Understanding part of a circuit - Page 1
Forum section: electronics / beginners

author: osch
author_rank: Newbie
timestamp: June 19, 2015, 11:25:55 pm
content: Hi! First post here...I have been working on repairing a heating control panel for a car the past weeks. I have reverse-engineered most of the circuit and drawn schematics, but i'm not quite sure what this part of the circuit does, or if my drawing is off.So i have two questions... Why is D14(the zener diode) there, and why is C3 there and switched by T15?After simulating it i still dont know about the zener, but my guess is that C3 has something to do with soft-start or something as its essentially disconnected when the ignition is off?EDIT: I should probaly include that the top part of the circuit seems to be over-voltage protection, and the emitter of T2(the ic voltage rail) is about 10.5V when the battery is at 12v

author: Tandy
author_rank: Frequent Contributor
timestamp: June 20, 2015, 12:04:57 am
content: It looks like an shunt regulator, Take a look at this page.http://www.radio-electronics.com/info/data/semicond/zener-voltage-reference-diodes/applications-circuits.php

author: osch
author_rank: Newbie
timestamp: June 20, 2015, 12:40:10 am
content: Hmm it does, but i still dont get why its feeding that transistor really :/Heres the part of the board im working with. The bottom image is flipped so the pins are in the same locations.EDIT: C1(The orange one) Is tied to the rail as the schematics is showing. It's too late to doublecheck T15 now, but i'm pretty sure there was no traces on top of the board that goes somewhere else, but i will take a look "tomorrow", getting late here.Quote from: Tandy on June 20, 2015, 12:04:57 am

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 20, 2015, 12:48:12 am
content: It is extraordinarily suspicious that there is no DC path for the emitter of T15.I would bet that the schematic reverse-engineering is incorrect.Now if C3 were on the base, and there was some DC path from T15 emitter to ground, then I could believe in the shunt regulator theory.

author: Deathwish
author_rank: Unknown
timestamp: June 20, 2015, 12:55:07 am
content: Overlaying the top on to the bottom picture the capacitor IS on the base of the transisor

author: SeanB
author_rank: Super Contributor
timestamp: June 20, 2015, 07:36:01 am
content: As well Q15 must be a PNP transistor, so this shunt regulator can work. If it is a NPN device the diode and resistor would have to swap places. It basically amplifies the current capability of the diode by the current gain ( up to the transistor device dissipation limits depending on mounting and ambient derating) of the transistor, so you can use a cheap close tolerance low power zener diode to handle a higher current.Not as good regulation wise as a IC regulator, but good enough for a power rail, and can handle large current pulses quite well, with the input side providing a roughly constant current drive and turning off on overvoltage to protect the regulator. Power rail will be around 4V for everything, and will keep it reasonably constant as the engine runs. As the rest is either CMOS or Siemens IIL logic that is perfect for them. CMOS does not care much about supply voltage and the IIL logic has an internal current source generator to run it's internal logic devices.

author: osch
author_rank: Newbie
timestamp: June 20, 2015, 01:30:45 pm
content: Success! Your voltage regulator theory was correct, and there was a path from the T15 emitter hidden under the positive side of C3!(aswell as a little schematic error in the overvoltage part)The zener is also not 3.5, but a 7v one after doing a bit more testing on it. So theres 7v over D14, and emitter of T15 is a 6.5v voltage rail which is used for the opamp comparators throughout the entire circuitThanks for all your pointersOn a sidenote, what current should i set my power supply to when reverse biasing zener diodes to check the value? On D2 i used 10mA and got up to 20v before my supply went to CC-mode, but on D14 it did it at 3.5 so thats the value i wrote down..