LM317T and BDW93C circuit not regulating voltage. - Page 1
Forum section: electronics / repair

author: Abril_Azcarate
author_rank: Newbie
timestamp: September 14, 2020, 09:53:34 pm
content: Hey guys, so I have the following circuit belonging to a marathon ESCORT-III from Saeyang microtech.This is the original circuit as transcripted from the board that comes with the thing. My problem is that no matter the position of the potientometer, the output of the bdw93 is high, like 23V, which tension is both in the emitter and the base. I removed the 3k3 resistor, isolating the LM317 from the bdw93c, with that configuration, the output of the LM317 varies normally, but the bdw still outputs 23V, and the base is also at that voltage, even with the base connected to nothing. I did test the BDW for continuity but it wouldn't seem to be the case. Do you have any ideas of what could be causing this issue?

author: fzabkar
author_rank: Super Contributor
timestamp: September 14, 2020, 11:41:37 pm
content: The transistor is faulty. I can't see any other explanation.

author: Abril_Azcarate
author_rank: Newbie
timestamp: September 15, 2020, 12:02:55 am
content: I did replace the transistor twice, with no effective resultt really.

author: DeepLink
author_rank: Regular Contributor
timestamp: September 15, 2020, 05:32:02 am
content: Not a typical power supply circuit - but there is something wrong with the circuit around the LM317The pot should go to the adjust pin and not to the outputAt least then the out of the LM317 will vary with the setting of the potBut the lack of a bulk capacitor after the rectifier seems odd

author: DeepLink
author_rank: Regular Contributor
timestamp: September 15, 2020, 05:44:05 am
content: Here is what I guess it should look like

author: coromonadalix
author_rank: Super Contributor
timestamp: September 15, 2020, 10:27:19 am
content: some lm 317 applications, it can help understand how it works ?https://www.homemade-circuits.com/lm317-with-outboard-current-boost/

author: Abril_Azcarate
author_rank: Newbie
timestamp: September 15, 2020, 12:18:35 pm
content: Yes, I correct myself, this is the actual circuit I'm working on, I must have made a mistake while drawing it... Hehe. But still, when I remove my. 3k3 resistor, the base still is at 23V and drives the motor at full speed. I tried grounding the base, but I got a spark, the motor shut off and I think I killed the bridge rectifier.

author: Refrigerator
author_rank: Super Contributor
timestamp: September 15, 2020, 02:19:43 pm
content: Quote from: Abril_Azcarate on September 15, 2020, 12:18:35 pmAre you sure it was the base that you shorted out, not any other pin? Which pin did you short to ground?

author: Abril_Azcarate
author_rank: Newbie
timestamp: September 16, 2020, 01:28:17 am
content: I'm quite sure I shorted pin 1 to ground.I did fix this though, it seems I had a bad batch of BDW93Cs so I got an older one and it now works.

author: Refrigerator
author_rank: Super Contributor
timestamp: September 17, 2020, 07:19:40 am
content: Could it have been that you had the BDW94C, which is the PNP version of the BDW93C?In that case it would make sense that i burned out once you shorted the base to ground.