current limiter / voltage regulator question

I believe changing the feedback network with a National audio power amplifier module can result in a V to I converter. D from BC

what would happen if you connected the 0 - 5V directly to your 15 ohm load ?

"all" you need to do is scale the 0-5V up to 0 - 30V the 15 ohm resistance will convert the voltage to 0-2 A

perhaps using a single-rail op-amp and a NPN power darlington in common emitter configuration.

Bye. Jasen

Look at the National Semiconductor LM12 - probably something of an overkill - or their LM675, which should be able to do the job. The LM12 is nicer - I think it was designed by Bob Widlar - while the LM675 is an improved second source of a nasty Thompson-CSF power op amp.

You could build either of these parts into a "Howland current source" which can certainly be driven by a variable voltage.

There are tidier solutions, particularly if you only need a uni-polar current source, but you'd need to learn more to get them to work.

-- Bill Sloman, Nijmegen

you only need a constant current source if you have a varying load such as a battery. with a fixed load a variable voltage supply will be fine .

Instead of using the LM117 "Precision Current Limiter", use the LM150 "Precision Current Limiter". It goes up to 3 amps. Yes it is a variable constant current source. You set whatever constant current you want, and as the load varies, it will maintain that current (provided the voltage supplying the constant current regulator is high enough).

What the last poster says is true, and recommendation of the LM150/350 is good, but in context of the original question, there are problems with this approach. (1) The OP stated that he needed to supply a fixed resistance with 0-2 amps. The "Precision Current Limiter will not go to zero amps. It can get close, but never to exactly zero amps. (2) The current limit pot that controls the current must be able to handle at least 2 amps... not an easy task for a common potentiometer. I suggest that the OP consider the circuit titled "Adjustable Current Regulator" in the LM150 datasheet for full realization of his requirements. The additional regulator and negative voltage source in that circuit lets the output current go down to zero amps, while still giving good control up to full output current.

Very good observations, Dave. When you don't go through the numbers, it is easy to overlook some of the details.

Item (1): That may or may not hurt the Op, based on what he is using it for.

Item (2). That would probably be a problem.

The "Adjustable Current Regulator" would be okay, if he has a negative supply. At least according to the drawing, he would need a -5 to -10 volt supply, too (even though a very low current negative supply). Or, am I reading it wrong?

I drew a diagram up, look at

good,

IIRC he had a 0-5V input, not a pot input.

here's one way to do it using an LM350 +33.5V --[RL]----[LM350A]---[0.1]--- 0V | 5W | Vin (0-5) --[2K4]---+--[100R]--- -1.3V The silly looking 33.5V supply is needed to accomodate the 30+3+0.2V drop under full load and stat under the 35-1.25V maximum under no load. it works on paper with ideal parts but 1% drift in the part translates to a

Bye. Jasen

"panfilero" wrote in message news: snipped-for-privacy@v45g2000cwv.googlegroups.com...

I made two circuits that may do what you want, using just two transistors and some resistors and diodes. The first one uses bipolar transistors, and the second uses MOSFETs, which seem to be more linear. You may want to use devices that are better rated for the purpose than what I found in the LTspice library. In each case, the voltage input varies from Vbe or VgsOn to about 2 volts above that, to vary the output from zero to two amperes. I tried it with loads from 1 ohm to 15 ohms and it stays close to 2 amperes (or wherever you set the current). You need a raw supply of 35 to 40 volts to get the 30 volts into 15 ohms. I used a one ohm sense resistor, which drops 2 volts at full current, but you might get by with 0.5 ohms or less. Temperature stability will be an issue with this circuit, but it might do the job. Of course, an op-amp could be used with current feedback to achieve near perfect regulation. The LTspice files follow:

Paul

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