Well, apparently I've been laboring under misconceptions about transistors.
I am working on a variable power supply, and here is the current setup:
LM317 with one fixed resistor for adjustment, and the other a pair of transistors (PN2222) in Darlington Pair formation.
The transistors are actuated by an MCP4251 digital potentiometer producing varying voltages via commands from the Arduino.
I figured that this would work, but apparently I was wrong. No matter what voltage I apply to the transistor pair, the LM317 seems to take it as fully grounded, and sets the voltage to its minimum (~2V).
I expected higher voltages applied to the transistor to equal lower voltages from the LM317, but apparently that's not so.
After that problem, I tried using two pins on the MCP4251 as a variable resistor, and not a voltage divider, to activate the transistor, but I get the same result.
Any advice? I realize this may be hard to follow, so here's a schematic:
Am I wrong in choosing a transistor for this job? What else could I use? I can't use the MCP4251 itself, because its pot terminals aren't rated for more than +/-0.3V of VDD.
Thanks for looking :/.
PS: Yes, I'm using the LM317 correctly, I just don't know why I can't get the transistors to work like I want.
Yea that looks pretty messed up. I guess first I would want a purpose of the circuit. Is it to allow the digital pot to control a 0-5 volt output source?.
"Am I wrong in choosing a transistor for this job?"
Yea, I think so. You would probably have better luck not having the transistors at all and going from the pot directly to the adjust pin on the regulator, but you would have to check up on a application note somewhere on how to control the regulator externally rather then the normal feedback resistor. Transistors are current amplifiers, not direct voltage amplifiers, so they have to be used in circuits using bias and load resistors to generate a voltage output.
What else could I use? I can't use the MCP4251 itself, because its pot terminals aren't rated for more than +/-0.3V of VDD.
Again it's not clear what the overall objective of the circuit is. What result range are you wishing for at the DVM terminals, and how much current capacity do you wish for? The key specification missing is the full voltage range you are looking for from the regulator circuit. With regulators like this there is problems getting lower then around 1.2vdc and your pot has a 5vdc input limit. So the complexity revolves around the range you want, as below 1.2 and above 5volts will require a non-simple circuit.
Thanks for looking :/.
no problem
PS: Yes, I'm using the LM317 correctly, I just don't know why I can't get the transistors to work like I want.
Again if the transistors were working as you want, what would be the result?
Possibly doing some research on available documents, you might find a circuit example that could apply:
;D I literally laughed out loud because that's so darn true!
I'm trying to build a variable power supply cheaply using the variable voltage regulator LM317.
The circuit I showed is pretty stupid looking because I'm trying round-and-about fixes :P. I never really thought about Transistors being current amplifying, but I guess that's true :P.
The range I was looking for was anywhere from about 2-40 volts, so that's why directly using the MCP4251's pots is out of the question. Is there anything that can amplify power in the same manner as a transistor, but with voltage rather than current?
I am with the same question, the only thing to switch between four fixed voltages, so I thought of using 4 transistors and 4 resistors according to its value, you will get a corresponding output. Perhaps also with a mosfet can be done, I have little idea. Any idea finest do my suggestion? Thank you.
Well my advice is that those inexperienced in electronics should not attempt to DESIGN but rather search out for already published circuits that perform the function you are after. Many times an existing circuit can either be used directly or used with only slight modification.
