BJT controlled relay repair - comfy chair

[plexus]

Hi. My dad bought an expensive reclining chair that has numerous vibration motors in it. the issue if the motor in the head rest. the chair uses a detachable wired remote with an 8 pin DIN. he was working with the remote and noticed that the head motor will not turn off. in the remote is a simple board with a anonymous µcontroller, with control pins that drive a resistor and a BJT. the BJT is wired out to the DIN with a diode across it on the board. as I suspected the BJT for the head motor was shorted. I am surmising they are using electromechanical relays in the chair to control the motors, although I can't confirm that as I can not see the motors or any circuitry in the chair due to the way the chair is constructed.

I can replace the BJT which is an ss8050 PNP 1.5W. I was wondering about using, perhaps, a MOV or some other better component to protect against back EMF - I am speculating this is what may have taken out the BJT. the diode on the board is ok.

What are you thoughts and any suggestions on ways to beef this circuit up so my dad doesn't have more problems like this?

thanks!

[james_s]

I would just replace the transistor, a diode across the relay coil is a very common protection method, sometimes parts just fail.

[plexus]

I hear ya. That's the plan, to replace the transistor. In terms of using a diode across a relay to catch back-EMF, if there is some length of wire between the switching transistor and the relay, does it matter if the diode is on the transistor end of the remote vs on the relay?

[james_s]

Oh, there isn't a diode already? I usually put them as close to the relay as possible.

[plexus]

Oh, there isn't a diode already? I usually put them as close to the relay as possible.

There is a diode in the design already however it's in the remote on the trasistor end. I am thinking about modifications to the design to make it more reliable and wondered if there was a reason to have it closer or on the relay as I am typically seen.

[james_s]

It might help to reduce EMI but in practice I doubt it makes all that much difference. I would say replace the transistor for now and then if it fails again look at modifying the design. It's quite likely just a marginal transistor that was of low quality to begin with.

[plexus]

It might help to reduce EMI but in practice I doubt it makes all that much difference. I would say replace the transistor for now and then if it fails again look at modifying the design. It's quite likely just a marginal transistor that was of low quality to begin with.

That is a good point. this is a cheap chinese product through and through. I ordered some replacement transistors and I think I will replace them all with the new ones which are name branded from digi-key. thanks for the idea.