Constant Current

[be80be]

I found this circuit on here I added some changes. One a 120 ohm 1 watt resistor to insure that the 2n7000 doesn't see more then 200 mA maybe over kill but. The V+ will be 24 volts or less

The 1 ohm 1/2 watt will be read by the pic's ADC to set the output to 4 to 20 mA

Would this work **broken link removed**
The read resistor will be read with a Pic down the line

Thanks Burt

 

[dougy83]

So the point of the circuit is just a current sink controlled by a PIC?

You have Q2 configured as a current limit (600mA with R2 = 1 ohm), so the 120R wouldn't be needed if you increased R2. Increasing R2 would also give you better resolution on your ADC reading of the current.

 

[be80be]

Then this should be better what I'm after is I want to control the current across the read resistor from 4 to 20 mA so i changed the 1 ohm to a 10 ohm and dumped the 120 View attachment 68937

 

[dougy83]

You should calculate the values for the resistor to suit your application. Before working out the values though, it will help if you can define the following:
ADC reference voltage and bit resolution: e.g. 5V, 10 bits
Output current resolution: e.g. 100 levels from 4mA to 20mA
PWM resolution and frequency: e.g. 8 bits + N bits dither @ 60kHz
Output current change rate: e.g. don't care

From these values you can work out the resistor values and how the current limit circuit will be configured.

Assuming that you want an analogue output current, and not a switching PWM, you should put some low-pass filtering on the PWM before driving the transistor (e.g. a capacitor on Q3 base-collector).

 

[be80be]

The 250 ohm read resistor is fixed adc 10 bit 60 khz PWM is ok

I'm just playing with parts I have laying here

 

[dougy83]

Well the value of the resistor affects the voltage across it; consequently the range of the ADC used will limit the sensitivity/accuracy of the current reading. e.g. with a 10R resistor, at 20mA that's 200mV, or 1/25 of the full scale range of the ADC operating at 5V. That's only 40 counts of the ADC, so that's <40 output current levels (unless you oversample the ADC).