Author Topic: The Power Supply Unit  (Read 4867 times)

Offline Gene

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Re: The Power Supply Unit
« Reply #60 on: March 26, 2024, 08:33:29 PM »
The problem with regulator diodes is that they drift as they warm up. They're NOT diodes. They're a JFET with a resistor connected appropriately. There is no high accuracy, temperature stable reference element in them.

If you want that with using a 2 transistor limiter, replace the feedback transistor with a TL431 shunt voltage regulator. Its a 3 pin, TO92 package.  It will require setting the sense resistor to a value where you develop 2.5V across it at the desired current.

The TL431 acts sort of like a voltage comparator comparing the sense input to an internal 2.5V bandgap reference (rock solid stable, temperature compensated).

Resistor between power and the "cathode" side of the thing. The cathode goes to the base of the main transistor. The anode side goes to ground.  The emitter side of the sense resistor goes to the 3rd terminal (shown as a terminal that comes off the body of the diode in the symbol that represents it).

In this configuration it acts like a voltage comparator. IF the voltage on the sense resistor exceeds 2.5V, current flows from the TL431 anode to ground dropping the voltage being presented to the base of the pass transistor, thereby reducing current to compensate. Conversely, too low a voltage causes less current to flow through it which raises the voltage on the base of the pass transistor and increases current flow.

Yeah, for all of a couple pennies a piece from Aliexpress.

If you download a datasheet for the TL431A (Ti.com, others), there should be an application circuit for a precision constant current sink later in the datasheet. Thats what you want.

Just be warned that the pinout of this thing is NOT the same as the transistor you'd be replacing it with!!!  Believe the TL431A datasheet (wink).

Offline aquataur

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Re: The Power Supply Unit
« Reply #61 on: March 27, 2024, 07:32:35 AM »
Those units tend to create the impression that you throw them at any voltage and they do their thing. Wrong.
As mentioned earlier, any CC circuit has to digest the power it dumps. You easily cook them.

And there is nothing "constant". The datasheet even tells you the current spread.
This "constant" refers to driving LEDs and such, where the term is relatively correct.

Nothing comes without penalty.