Final Update: See the final 2 images. Currently running a load of laundry.
Transistor: NPN - 2222A
Relay: Universal Dorman Automotive Relay
Battery: 9V Duracel
The double capacitor volt reduction didn't work. (Image 6 using 10k ohm top and 1k ohm bottom)

Original Post:
What transistor would work with 14.5v @ 1mA to Base, 9V Battery (+) Collector, Emitter to Relay (Prong 85), Relay Prong 86 to 9V Battery (-). See diagram below.

I have a washing machine PCB that is on its last limb. The new machine is on it's way, but still a month away from delivery. The problem is the drain pump won't drain the water. The pump itself is fine. Initially I thought it was the relay that was bad, so I removed it. However after further inspection I discovered that the low level current being sent to the relay is severely low. Whatever component that sends that low voltage (14.5v) signal to the relay is damaged. It still sends 14.5v as expected, but only 1mA (milliAmp). I measured with a multimeter. This is not enough to energize even a small relay. 14.5v x 1mA = 14.5mW I believe that is correct. The spec sheet on the relay says it needs 200mW for its coil to activate the relay, so we are not even close to that with only 14.5mW.

After removing the relay, I soldered on 4 wires to the PCB to where the relay connected.

2 wires for low level current (14.5v) the signal.

2 wires for the high level current (~150v) this is the completion of the positive current for the drain pump.

I've replaced the relay that I removed with an automotive relay. The high level current connects to 30 and 87 just like on the previous relay. For 85 and 86 however, I've wired in a 9V battery and a toggle switch. 9V positive goes to toggle then to 85. 86 goes to 9V negative to complete the circuit. When I turn the toggle switch on it activates the drain pump. Yay!

This is an ok solution for the next month, but I was hoping you could help me identify a transistor that might work in place of the toggle switch. The Base would be powered by the 14.5v 1mA low level current from the PCB. The collector would be attached to the 9V battery positive. The emitter would be attached to 85 on the relay. And finally 86 on the relay would be attached to the negative 9V battery.

As of now, I have to Homer Simpson style manual override the drain pump 3 times per wash cycle. A transistor would be helpful if you think it would work with the given power levels.

Washer Repair: What I have: What I want: Updated Diagram: Top 2 wires are high voltage (Relay prongs 30 & 87) Bottom 2 wires are the low voltage signal to activate the relay (Relay prongs 85 & 86). Updated diagram with a voltage drop. This ended up working: (currently running) Actual Implementation (the toggle switch is no longer connected):

  • \$\begingroup\$ Can you see them now as html img links? Copy paste was not working, it was saying over 2MB, but the images are all 515KB each. Upload was not working, but Imgur could not be contacted. \$\endgroup\$
    – James B
    Aug 19 '20 at 16:37
  • 1
    \$\begingroup\$ Your 9V battery isn't going to last very long driving a relay coil. And the automotive relay isn't rated for switching line voltage. \$\endgroup\$
    – Dave Tweed
    Aug 19 '20 at 16:39
  • \$\begingroup\$ Basically any cheap NPN transistor. You do need to learn how to wire it correctly though. \$\endgroup\$
    – user253751
    Aug 19 '20 at 16:46
  • \$\begingroup\$ Going to try something like this: youtube.com/watch?v=getN1wMYSXY Maybe using this NPN transistor: google.com/… \$\endgroup\$
    – James B
    Aug 19 '20 at 17:03
  • 2
    \$\begingroup\$ In any case, that circuit won't work as there's no return path for the current coming from that 14.5V output. \$\endgroup\$
    – Finbarr
    Aug 19 '20 at 17:22

Using a Mosfet would be a solution since you cannot provide many amps.

Transistors work using current to turn on.

Mosfets use just voltage (and currents like some nAmp or uAmp). Thats why mosftets sometimes get activated when touching them. They are sensitive to anything.

And using a mosfet would also not require a Resistor since you dont want to limit the current it draws. (But i suggest you put one resistor anyway on the gate (aka Base))

Also, you should use a flyback diode as @csabahu recommended, or else you may fry your mosfet or transistor in no time (it may not last a day)


Try it this way, it will probably work: mosogep

  • \$\begingroup\$ Thank you csabahu. I understand the diode additions to protect the components, but this is only to last 1 month so I don't really care about ruining the transistor or relay. I tried a 10k top and 1k bottom resistor and it didn't work. Perhaps it wasn't enough voltage. Anyways, I read the base to emitter voltage in the final diagram as 4.5v so that seems good. \$\endgroup\$
    – James B
    Aug 20 '20 at 20:48
  • \$\begingroup\$ The base emitter voltage is could be max. 0.7V, as long as the transistor is good. If there is 4.5V, it is either badly connected or already faulty. \$\endgroup\$
    – csabahu
    Aug 20 '20 at 21:10
  • \$\begingroup\$ "I don't really care about ruining the transistor" This is not like runining it in years. If you're unlucky enough, it will work 5 times and stop. Do you want to be replacing the transistor every 5 washes? Probably not :) \$\endgroup\$ Jan 25 at 20:42

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