I need to design a constant current source of 0.1 mA. The load varies between 1Mohm and 1 Kohm. Using a constant voltage source, is it possible to design a constant current source? Any help in this is greatly appreciated.
--Confused Soul
I need to design a constant current source of 0.1 mA. The load varies between 1Mohm and 1 Kohm. Using a constant voltage source, is it possible to design a constant current source? Any help in this is greatly appreciated.
--Confused Soul
0.1mA thru 1Meg is 100V. What supplies do you have available?
...Jim Thompson
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et |
Hints: check out common bipolar-transistor current-source circuits, with opamp feedback. Think about using two MPSA42 (npn) or MPSA92 (pnp) transistors, wired in the Darlington configuration.
-- Thanks, - Win
Good enough approximations may be possible. Is the load resistance connected to a positive supply, a negative supply, ground, or none of the above?
What supply voltages are available?
What kind of frequency response must this current source have?
0.1 mA through 1 Mohm implies 100 volts drop across that resistance, so a voltage supply greater than 100 volts will be necessary to feed the current regulator.-- John Popelish
Looking at the previous replies, I kinda feel dumb that I didnt even think about the possible voltage required. 100 V seems impractical in my case. It is possible for me to change the constant current value to
0.01 mA or even 1 uA instead of 0.1 mA. In that case, what kind of circuits can be used for building a constant current source.I read in sci.electronics.design that confused soul wrote (in ) about 'Constant Current Source Design', on Mon, 24 Jan 2005:
Yes. It sounds like homework to me. We don't do homework but we may give hints to polite enquirers.
Have you given all the data? How much variation in the 0.1 mA is allowed? Do you have to use just a voltage source and resistors, or are transistors allowed?
0.1 mA through 1 Mohm means that there is 100 V across the 1 Mohm. If you had a 1 kV supply with a 9 Mohm resistor in series, how would that work out? Do you think that's a practicable solution?-- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory.
I read in sci.electronics.design that confused soul wrote (in ) about 'Constant Current Source Design', on Mon, 24 Jan 2005:
A few weeks ago we had a thread here in which a FET-based current source could give output source impedances of gigohms. That should be OK for you! If you can't search the group archive, maybe someone will give you a message ID to look up or a copy of one of the relevant posts.
-- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory.
With a bipolar op-amp such as the LM324, there's an error component from the input bias current. At 1uA, it could be as much as 50% error (typically 4%). It also varies with supply voltage and somewhat with temperature. There are better op-amps.
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com
A FET-based solution is well worth considering!
-- "What is now proved was once only imagin'd." - William Blake, 1793.
Have a look at the Fixed Current Source schematic on page 10 of
The High Compliance Current Sink on page 12 will do the same trick if you need to sink instead of source the current. Again just substitute the NPN transistor with an N-channel MOSFET and recalculate for your needs. The current sink circuit has the advantage that it can be made to work with any voltage as long as you use the right MOSFET.
You can keep the transistors if you don't like MOSFETs but then you will have a small error contribution from the base current. Old datasheets are abundant with similar circuits.
HTH
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