[SOLVED] PC fan speed control with transistor and pot - Page 1
Forum section: electronics / beginners

author: Dr_Krieger
author_rank: Contributor
timestamp: August 03, 2014, 03:46:47 pm
content: Usinghttp://pcbheaven.com/circuitpages/images/linearfancontrol_1269972793.pngto start basically; I built the circuit on vero-board and tested it; having just one fan powered, it spins at a slow speed, and turning pot makes no change.My DMM has been dropped one too many times and now displays garbage unless I apply pressure on the LCD, so taking measurements is a pain in the ass, if I had to guess though, the voltage would be around ~2v across Fan+ and ground; the pot is replaced with a 10k variable resistor right now.The fan I tested it with has 4 blue LEDs and they just barely light up.R1 is supposed to be 5k potR2 is 100ohmR3 is 10k ohmC1 is 10uFC2 is 470uFQ1 is supposed to be BD243, I've got a 2SC1384 in it's placeIf I replace Q1 with a transistor I grabbed from a dead CFL (DD128F, can't find a datasheet for it), the fan speed is faster, but same problem of no control. :/It's supposed to be an "emitter follower" setup for current gain, for the fan(s), so why would changing the transistor change the voltage?Any help would be appreciated. :3

author: wraper
author_rank: Unknown
timestamp: August 03, 2014, 03:57:09 pm
content: Suppose either you connected something incorrectly or just connect transistor pins in wrong order

author: Joule Thief
author_rank: Regular Contributor
timestamp: August 03, 2014, 04:00:13 pm
content: Quote from: Dr_Krieger on August 03, 2014, 03:46:47 pmNot to be a wise guy, but if you are going to spend any more time and effort making the circuit work, correct the meter issue first. Other than Google, a DVM is your best friend.

author: David Hess
author_rank: Super Contributor
timestamp: August 03, 2014, 04:19:14 pm
content: That circuit will work but transistor current gain will be a problem unless the voltage divider uses lower value resistors which may explain why swapping the transistor changed the behavior so much.

author: Dr_Krieger
author_rank: Contributor
timestamp: August 03, 2014, 04:25:02 pm
content: I know, I know, I feel horrible for not getting a proper DMM yet, but I'm broke and haven't had a real need to replace it until now.I've got access to an even crappier cen-tech clamp meter that has really low resolution but can at least tell me 1v 2v, etc. :/I'll take a few measurements and post again@David: thanks for the feedback, after I get some measurements, I'll check different resistor values.

author: Dr_Krieger
author_rank: Contributor
timestamp: August 03, 2014, 04:33:06 pm
content: OK, measuring at every point, it's 12v; except across emitter to ground, where it's 2v; regardless of "pot" setting.So, I guess I must have something not connected right?

author: mij59
author_rank: Frequent Contributor
timestamp: August 03, 2014, 04:40:50 pm
content: What's the current of fan.You can't replace a BD243 with a 2SC1384, its will run way to hot.Are you sure the 2SC1384 is o.k. ?

author: David Hess
author_rank: Super Contributor
timestamp: August 03, 2014, 04:42:48 pm
content: If it is 12 volts from the base to ground and 2 volts from the emitter to ground, then the transistor has failed.

author: Dr_Krieger
author_rank: Contributor
timestamp: August 03, 2014, 04:53:41 pm
content: Quote from: mij59 on August 03, 2014, 04:40:50 pm0.24A according to the labelNo I'm not sure the transistor is ok; it was taken from a dead ATX power supply.@David: thanks. guess I'll be hunting for another resistor from this power supply

author: mij59
author_rank: Frequent Contributor
timestamp: August 03, 2014, 04:57:40 pm
content: Quote from: Dr_Krieger on August 03, 2014, 04:53:41 pmGet a TO220 transistor and a small heat sink.

author: Dr_Krieger
author_rank: Contributor
timestamp: August 03, 2014, 05:21:58 pm
content: OK, replaced the transistor (using TO220 from CCFL) and I get 12v base to ground, collector to ground, and emitter to ground, and no change with the variable resistor I'm using for pot.(btw, I did in fact have base and collector swapped before and that has been fixed.)

author: rob77
author_rank: Super Contributor
timestamp: August 03, 2014, 06:18:00 pm
content: does the voltage on the base change when you turn the pot ? 12V base voltage is suspicious when considering the schematic you linked.

author: Dr_Krieger
author_rank: Contributor
timestamp: August 03, 2014, 06:56:36 pm
content: Quote from: rob77 on August 03, 2014, 06:18:00 pmNo, voltage doesn't change at base; but on the pot I'm using (replaced the 10k variable resistor with a 20k pot), the voltage changes going to R3. (A on the pot in the schematic), swapping the A and S wires on the pot yields the voltage on the wire going to R3 going from 12v-8v-12v while turning the pot.So, I'm starting to get really confused. :/

author: rob77
author_rank: Super Contributor
timestamp: August 03, 2014, 07:13:17 pm
content: just keep the 2 resistors and the pot - disconnect everything else. you have to have different voltages on "S" when turning the pot. if not, then you wired the pot wrong.

author: Dr_Krieger
author_rank: Contributor
timestamp: August 03, 2014, 07:21:38 pm
content: OK, with the S wire not connected to the pot, and R2 and R3 wires going to A and E pins on the pot, I get variable voltage on S pin between 4-12vIf I touch the wire going to base to the S, voltage goes to 12v.

author: rob77
author_rank: Super Contributor
timestamp: August 03, 2014, 07:33:48 pm
content: then your transistor is blown. even if you would use PNP instead of NPN transistor by mistake, the voltage wouldn't go up to 12V on the base. most probably the collector-base junction is destroyed in your transistor.

author: Dr_Krieger
author_rank: Contributor
timestamp: August 03, 2014, 07:48:24 pm
content: Fuckin' hell; time to hunt for another transistor (or 5 :/) I guess

author: Dr_Krieger
author_rank: Contributor
timestamp: August 03, 2014, 08:24:23 pm
content: Got a beefy 3DD4515 connected in and it's working like a charm.