# Problem with varible power supply

#### Dzoro

Joined Feb 1, 2019
154
i have built this varible power supply but the pot for adjusting the constant current its not working, is that beacose i didnt put the pnp transistor bc557 or something else

Thankyou dzoro

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#### crutschow

Joined Mar 14, 2008
24,397
the constant current its not working, is that beacose i didnt put the pnp transistor bc557 or something else
Likely something else.
What's the purpose of T1?
The limit takes effect when the voltage from CL to CS created by the T3 emitter shunt resistor (can't make out its reference no.) due to the output current equals about 0.65V.
What's the value that you used for that resistor?
What current limit do you want?

#### AlbertHall

Joined Jun 4, 2014
9,328
is that beacose i didnt put the pnp transistor bc557
Do you mean that you didn't fit a transistor in the position for a PNP?

When you say it isn't working do you get voltage but there is no limit on the current?

#### Dzoro

Joined Feb 1, 2019
154
Likely something else.
What's the purpose of T1?
The limit takes effect when the voltage from CL to CS created by the T3 emitter shunt resistor (can't make out its reference no.) due to the output current equals about 0.65V.
What's the value that you used for that resistor?
What current limit do you want?
I used 0R33 ohms just like in the schematic but i want the current to be adjustable from the minimum it can go to the maximum it can go

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#### Dzoro

Joined Feb 1, 2019
154
Do you mean that you didn't fit a transistor in the position for a PNP?

When you say it isn't working do you get voltage but there is no limit on the current?
Yes i get varible voltage by turning the pot
Do you mean that you didn't fit a transistor in the position for a PNP?

When you say it isn't working do you get voltage but there is no limit on the current?
Yes i get varible voltage when im turning the 10 k pot but when i turn the 1k pot doesnt work for the current limiting

#### crutschow

Joined Mar 14, 2008
24,397
I used 0R33 ohms just like in the schematic but i want the current to be adjustable from the minimum it can go to the maximum it can go
That pot, as configured, will have little effect on the value of the limit, as you have found.
0.33 ohms will give a limit of about 2A.
What current limit range would you like?
What is the voltage and current rating of the transformer you used?
Even with a modified circuit, the current limit would have a limited range of adjustment, unless you significantly modify the circuit, such as with an added op amp.

#### Dzoro

Joined Feb 1, 2019
154
That pot, as configured, will have little effect on the value of the limit, as you have found.
0.33 ohms will give a limit of about 2A.
What current limit range would you like?
What is the voltage and current rating of the transformer you used?
Even with a modified circuit, the current limit would have a limited range of adjustment, unless you significantly modify the circuit, such as with an added op amp.
I used a smps 20v 5A

Joined Feb 20, 2016
3,323
Why did you not add the transistor etc?
That circuitry around T1 creates a constant current source and so will develop a voltage across P3 to set the current.
Build the circuit as shown and see if it works.

#### Dzoro

Joined Feb 1, 2019
154
Why did you not add the transistor etc?
That circuitry around T1 creates a constant current source and so will develop a voltage across P3 to set the current.
Build the circuit as shown and see if it works.
Okay thanks today when i come from school i will try that and i will give feedback about how it was

#### crutschow

Joined Mar 14, 2008
24,397
That circuitry around T1 creates a constant current source and so will develop a voltage across P3 to set the current.
Ah, thanks for that.
Now I see what T1 does.
It provides a base-emitter voltage bias to the internal current-limit transistor, which varies the current-limit point with the adjustment of pot P3.
Likely somewhat temperature sensitive, but should work.

#### Dzoro

Joined Feb 1, 2019
154
I have put the T1 tranzistor with the led and the resistors but before the minimal voltage vas 1.2 volts now is 13 volts and the current limit is adjustable from 0 to 200 mA at short circuit with a darlington pair of a bd139 for driving the power transistor mjw3281A but doesnt work as expected, but i found a better simpler circuit that i think it should work better than these one hire is the new circuit

#### AlbertHall

Joined Jun 4, 2014
9,328
I don't see how the current limit in the new circuit works. There is nothing to actually sense the output current.
Anybody disagree?

#### crutschow

Joined Mar 14, 2008
24,397
I don't see how the current limit in the new circuit works. There is nothing to actually sense the output current.
It's the 0.33 ohm T3 emitter resistor which generates a voltage sensed by inputs CL and CS.

#### crutschow

Joined Mar 14, 2008
24,397
i found a better simpler circuit that i think it should work better than these one hire is the new circuit
No.
Whoever drew that circuit doesn't understand how the current-limit circuit works, and obviously never built (or simulated) it.
You need a resistor in series with the output for inputs CL and CS to sense the resulting voltage for the limit to work, as your original circuit does..
The limit occurs when the voltage between CL(+) and CS(-) is reaches about 0.65V.

Adding the T1 transistor should not have affected the output voltage, so double-check all your connections.

#### AlbertHall

Joined Jun 4, 2014
9,328
It's the 0.33 ohm T3 emitter resistor which generates a voltage sensed by inputs CL and CS.
I meant in the new circuit - so we agree!

#### crutschow

Joined Mar 14, 2008
24,397
I meant in the new circuit - so we agree!
Sorry, I missed the "new" in your post.

#### crutschow

Joined Mar 14, 2008
24,397
Below is The LTspice simulation of a circuit similar to your first circuit.
The first shows the change in current limit with U3 pot rotation, and the second shows the output voltage change with U2 pot rotation.