I need to make a current source that is adjustable from 300 ma to 1200 ma. I need to use a multiturn pot so using a LM317 and a series resistor won't work. What would be the best way to do this.

Thanks
Geoff

 

Hello,

Take a look at the schematic on the following page:
http://www.hobby-electronics.info/course/html/OpAmp-Current-source.html

When you use a single powersupply the opamp must be a rail to rail type.

Bertus

 

Thats a real easy circuit although I should have mentioned that I only have access to one side of the load the other end is grounded. I can't see how I could do that with this circuit. Any ideas?

Thanks
Geoff

 

You do not have access to the ground?

In order to do current regulation, you will need to feed the extra to ground. You may have a problem.

Do you not have access to the power supplies ground?

 

What is the resistance of your load, or if it is a variable load, what does it vary between?

 

• That circuit can be re-configured using a PNP transistor to regulate a current through a load which is connected to ground.
• You may need choose carefully the transistor and heatsink which will survive the heat disspation.
• What voltage are you working with?

Regards,
Ifixit

 

The supply is 12 volts and the load is around 6 ohms. It is not going to be on for a long time, maybe 5 minutes or so and then off for days. How would you rework the diagram to accomodate a PNP power transistor. I cannot isolate the load, one side is permanantly tied to ground.

Thanks
Geoff

 

See the attached for one possible solution.

It's not a constant current regulator; it's a simple PWM switcher.

However, with your stated load resistance of about 6 Ohms, this PWM circuit will supply roughly 300mA to 1.2A depending upon the setting of pot R2.

The advantage of using something like this is that your total power dissipation will be much lower than if you used a linear regulator, as practically all of the power will be dissipated in your load instead of much of it in the regulator.

 

Here is a linear solution in case you decide to go that way. This circuit regulates current, not voltage. The current is independant of the value of the load (R6).


Notes:

1. You can change R2 & R4 if your ten-turn pot is of a different value.
2. You can select a different, but similar transistor if you like.
3. A heatsink(HS) for the transistor is required. 5 or 6 square inches of black anodized aluminum should do. To test HS effectiveness, measure HS temperature.
4. Keep the HS temperature below 100°C for good reliability. The bigger the HS, the lower the temperature.
5. Protect the circuit with a fuse if this is in an automotive inviroment.
6. The opamp needs to be a rail-to-rail type.
7. The accuracy is dependant on the 12V supply being 12.0 Volts.

Playsafe,
Ifixit

 

Thanks guys. Whow 2 different ways. I understand the linear but got a question on the PWM one.

Couldn't you just feed a vaiable duty cycle signal from the 555 through Q1, Q2 to M1. What does L1 & D3 do.

Thanks again
Geoff

 

I need to make a current source that is adjustable from 300 ma to 1200 ma. I need to use a multiturn pot so using a LM317 and a series resistor won't work. What would be the best way to do this.

Thanks
Geoff

You can still use a LM317 as shown in the schematic I posted. With the reostat at minimum resistance the current would be1200mA and at maximum resistance the current would be very near 300mA.