I am looking for a way to take 18V and digitally regulate it between 6 and 16V. My original idea was to use an arduino to control a digital pot in an LM317 regulator circuit but most digital pots I find have a small wiper current. I need at least 0.5A to be safe. Any recommendations on a digital regulator?
I need at least 0.5A to be safe.
You sure of that?
The wiper current is not going to be the output current of the regulator you know.
You can go with 7805 /7809or7812 for regulator IC.
which can carry current 1A.
You can go with 7805 /7809or7812 for regulator IC.
BUT it is not a variable voltage regulator. The OP said:-
I am looking for a way to take 18V and digitally regulate it between 6 and 16V
You can not do that with any of those chips.
Grumpy_Mike:
You sure of that?
The wiper current is not going to be the output current of the regulator you know.
That's a good point. The circuit needs way less than half an amp to operate but the regulator may have a higher rating, causing the wiper amp rating to be even higher than that. I could use a DAC to regulate current and voltage down to spec but am unsure how to go about that.
but the regulator may have a higher rating, causing the wiper amp rating to be even higher than that.
No, I think you are not understanding the circuit.
Please post a schematic of what you are proposing.
Grumpy_Mike:
No, I think you are not understanding the circuit.
Please post a schematic of what you are proposing.
Thanks Grumpy Mike.
That is the idea. But want to control it with a digital pot...
The maximum current the pot will take in that circuit is given by ohms law. It is 16V ( maximum output voltage you want ) divided by the value of R1 which is 240R. This gives a maximum of 66mA.
However, that circuit is very wrong there is no way adjusting that pot will change the output voltage. See the data sheet for the LM317 for the correct circuit. Also review the value of R1.
What digital pot are you thinking of using? Most can not tolerate voltages outside their power rails.
Grumpy_Mike:
The maximum current the pot will take in that circuit is given by ohms law. It is 16V ( maximum output voltage you want ) divided by the value of R1 which is 240R. This gives a maximum of 66mA.However, that circuit is very wrong there is no way adjusting that pot will change the output voltage. See the data sheet for the LM317 for the correct circuit. Also review the value of R1.
What digital pot are you thinking of using? Most can not tolerate voltages outside their power rails.
I am open to any digital pot that is easily compatible with an Arduino. I have a few MCP4241s around. Unsure if I could use them
I posted the same question on this thread and got this response. Can someone verify this?
"the digital pot should work, you just need to get a nice high voltage one. the AD5290 datasheet even shows an example of using the digital pot as a controllable powersupply. the current going out the regulator can be up to 0.5A, but the current through the pot needs to be limited to 1mA or so. if you use a 1k resistor between the output and the adjust pin, that should limit the max current to 1.2mA. with a 10k pot in series with a 4.7k resistor, you would be able to swing the output voltage from 5.9V to 15.9V.
i keep thinking there must be some LED or motor controller chip that does this, but most of them are PWM, so not so great for keeping a nice DC voltage.
building your own high current DAC is also a good way to go, but might be more complicated. you could use the PWM out of the arduino, lowpass it with a resistor and capacitor, and send it to an opamp to be ampified. this could either be an opamp with a high power transistor in the feedback loop, or maybe an LM386 or other speaker driver amp would work."
have a few MCP4241s around. Unsure if I could use them
The data sheet says:-
Voltage on VDD with respect to VSS ............... Voltage on CS, SCK, SDI, SDI/SDO, and
-0.6V to +7.0V
So no.
The AD5290 can handle the voltage but not the current, so you need to feed the output to an emitter follower before you apply it to the regulator.
Grumpy Mike, You're a tremendous help but I am a bit confused
You are saying that I need to put an emitter follower on the output of the pot and connect to that to the junction (according to the corrected image above) of the 240 ohm and ADJ pin of the LM317?
I watched a few videos about emitter followers and found that they try to match the high impedance input to a low impedance output. Correct?
NO .... Replace R1 with an emitter follower.
An emitter follower has a voltage on the emitter that is on the base minibus 0.7V.
Yes it is a low impedance output for a high impedance input. You need a high impedance input to feed your digital pot into to keep the wiper current down.
Grumpy_Mike:
BUT it is not a variable voltage regulator. The OP said:-You can not do that with any of those chips.
So is this wrong?
Based on what I can see when I google "78xx variable voltage regulator circuit", all seem to be some variation of what is described on that page; granted, you can't go below 5 volts with any of them (from what I read). I've never actually tried any of the circuits, but it seems strange to me that so many are saying it's possible to use (abuse?) a 78xx regulator in such a manner...