I have a faulty solenoid valve from a hydraulic levelling system in an 1988 US motorhome. I sourced KT825G BDX66 T03 transistors to repair. I’ve been having issues with this in that when the return spring is fitted the plunger doesn’t move, without the spring it does. I recently noticed that the transistor is rated at 70V, not the 12V I was expecting. Could anyone tell me how that might affect the operation and/or if that might have caused the problem I have now?

Thanks!

 

A 12V battery on charge is 13.2V+ so a 12V transistor would not be a good idea. The 70V is the maximum voltage which the transistor can withstand but it could be used with any voltage down to less than 1V! It is wise to use components well within their maximum ratings. They last longer that way.

I don't think the rating of the transistor is cause of the spring/plunger problem.

 

I recently noticed that the transistor is rated at 70V, not the 12V I was expecting. Could anyone tell me how that might affect the operation and/or if that might have caused the problem I have now?

That rating specifies the maximum voltage the transistor will withstand, not the voltage at which it must be used.

 

It sounds like the solenoid is not powerful enough to overcome the power of the spring. I would try and isolate the solenoid, and then see if it works if you connect 12V directly to it, if it does you are dropping too much voltage in the switching circuit (maybe use a MOSFET instead of a BJT transistor, if not you need to try and replace the solenoid or get more oomph out of it! One possibility is to take some of the turns of wire off the solenoid, but this would be a desperate step perhaps!

 

All other things being equal (they never truly are, but ... ), a transistor with a higher voltage rating will have a *slightly* higher saturation voltage. It should not be enough to affect the operation of your system.

What is the part number of the original transistor? If it has higher gain (maybe a Darlington?), that could cause the symptoms you are seeing.

ak

 

Related to this ?
https://forum.allaboutcircuits.com/threads/12v-dc-solenoid-valve-has-blown-diode.143115/

 

A 12V battery on charge is 13.2V+ so a 12V transistor would not be a good idea. The 70V is the maximum voltage which the transistor can withstand but it could be used with any voltage down to less than 1V! It is wise to use components well within their maximum ratings. They last longer that way.

I don't think the rating of the transistor is cause of the spring/plunger problem.

Thanks AlbertHall, that's cleared that up for me

That rating specifies the maximum voltage the transistor will withstand, not the voltage at which it must be used.

Thanks OBW0549

It sounds like the solenoid is not powerful enough to overcome the power of the spring. I would try and isolate the solenoid, and then see if it works if you connect 12V directly to it, if it does you are dropping too much voltage in the switching circuit (maybe use a MOSFET instead of a BJT transistor, if not you need to try and replace the solenoid or get more oomph out of it! One possibility is to take some of the turns of wire off the solenoid, but this would be a desperate step perhaps!

Its the same solenoid/spring combination as has been working for 30 years. But what you've said has made me think it could be a mechanical issue so thank you.

All other things being equal (they never truly are, but ... ), a transistor with a higher voltage rating will have a *slightly* higher saturation voltage. It should not be enough to affect the operation of your system.

What is the part number of the original transistor? If it has higher gain (maybe a Darlington?), that could cause the symptoms you are seeing.

ak

Thanks ak. It's KT825G BDX66 but I am leaning towards other solutions after having read previous replies.

Related to this ?
https://forum.allaboutcircuits.com/threads/12v-dc-solenoid-valve-has-blown-diode.143115/

Yes mvas! All the help I got from this forum allowed a successful repair. Why it stopped working again I really don't know. I suspect a short somewhere, it's a tight physical layout. Thanks

 

Thanks ak. It's KT825G BDX66

I meant the part number of the original device.

ak

 

I meant the part number of the original device.

ak

Thanks ak, I'm going to investigate possible mechanical issues before anything else. Cheers

 

It looks like darlington pnp
possibly doing a manual over ride is a way to gain insight into test point location on the controller.

 

I have a faulty solenoid valve from a hydraulic levelling system in an 1988 US motorhome. I sourced KT825G BDX66 T03 transistors to repair. I’ve been having issues with this in that when the return spring is fitted the plunger doesn’t move, without the spring it does. I recently noticed that the transistor is rated at 70V, not the 12V I was expecting. Could anyone tell me how that might affect the operation and/or if that might have caused the problem I have now?

Thanks!

Transient suppressor devices for automotive systems are often rated to clamp at 68V - so your transistor has a smidge of safety margin.

The solenoid also generates a back emf when the transistor switches off - the solenoid is one of the types of components that generates the transients the transistor needs protecting from.