Power supply current limiting

[lti]

Re: Power supply current limiting

I measured 0.42V on the base. With the output set to 18V, the base voltage increases to 0.6V, but no voltage is dropped across R4. Before I made post 33, it seemed to work properly and the voltage at the base of Q1 was 0V until the current limit was reached.

[budm]

Re: Power supply current limiting

Looking at Video part 2 (@32:37 and on) and part 3 sch @ 00:56 and compared to your circuit (post 8) is not the same, you built your own LT3080 which I am not even sure if it will work like the LT or not.
As you can see in the SCH, the set pin has two 1K resistor with transistor connected in the junction of the two resistors and operate in linear mode (13:35 part 3, those two resistors are also part of the V set feedback circuit.
Right now that Q1 is in either on or off as instead of in linear mode to put the circuit into the constant current mode and maintains the set current.
Your Q1 act like shutdown, see page 17, also see typical application circuits.

[rievax_60]

Re: Power supply current limiting

Here is the updated schematic.

I only had a single-output transformer with an output of 18V, so I used another transformer for the negative supply. Might need more bulk capacitance also. (I have two 4700uF caps for that, not sure if it is enough though!)

The pass transistor is one I had on hand. I don't have any 2N3055's, but I think those should work fine also. It needs to be put on a decently sized heatsink, and with some thermal paste too. (Maybe a fan too, that depends on heatsink size, ambient air temp and flow)

The LM4040-25 is a 2.5V voltage reference.

I set the circuit up to do 0-18V, and 0-1A.
You might get higher or lower max V/A output readings because of component tolerances. (Also the max output voltage is actually 19.5V (Close enough to 18V xD).)

Be sure to trim the current sense null/trim.

You could try using other op-amps too, maybe like an LM324.

Note: Whoops, looks like I have two R5s and two R9s... the program I used doesn't automatically number the components, so I must have missed those after copying and pasting... (However, the resistances are correct)

Any comments/ideas, from the more experienced engineers here?

A capacitor input voltage doubler could have been used to produce the negative rail instead of the extra transformer.

[rievax_60]

Re: Power supply current limiting

I think that current limiting circuitry will be easier to design with the shunt in the ground rail. There will be no longer need for precession resistors to achieve high CMRR.

[ben7]

Re: Power supply current limiting

I measured 0.42V on the base. With the output set to 18V, the base voltage increases to 0.6V, but no voltage is dropped across R4. Before I made post 33, it seemed to work properly and the voltage at the base of Q1 was 0V until the current limit was reached.

Could you make a diagram of the circuit you have now? (Since it has changed ... or has it?)

[lti]

Re: Power supply current limiting

Looking at Video part 2 (@32:37 and on) and part 3 sch @ 00:56 and compared to your circuit (post 8) is not the same, you built your own LT3080 which I am not even sure if it will work like the LT or not.
As you can see in the SCH, the set pin has two 1K resistor with transistor connected in the junction of the two resistors and operate in linear mode (13:35 part 3, those two resistors are also part of the V set feedback circuit.
Right now that Q1 is in either on or off as instead of in linear mode to put the circuit into the constant current mode and maintains the set current.
Your Q1 act like shutdown, see page 17, also see typical application circuits.

I wasn't trying to copy the circuit of any voltage regulator. I don't see how to get Q1 into linear mode.

Could you make a diagram of the circuit you have now? (Since it has changed ... or has it?)

It's a even bigger mess than the last circuit.

V4 is actually the 9V regulator, while V2 and V3 are potentiometers connected to the output of the regulator. I will eventually have something connected to Q2 to turn on that LED. D6 is red and D7 is green.

If I put the shunt in series with ground, I could get rid of the second reference, but the output voltage would drop slightly as the load increases.

[lti]

Re: Power supply current limiting

For some reason, swapping the inputs of U3 and replacing the transistor with a diode causes it to stop oscillating. However, the voltage will only go down to about 1.1V in constant current mode in LTspice, so that isn't a very good solution.

[Agent24]

Re: Power supply current limiting

Maybe you can get some ideas from this design:

http://archive.siliconchip.com.au/cm...4/article.html
http://archive.siliconchip.com.au/cm...0&a=112014&i=3