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Digital current source.

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Sceadwian

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Using as simple a setup as possible, what would my options be for a variable digitally controlled current source be? The main current driver would be a power mosfet. I'm thinking along the lines of 0-1 amp 256 steps. 0 doesn't need to be one of them but not more than 5ma on the lowest setting. Linearity would be preferred but not required.

Anyone else have alternatives? Something I could do with discrete components? I'd rather not buy a digital pot, and I wouldn't mind 0-5 amps with 11 bits of range.
 
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MikeMl

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Look at this post. Put an inexpensive 10bit DAC where the 0-1V source is. If your chosen DAC puts out 0 to 4, etc, a simple resistive voltage divider on the non-inverting input would fix that, or change the feedback resistor to match the DAC voltage.
 

Sceadwian

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I do have an 8 bit DACport that isn't doing anything that I think has a VREF input.
 

MikeMl

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Sceadwian

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It's an AD558JN, looks like I can select a 2.56 volt output range, not sure if I can tweak that didn't read that far into the PDF.
 

MikeMl

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Place to start.
 

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Roff

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You can also use PWM with a lowpass filter for your DAC if you don't need fast response time.
 

Hero999

Banned
Does it need to be an SMPS or will linear do?

What's the maximum open circuit voltage?

Do you have any special requirements for transient response or ripple?
 

Sceadwian

Banned
Linear is fine, Mike's circuit pretty much sums up what I'm looking for, an op amp fed from a DAC or PWM source using a current sense resistor from the fet. I have a few 1 ohm resistors and an ompamp that iwll give me more than enough voltage. 0-12 maybe though I'd be intersted in any additional circuits you'd like to post. The FET I have can handle 20+ watts and I have heatsinks so at 12-20 volts 1amp even under short circuit conditions isn't gonna hurt anything. I've done a similar circuit to mikes using a POT instead of a DAC, but I was looking for any other methods people can post.
 
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Roff

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Linear is fine, Mike's circuit pretty much sums up what I'm looking for, an op amp fed from a DAC or PWM source using a current sense resistor from the fet. I have a few 1 ohm resistors and an ompamp that iwll give me more than enough voltage. 0-12 maybe though I'd be intersted in any additional circuits you'd like to post. The FET I have can handle 20+ watts and I have heatsinks so at 12-20 volts 1amp even under short circuit conditions isn't gonna hurt anything. I've done a similar circuit to mikes using a POT instead of a DAC, but I was looking for any other methods people can post.
You need an op amp whose input common mode range includes ground. Also, Mike's circuit is a sink, not a source, but apparently that's what you want.
 

chaitu_b99

New Member
hi

Hi my project is voice rocognizeg car iam using HM2007,please tell me how the voice are stored and which form they are storerd in SRAM,please reply
 

Sceadwian

Banned
chaitu, how dare you try to hijack my thread! =) How about trying to read the documentation for your chip?

Current sink is fine, There's a bit of resistance to ground on the sink but I don't think it'll hurt anything I'd be doing with it. It's just a generally adjustable current sink (corrected now thanks =>) for manually charging batteries testing LED's or any other simple constant current uses. I'm probably going to use an ATX power supply as soon as I mod a new one so the opamp will have a -12 supply if I need it to.
 

ericgibbs

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Hi my project is voice rocognizeg car iam using HM2007,please tell me how the voice are stored and which form they are storerd in SRAM,please reply
Please start your own new thread.

 

MikeMl

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Sceadwian

Banned
I just rumaged through my bits box. I have an LM358.
<two minutes later>
Apparently I have more than one 358N because I just checked the breadboard I left the pot controlled current source on that's using one as well. I think I have a few more floating around I got them from an analog IC pack I bought ages ago. Bunch of random stuff in there regulators opamps and comparators or what not. So the only difference between the circuit Mike made and the one on the breadboard is I'm using a POT instead of a DAC and my fet is an IRF510. It was my first analog circuit =)

One thing, if this circuit is completely unloaded what happens when I connect it to say an LED? Will the 358 be able to drop the voltage in the FET fast enough to keep an initial current spike from going through? If I use a 12 volt supply that's gonna fry a 3.1v LED in a hurry if it can't keep up. I think I've heard that it sometimes takes time for an opamp to come out of saturation? Am I making sense here?
 
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Hero999

Banned
If you're worried about current surges then stick a choke in series with the load.
 

Sceadwian

Banned
<smacks forehead>
I could also simulate it and see... God I'm dull sometimes.
 
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