How to build a constant current source?

I have been frustrated for months working through an introductory circuits book, as I want to build many of the circuits, but many of them include constant current sources. Why is it so easy to find voltage sources at your local store (i.e., batteries!), but searching for current sources on the web leads to a complicated bunch of circuit diagrams?

Here is my naive question: using thevenin-norton equivalent circuits, couldn't I transform a battery (i.e., voltage source) into the desired equivalent current source using Vth=InorReq. That is, can I put a voltage source in series with a resistor (as opposed to its equivalent, a current source in parallel with the same resistor)? What is the problem with doing that?

Does anyone know where I could buy a cheap but reliable current source? Why is this so hard?

Reply to
thomson.eric
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Your desire to build many circuits but not any of the simpler current sources is a puzzling combination.

Maybe nothing. How much of a current source do you want? How constant does it have to be versus terminal voltage?

Your subject indicates you want to build a current source. If that is out of the question, one of these parts,

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together with a battery could be called a "current source".

You can use a JFET, with its gate connected to its source, as a current source. Put a variable resistor in series with the source to adjust its value downward. Somebody used to sell a two-terminal current source which was just a JFET hooked up that way.

It's not.

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--Larry Brasfield
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Reply to
Larry Brasfield

If a current sink will do, then you can do it with an NPN transistor, a couple of diodes, and a couple of resistors.

"output" | |= x mA | C +V ---[R1]--+--B | E V | --- | | [R2] V | --- | | | GND GND

Where R2 is sized to give .6V at your desired current, R1 is maybe

1K or so, and you should recognize the two diodes. I think a 2N4401 can go up to almost a half an amp.

Turning that into a source, with a PNP transistor, is, of course, left as an exercise for the reader. ;-)

Have Fun! Rich

Reply to
Rich Grise

You can buy a constant current source. I have a cheap one myself. It is basically a power supply with a voltage source and current limit. Turn the current to 0, short the output, turn the voltage all the way up, and then adjust the current to your liking. You can get them here:

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You can also build a constant current source, given a voltage source. An LM317 voltage regulator makes a good one. It tries to keep the voltage between it's output and adj terminal at 1.24V. Thus, if you put a resistor of value R on the output, and connect the adj terminal after it, before the load, the LM317 will try to adjust the voltage so that there is a constant 1.24/R A across the load. If you use an R = 100, for example, then it will try to keep 12.4mA across the output by varying the output voltage. Note that it will only be able to adjust the voltage within the limits of the original voltage source, minus some for whatever the LM317 + resistor needs, probably 3 or 4 volts.

--
"I\'m mad as hell, and I\'m probably going to just sit here and take it!"
Reply to
Bob Monsen

Wow, you have all been very helpful. I will print these suggestions out and try to figure out which would be best. As far as specifics, I would be happy with something that could generate a single DC current of ~100mA. In the best of possible worlds, I would love something that would let me, through a turnpot, vary the DC current between 0 and 1 Amp. I don't want anything fancy, just something so that I can play with my breadboard while working through an electronics book.

I guess we are lucky to have batteries. I wonder why no companies sell a cheap current source that takes AC in or even uses batteries to power it. They could probably make a KILLING with all the EE departments around the country! Maybe I'll make one and provide the supply!

Reply to
thomson.eric

Voltage sources produce a fairly constant output voltage for all loads from infinite resistance (zero current) to some minimum resistance (more than zero current), but they are only approximate voltage sources, because their output voltage does sag and current passes through them. This is because they have some internal resistance.

Practical current sources hold a very nearly constant current for all loads from zero ohms (zero voltage drop) to some maximum value of resistance (some maximum voltage drop called the compliance voltage).

Ideal current sources would produce a constant current, regardless of the load resistance, including infinite resistance, by producing a compliance voltage up to infinity volts.

It is a lucky accident that we have discovered chemical cells that produce something close to a voltage source over useful ranges of load current. There is no similar simple power sources that provide a constant current output.

The problem is that it is not a current source, but a current source in parallel with a fairly low value of resistance. Mathematically transforming a voltage source that has a little series resistance to the equivalent current source with parallel resistance does not actually alter the characteristics of the source, at all.

Almost all practical current sources are actually active variable resistances that waste all the extra voltage from a voltage supply that is not needed to force the specified current through the load. This active circuit has to be designed to pass the required current, while measuring it, in some way, and using some sort of feedback, vary an active resistance as needed to hold the measured value of current, constant, till the load uses up all the available voltage. These active current regulators are specified with their effective parallel resistance (equivalent to a perfect current source in parallel with a resistor), their voltage compliance, their frequency response, and sometimes how long it takes for them to recover current regulation after being exposed to an excessively high load resistance and then the load falls to within the normal operating range.

Name your specifications, and people, here will help you design current sources (regulators) that meet your needs.

Reply to
John Popelish

Take a look at page 16 of the data sheet for the very common voltage regulator LM317:

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All you need for a variable current regulator is an isolated voltage source (battery or wall wart) and a variable, low value resistor, or a selector switch that selects various low value resistors.

The current setpoint is 1.2 volts/resistance. The LM317 may well need a heat sink for currents above 100 mA. At 1 amp, the resistor will also produce some significant heat ((1.2 volts^2)/resistance).

Reply to
John Popelish

Exactly, my main motivation. Secondarily, I am a neuroscientist. We inject current into neurons all the time, and I want to stop treating the stimulator as a black box!

Reply to
thomson.eric

Have you ever gone through basic electronics text book? They are full of hypothetical circuits that contain voltage sources and current sources, to demonstrate the analytical principles. If you are an experimentalist, you need both sources to verify that they are not lying to you.

Reply to
John Popelish

Nobody as asked:- Why do you want a constant current supply?

Most of the electronics experiments you will encounter will require constant Voltage and these supplies are every where because everyone uses them for most everyting.

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John G

Wot\'s Your Real Problem?
Reply to
John G

Whose neurons? Hey has anybody seen my foil hat!!?

Reply to
kell

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