Device PWM output protection circuit - Page 1
Forum section: electronics / beginners

author: Miha
author_rank: Contributor
timestamp: January 26, 2023, 08:50:01 am
content: Hello.I need to make protective PWM device output.Protection from:- direct power connection 0..20V- direct groundingMy schematic in the picture below.I ask for critic.Also thoughts on how to reduce the cost of the circuit.

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 28, 2023, 01:29:09 am
content: Quote from: Miha on January 26, 2023, 08:50:01 amWell, it's already protected against the output being shorted to ground.Depending on what is connected to PWM_OUT, you could add a resistor in series with the fuse (FU1) to limit the current if +20V was placed on the PWM_OUT. Also replace VD5 with a 12V zener diode. That would protect VT1 and the added zener from over current in that scenario. Resistor depends on what your driving with the PWM. Zener wattage rating depends on the added resistor value.

author: Miha
author_rank: Contributor
timestamp: January 29, 2023, 04:11:33 pm
content: Thanks for answer.I think that the fuse can limit current and save transistor.Not necessary to use zener, because VD4 (in power stage) will limit voltage near 20V.Another thing against the resistor - I will need (with a 100 ohm resistor) its power about 4 watts.This is a rather expensive and large resistor. Larger and more expensive than a resettable fuse.

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 29, 2023, 08:38:12 pm
content: QuoteI doubt it. The BC817 is rated for a max IC of 0.5A and a max Pd of 0.46Watts. If 20V is applied to the output, then the voltage at UZP_OUT will be around 19.3V resulting in a base current of 1.86 ma into VT1 when VT2 is off. Datasheet for BC817 puts beta at 250-600 for an IC of 100ma so the collector current will begreaterthan 100ma... But at only 100ma, the poor little BC817 will be dissipating 2 watts with 20V across it. Beta drops with increased collector current, so it's hard to say exactly how much current will flow, but we know that the MF-NSMF012 fuse can pass up to 1A for 0.2 of a second before opening... I think the BC817 will die before the PTC fuse saves it.... I would experiment to see what happens.

author: Miha
author_rank: Contributor
timestamp: January 30, 2023, 03:49:45 pm
content: Now I completely agree that there is an issue of uncertainty in the circuit and I need to either test or improve the design.Thanks for the advice!

author: Zero999
author_rank: Super Contributor
timestamp: January 30, 2023, 11:08:29 pm
content: What frequency?How many channels?Is it acceptable if the output voltage is a little below 20V?How about the CD4504? It has six channels. It's only specified to run off 18V, but it has an absolute maximum rating of 20V, so put current limiter circuit in the power supply, which will also drop the voltage a bit.https://www.ti.com/lit/ds/symlink/cd4504b.pdf

author: Miha
author_rank: Contributor
timestamp: January 31, 2023, 06:12:32 am
content: Quote from: Zero999 on January 30, 2023, 11:08:29 pmOnly one channel and no more than 3 kHz (1..99% PWM - 3 us minimum pulse width).Yes. It was CD4049UBD in this place, but it relatively large and not so cheap. And require same (or better) output protection circuit. So we need to change.