what is this transistor doing  (circuit attached) - Page 1
Forum section: electronics / beginners

author: mr ed
author_rank: Regular Contributor
timestamp: February 03, 2024, 02:34:05 am
content: circuit is charger regulator for lead acid battery. I think red part its temp compensation. If it is, i only understand base emitter voltage drops with temp.battery is 8v. circuit measured 5v at pin 1 on reg reference input.

author: zilp
author_rank: Frequent Contributor
timestamp: February 03, 2024, 03:39:49 am
content: Quote from: mr ed on February 03, 2024, 02:34:05 amThat's a shunt voltage regulator. Think of it as a voltage divider made up of R999 and R927 in parallel with Q906, where, if the voltage across R927 reaches ~ 0.6 V, Q906 starts conducting and thus prevents the voltage from rising any further.Effectively, that simply makes the voltage at the emitter a few volts lower than at the collector (could be calculated from the resistor values, but R999 isn't quite readable), or just (close to) zero if the input voltage is lower than the drop.

author: mr ed
author_rank: Regular Contributor
timestamp: February 03, 2024, 04:04:20 am
content: its one meg on top. resistance is high, all uA. about 2uA. This is deep into the knee area of B-E junction? The battery voltage goes through this stack of resistors and the transistor regulator. if the regulator action was any good, it would not regulate the charge voltage.Now i think its a regulator whos primary purpose is temp comp.But, the lead acid battery float voltage goes down with increasing temp, this seems to increase it.

author: mr ed
author_rank: Regular Contributor
timestamp: February 03, 2024, 04:28:44 am
content: I have it backwards. Transistor current increases with temp. So the hot transistor will raise the reg. chip pin 1 chip feedback input, and the regulator will compensate batt output downward to keep the battery happy at a lower float. Makes sense?

author: zilp
author_rank: Frequent Contributor
timestamp: February 03, 2024, 04:57:54 am
content: Quote from: mr ed on February 03, 2024, 04:04:20 amI'm not sure what you mean?!Haha, sorry, I must have misread this ... well, yeah, it's probably not far above it, though no idea how deep really?! Certainly doesn't make for a particularly stiff regulator, that's true.Quote from: mr ed on February 03, 2024, 04:04:20 amNo, it doesn't regulate charge voltage, it regulates the voltage drop between collector and emitter.Quote from: mr ed on February 03, 2024, 04:04:20 amI mean, I haven't really thought much about what the purpose of that regulator is in that circuit, and it's kinda hard to know given that we can only see a small part of the circuit anyway, but my first guess certainly would be more in the direction of "limit voltage to acceptable levels for the input".

author: pqass
author_rank: Super Contributor
timestamp: February 03, 2024, 06:03:41 am
content: That's aVbe Multiplier(a.k.a. Rubber Diode).It behaves like a diode with selectable (via resistors) forward voltage drop ("shunt regulator" in an earlier reply). SeeW2AEWvideo #198.pin 1 should be = (BATT-Vbe drop)*(Rlower*(Rupper+Rlower))

author: srb1954
author_rank: Super Contributor
timestamp: February 03, 2024, 07:09:27 am
content: Quote from: mr ed on February 03, 2024, 04:04:20 amCorrect. It is a Vbe multiplier circuit. With the resistor values shown the voltage drop across the transistor is approximately 5xVbe and the temperature coefficient of that voltage drop is approximately 5 x -2mV/℃ = -10mV/℃.With lead-acid cells is is recommended to decrease the float charging voltage by -2.3mV/℃ for optimum life so, with the -10mV/℃ compensation from the circuit, it is about right for a 4-cell battery or an 8V battery pack.QuoteNo it doesn't. The reducing voltage drop from the Vbe multiplier in the upper leg of the battery sense divider causes the regulator control input to see an increasing voltage and consequently, through feedback action, turns down the regulator output voltage to compensate.

author: mr ed
author_rank: Regular Contributor
timestamp: February 03, 2024, 10:13:58 am
content: "Vbe multiplier" with a gain of 5. Without a name I had no luck finding any description.https://youtu.be/Obh_PIC2qqo?si=XyBOou29l614Jdb0Thanks all.