using heat transfer compound are a must. The total wattage of the 2.5 Ohms resistor is 0.63W but I would recommend using either one 2W resistor or four 10 Ohms 0.5W resistors.

andreww · 26th September 2005, 11:59 PM

Thanks guys for your replies, greatly appreciated.

The other part that i would like it to do, is adjust the voltage automatically when the resistance / conductivity of the load changes to maintain the 500mA output. Or does the circuit above do this??

So as Resistance of the load goes down (conductivity increases) the voltage reduces to maintain the current at 500mA

cheers

Andrew

eblc1388 · 27th September 2005, 07:29 AM

The circuit I posted above will adjust its output current automatically to give a stable 500mA output for any load from 20 Ohms to 80 Ohms.

It doesn't matter if the resistance changes within above range, the current will be kept constant at 500mA regardless.

Oznog · 27th September 2005, 07:55 AM

What are you trying to drive? What is the voltage going to be like?

andreww · 27th September 2005, 12:28 PM

Thanks guys,
In its current form it takes about 40vdc to get the process started at 500mA, then as the process continues the resistance of the load drops, and voltage has to be turned down or current will run away. At this point it requiers human monitoring to reduce the voltage to bring the current back into range approx 500mA.

As far as resistance of the load i have not taken any measurements at this stage.

cheers

Andrew

26th September 2005, 11:55 AM	(permalink)
andreww New Member	Current Reguation HI all, I'm after some help /advice if anyone can help please. What i want to do is keep a constant current output of 500 milli amps on a load that is changing is resistance / conductivity. At the moment when the resistance changes, the current runs away. Supply voltage is between 12 - 40 volts, and the current draw needs to stay around 500ma. In this situation the resistance starts off high - more voltage needed to keep current at 500ma but as the resistance starts to drop i need to be able to regulate the current at 500ma by reducing voltage. I've read about current sensing, current regulators, but i'm not sure what i need. Any help would be greatly appreciated. thanks in advance Andrew

27th September 2005, 07:29 AM	(permalink)
eblc1388 Experienced Member	The circuit I posted above will adjust its output current automatically to give a stable 500mA output for any load from 20 Ohms to 80 Ohms. It doesn't matter if the resistance changes within above range, the current will be kept constant at 500mA regardless. __________________ L.Chung

27th September 2005, 07:55 AM	(permalink)
Oznog Experienced Member	What are you trying to drive? What is the voltage going to be like? __________________ I thought what I'd do was I'd pretend I was one of those deaf-mutes.

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26th September 2005, 11:59 PM	(permalink)
andreww New Member	Thanks guys for your replies, greatly appreciated. The other part that i would like it to do, is adjust the voltage automatically when the resistance / conductivity of the load changes to maintain the 500mA output. Or does the circuit above do this?? So as Resistance of the load goes down (conductivity increases) the voltage reduces to maintain the current at 500mA cheers Andrew

27th September 2005, 12:28 PM	(permalink)
andreww New Member	Thanks guys, In its current form it takes about 40vdc to get the process started at 500mA, then as the process continues the resistance of the load drops, and voltage has to be turned down or current will run away. At this point it requiers human monitoring to reduce the voltage to bring the current back into range approx 500mA. As far as resistance of the load i have not taken any measurements at this stage. cheers Andrew