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Constant current mode for DIY power supply
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T3sl4co1l:
Oh hey, I do have an example handy!



You'd flip this low-side sink upside down (NPN --> PNP, N-ch --> P-ch, flip diodes, flip -V for +V supply) for the high side source, but otherwise it shows the "ring of two".

Q1's source current (can also be a BJT with basically no changes) is set by Q2's Vbe, so that:
Is = Vbe / R(R5)
Although, it's a bit more than that, because of the R3+(R4+R6) voltage divider, which you don't usually see in this circuit.  R4 is an NTC thermistor (B ~ 3000) which compensates for Q2's voltage coefficient, keeping current stable as temperature rises (whereas otherwise it falls with rising temp, which really isn't too bad for a protection circuit as is).

In general, R8 and C2 values will change with component choice, so just play around with that when you drop in other components.  The playing is driven by the step response -- note the pulse source on the right for simulating that, as well as L1 which represents wiring inductance (about a meter's worth of cable) and R7+C1 (which dampens L1 and the CCS).  (Ignore R1, it's just there to measure drain current in the simulation.)  And, R2 also depends on devices; in this case it was relatively low to get reasonable response time into Q1's gate capacitance, but it can be set based on hFE(sat) when using a BJT for Q1.

Tim
The Soulman:
I'd use a second lm317 for current limiting after the voltage regulation.
The datasheet likely will have examples, but not much is needed.
You can either use a pot to make it adjustable or a multi position switch to switch between predetermined settings,
such as 10mA, 50mA, 100mA and 500mA.
The lower amperage's also come in handy for led testing, the higher also for measuring low ohm values in combination
with a voltmeter, handy for testing switches and connections.
 
not1xor1:

--- Quote from: The Soulman on February 19, 2019, 12:14:30 pm ---I'd use a second lm317 for current limiting after the voltage regulation.

--- End quote ---

you can't put it after
current limit must precede voltage regulation in case of LM317 voltage regulator and BTW another LM317 would add a 1.25 + 1.5 or more volts of dropout
aiq25:

--- Quote from: The Soulman on February 19, 2019, 12:14:30 pm ---I'd use a second lm317 for current limiting after the voltage regulation.
The datasheet likely will have examples, but not much is needed.
You can either use a pot to make it adjustable or a multi position switch to switch between predetermined settings,
such as 10mA, 50mA, 100mA and 500mA.
The lower amperage's also come in handy for led testing, the higher also for measuring low ohm values in combination
with a voltmeter, handy for testing switches and connections.

--- End quote ---
I might try to make it with this but I also wanted to learn by using a different approach. Still not decided on which way to go though.
German_EE:

--- Quote from: not1xor1 on February 20, 2019, 05:56:52 am ---
--- Quote from: The Soulman on February 19, 2019, 12:14:30 pm ---I'd use a second lm317 for current limiting after the voltage regulation.

--- End quote ---

you can't put it after
current limit must precede voltage regulation in case of LM317 voltage regulator and BTW another LM317 would add a 1.25 + 1.5 or more volts of dropout

--- End quote ---

Why should this be the case? In its simplest form a fuse is a current limiter and if you exceed the rated current the supply will 'disconnect'. It should be possible to build some form of electronic circuit that will disconnect the output of a regulated supply at a specific current draw and put this after the voltage regulation.
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