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13.8V 20A power supply-Looking to add adjustable current limit?
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spec:

--- Quote from: Doctorandus_P on January 30, 2019, 05:00:34 am ---You have 3 emitter resistors to stabilize the current through your power transistors.
The voltage over these resistors varies lineairly with the output current.
You can use 3 resistors(100 ohm to 1k or so) to average the voltages over these resistors.
Then you can use this voltage for your current limiting.
The easiest way  is to use an PNP transitor with the Emitter connected to V+ and the base to the common point of these 3 resistors. This transistor wil be opened when there is >600mV over your power resistors.
--- End quote ---
That is a neat idea but the OP wants current limits of 1A to 20A. How would that be realized with your approach?


--- Quote from: Doctorandus_P on January 30, 2019, 05:00:34 am ---But your LM324 probably can not deliver enough base current for your power transistors. Usually the power transistors are used in a darlington or shizalsky configuration.
--- End quote ---
You didn't notice that the output transistors are Darlingtons. :)
Doctorandus_P:

--- Quote from: spec on January 30, 2019, 05:46:26 am ---
--- Quote from: Doctorandus_P on January 30, 2019, 05:00:34 am ---You have 3 emitter resistors to stabilize the current through your power transistors.
The voltage over these resistors varies lineairly with the output current.
You can use 3 resistors(100 ohm to 1k or so) to average the voltages over these resistors.
Then you can use this voltage for your current limiting.
The easiest way  is to use an PNP transitor with the Emitter connected to V+ and the base to the common point of these 3 resistors. This transistor wil be opened when there is >600mV over your power resistors.
--- End quote ---
That is a neat idea but the OP wants current limits of 1A to 20A. How would that be realized with your approach?

Can be done in a multitude of ways.
You can use one of those INA high-side current sensor chips to measure that voltage.
You can build a simple current mirror with 2 transistors.
You can use an opamp in which the inputs work (slightly) above the Power supply rails. LM324 is not fit for this, but another old jelly bean part such as the TL072 can do this easily.
It would also need a potentiometer to compare it to.

A fun (and unusual) circuit to implement the output of the current limiting section is to put the transistor of an optocoupler in series between the bases of the output transistors and the output of the voltage regulator opamp. Normally it is in saturation, but when current limiting is needed the transistor is closed.


--- Quote from: Doctorandus_P on January 30, 2019, 05:00:34 am ---But your LM324 probably can not deliver enough base current for your power transistors. Usually the power transistors are used in a darlington or shizalsky configuration.
--- End quote ---
You didn't notice that the output transistors are Darlingtons. :)

--- End quote ---
If OP is using darlingtons, he should have drawn them. That is what schematics are for.
edavid:

--- Quote from: spec on January 30, 2019, 05:46:26 am ---
--- Quote from: Doctorandus_P on January 30, 2019, 05:00:34 am ---But your LM324 probably can not deliver enough base current for your power transistors. Usually the power transistors are used in a darlington or shizalsky configuration.
--- End quote ---
You didn't notice that the output transistors are Darlingtons. :)

--- End quote ---

The minimum beta of those 2N6051 Darlingtons is 750, so at 20A output the LM324 would have to sink 27mA, which is outside the datasheet worst case spec.

But all this is moot because OP said he is now going to use 2SD111 NPNs, which have a minimum beta of 10  :-//
Xnke:
Fortunately for the confused and irritated, most of this thread is disused now. I was able to trade with a local who has helped me out with getting some parts and a good, proven design, one he uses and has used for 15+ years, and he was able to go through the design with me so I understand it now.

Thanks for the help, and for explaining why the PNP/LDO style has it's pitfalls and snares to watch out for-I will still have to find a use for these 2N6051's at some point!
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