EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: ledtester on September 14, 2020, 06:40:48 am
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So there's this "LM317 Kit Build Speed Run" contest going on on Youtube. The goal is to clock the fastest build time of a simple LM317 power supply kit.
Here's the schematic for this particular kit (from http://www.icstation.com/lm317-adjustable-step-down-module-125v-acdc-power-supply-module-p-2713.html (http://www.icstation.com/lm317-adjustable-step-down-module-125v-acdc-power-supply-module-p-2713.html) )
[attach=1]
I was wondering about the role of Q1. It seems that during normal operation it wouldn't be on. My guess is that it is used to discharge C5 when the transformer power is disconnected. Comparing it to the reference implementation in the LM317 datasheet, it seems it would serve the same role as the protection diode D2. If so, does it offer any advantages over the D2 diode approach?
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Its base is shorted to the emitter by the pot. You can fiddle with the transformer all day long and it won't discharge anything.
It discharges C5 when the pot loses connection.
IIRC, diode D2 may or may not be required, depending on output voltage and capacitance. See the datasheet for details.
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First of all C4 is way too big and is killing the regulation... I would use a 100uF (max 220uF) for C4.
Let's say the adj pin of the regulator is wired to a potentiometer and to the ground.
When there is a break between the ground and potentiometer (that could happen on some potentiometers at the ends) the load will see ~(Vin-3) volts which is something not desirable.
Q1 will prevent that situation to occur, e.g. wiper is disconnected then Q1 turns on and Vout will be reduced to ~ 2.1 volts only.
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It discharges C5 when the pot loses connection.
Exactly but it does more than that. Q1 pulls the regulator's adjustment pin low if the wiper on the poteniometer fails so that the output voltage falls to a minimum, about 1.25 volts + 1*Vbe or 2 volts, instead of rising to maximum which could damage the load. It protects against an open wiper which is the most common failure of a potentiometer.