2mA constant current source

[wilbee]

hi people.
I am not very experienced in electronics
But I need to build a constant current source for a current of 2mA.

I was hoping if anyone can direct me to a reference where I can look up some circuits which can do this. Any help would be greatly appreciated
Thanks
wil

 

[sbayeta]

Try the LM134. Look for the datasheet, it's simple to use.

 

[wilbee]

thanks so much
:lol:
wil

 

[mechie]

Easy Constant Current

Take a look at this circuit (same circuit shown using an NPN and PNP transistors).
If you want a cheap and cheerful CC circuit...

The zener diode will produce a constant voltage,
Choose a sensible zener voltage, then calculate R1 to suit it.
The transistor (either circuit - both the same!) will be biassed to such a point that the voltage across its base-emitter junction will be about 0.6v.
There will now be a voltage drop across R2 which is equal to ZD1 - 0.6v, as this is a constant voltage and R2 is a constant resistance it follows that there will be a constant current through it! (Ohm's Law).

As long as the load (R Load) is a low enough resistance to allow this current through when the supply voltage MINUS the R2 voltage is across it.

Does that make sense ?

 

[wilbee]

Re: Easy Constant Current

I may be asking a silly question
but i need to use the current to stimulate some animal tissue. therefore how am i going to "tap" into the circuit and make use of this constant current source in the transistors configuration?

many thanks
wils

 

[Roff]

Re: Easy Constant Current

I may be asking a silly question
but i need to use the current to stimulate some animal tissue. therefore how am i going to "tap" into the circuit and make use of this constant current source in the transistors configuration?

many thanks
wils

Your two probes will be in place of the resistor RLoad. In other words, the tissue is RLoad. The problem you may have is that the current source will saturate if the resistance of the tissue sample causes the voltage across it to approach the voltage on the emitter of the transistor. If this happens, the current through the tissue sample will be less than you calculated. The only solution is to pick a higher value for ve. Make sure your transistor has a breakdown voltage higher than ve.

 

[wilbee]

Re: Easy Constant Current

First thank you very much for your replies. they have been very helpful.

the tissue i will be testing from experience it should have a resistance in the range of kilo ohms. however, i need to stimulate it with pulses of dc current at a freq of 3Hz and duration of 100ms. so which of the LM134 and the transistor configuration is a better option? I have read the LM134 datasheet, its output impedence do seem to fall quite rapidly with freq.
thank you so much
wils

 

[Roff]

I have simulated this circuit. I should do what you need. Note that I made the positive supply rail GND, instead of the negative rail as is usual in 555 circuits, so that one of your probes will go to circuit GND, in case you have a grounded power supply.
This should work for tissue resistances up to about 6k ohms.

 

[wilbee]

would it be possible?

After doing more tests, i found out that the cell's resistance is extremely freq dependent. And i think that i need to take it as around 20kilo ohmns and if i wanted to pass a current of 2mA through it. that means i would need a power source of over 40V. If i wanted a higher output like 5mA i would need source of around 100V. if i need to run from the mains there is no problem. :shock:

Also, i wanted to ask if i want the stimulator to output a current which can be varied from 2mA to 5mA with some kind of detection device which i can monitor the ouput on a scope. In addition, allows me to change the polarity of the current.
Can it be done :?:
I know there is a lot here. but it would be very very helpful.
and your help is very much appreciated

many thankx :!:
wil

 

[Roff]

OK, this outputs 2ma to 5ma, handles up to 20kohm tissue sample, and provides a place to monitor the current with a scope. If you want to reverse the current, reverse the probes. You can do it manually or with a DPDT switch or relay.
The MPSA43 will dissipate up to 600mw instantaneously, but if you keep the frequency and the duty cycle as designed, the average power is only about 200mw max. If you slow down the frequency and/or increase the duty cycle, you might need a heat sink.
The 120v supply does not need to be regulated, and a few volts of ripple won't hurt anything. Remember that 120 volts can be LETHAL!

 

[wilbee]

thx so much!!!! :roll:

 

[jozeh]

This circuit will be really useful for me! thx so much.

I have a doubt, is the 120 v supply a dc source? Can I use AC?

 

[atferrari]

Check the date of the htread.

 

[crutschow]

This circuit will be really useful for me! thx so much.

I have a doubt, is the 120 v supply a dc source? Can I use AC?

No. You need to use an isolated power supply to turn mains AC into DC.