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How to preset current regulation & short circuit protection ? Answered

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Found this awesome diyr Linear power supply design with voltage & current controlling-

Foe more details--http://electronics-diy.com/power-supply-lm317-2n3055-0-28v-6-8a.php

1>Since it has no short circuit protection apart from the fuse,then how to preset current just like bench power supply by shorting the the leads ?

2>Without short circuit protection will the  2 two 2N3055 blow up ?

3>Please assist that what should be done for the modification.

3 Replies

steveastrouk (author)2015-02-23

Like -Max- says, this isn't really a good circuit. You could do with finding something with real current control if you need it.

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-max- (author)2015-02-22

To answer the question of if the transistors will 'blow up' w/ short circuit, well, it's difficult to say. If we were to apply a constant voltage to the base, and attempt to chort the output, forcing it to be zero, while the base is still at 12.6V, the transistor will have hugely excessive current because the PN junction is forward biased. (the connection between the base and emitter looks like a diode) and excessive current will flow through everything.

If the transformer can deliver lots of amps, and the heatsink is not big enough, there is a good chance of letting out the magic smoke from something. So that is not a good idea. However, that variable resistor, p2+, will drop the voltage significantly if that short circuit happens, and the transistor will be turned on, but not quite as hard. Excessive current will still flow if it is a relatively low value, like 100 ohms. Also, if the transformer cannot deliver those amps, then it will be the weak point in the circuit during a short circuit condition and overheat, and possibly burn out, leaving everything else intact.

To properly regulate current, you need to somehow measure the current on the output. It does not really matter where in the circuit you measure the current, though I generally prefer right after the pass element, in this case, the NPN transistors. The most common way to do that is to add a shunt resistor of like 0.1 ohm between the transistor outputs and the actual output going to the load, and measure the voltage drop across it, then use a differential amplifier to amplify that voltage drop across the resistor and make a voltage relative to ground instead of across the high side resistor. Then that can be used with additional processing circuitry to force the transistor off when more than the specified current flows.

Below I have a design that I have been working on and tweaking to get to work. You can precisely set both the voltage and current, and so the voltage will be maintained until the current draw increases to the point that overcurrent protection kicks in and lowers the voltage until only the set max current flows. It is not perfect, and in simulators, it seems to have a terrible parasitic oscillation at really low voltage outputs :(. Haven't made it fo' real yet though.

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-max- (author)2015-02-22

This looks like just a glorified emitter follower regulator. There is no feedback loop to maintain regulation on the output, and the only feedback occurring is a small loop right on the LM317, to give the base of the transistors a constant regulated voltage.

The way this works is simple. When the voltage between the base of the transistors and the emitter is above 0.6V, the transistor is turned ON. When the voltage is lower than that, the transistor is OFF. So because of this, say we make the voltage at the base 12.6V. The voltage at the emitter will rise up and up as it charges a capacitor and current goes through a resistor, and once the voltage is at 12V, the voltage difference between that emitter and base is only 0.6V, so the transistor is beginning to turn off. Thus, a poor and crude regulator is formed. It will do fine for non-critical applications, but do not expect the output voltage to be super stable.


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