Is this schematic a linear regulator? - Page 1
Forum section: electronics / projects

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: March 30, 2025, 06:32:59 am
content: This is a motor speed "resistor" for an AC blower fan in a Kia.Sorry for the weird layout.It has two connections that go back to the Climate Control module.I had assumed they were PWM controlling the fan, but now I'm not so sure.Thanks!EDIT: R3 is just the motor fan, drawn as a place holder. G, D and S are the labelled pins of the connector that plugs into this module.

author: Xena E
author_rank: Frequent Contributor
timestamp: March 30, 2025, 09:16:56 am
content: Could be the power stage of either.You could scope the G connection whilst operating if you want to find out.X

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: March 30, 2025, 09:22:56 am
content: Quote from: Xena E on March 30, 2025, 09:16:56 amThe fan runs at full speed no matter what speed its set too, so scoping it only tells me that the fan is on.

author: Xena E
author_rank: Frequent Contributor
timestamp: March 30, 2025, 09:39:13 am
content: Quote from: AmmoJammo on March 30, 2025, 09:22:56 amSooo sorry, not clairvoyant.In the conjunction that the module you posted the schematic of is disconnected and you scope the AC controller output and try to vary the fan speed what do you get?If it just goes to supply and stays there, it's the controller that is faulty not the module. On the other hand, if its a PWM signal or varying voltage, (less likely its Analog control I'd have thought), the mosfet in the module is dead short.X

author: jwet
author_rank: Frequent Contributor
timestamp: March 31, 2025, 04:27:53 pm
content: It is a PWM driver stage. That is a terrible drawing- I redrew it and it makes more sense.The extra parts do two things- slow down edges (RC) and the diode zener protects the gate. The FET must be shorted and there should be a PWM signal to the G Terminal.

author: tooki
author_rank: Super Contributor
timestamp: March 31, 2025, 06:40:39 pm
content: Quote from: jwet on March 31, 2025, 04:27:53 pmAssuming OP's schematic is accurate, even if oddly drawn, then your schematic does not match exactly. R1 is in the wrong place (and consequently, so are the lone diode and the cap), and the two series diodes are backwards.

author: tooki
author_rank: Super Contributor
timestamp: March 31, 2025, 07:06:01 pm
content: I took a quick stab at redrawing in LTspice (where I don't have motor and fuse symbols handy, unfortunately, so I've pressed a load and resistor into service). Hopefully I didn't make any mistakes myself!The back-to-back diodes D1 and D2 don't make much sense where they are. OP, is it possible that these diodes actually go across the motor? (So between +14V and D.)

author: jwet
author_rank: Frequent Contributor
timestamp: March 31, 2025, 08:23:33 pm
content: Your schematic is more correct- thanks. All the diodes, I think are a gate clamp, but only the OP can tell, single one to keep the gate from going low, the two diodes to keep the gate below the zener. I was just trying to make some sense out of it.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: March 31, 2025, 09:08:21 pm
content: To the TS question: No, It doesn't seem that this is a "linear" regulator.In this time and age, I would venture that 99+% of all DC motor speed controllers are PWM.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: April 01, 2025, 08:08:33 am
content: Thanks for the input guys! I'll have another look at the PCB, my schematic was mostly drawn the way it was, as that's how the board is laid out.All traces are on the top except one that goes from the other end of the capacitor to the drain of the mosfet.The fuse is a thermal fuse that's coupled to the heatsink.The mosfet does not appear faulty, I've bench tested the whole device, and supplying 5v to the gate appears to fully switch the fet on, supplying a lower voltage however, and I can measure a proportionally lower voltage on the output.I shall now attempt to redraw the schematic!

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: April 01, 2025, 08:36:27 am
content: Quote from: tooki on March 31, 2025, 07:06:01 pmI redrew it from the board, then came back and looked at your schematic. SURPRISE!