diode across solenoid coil

[kenlt30]

I had the diode go bad on my WM hydraulic motor solenoid. All hydraulic functions stopped working. My first thought was to adjust the micro switch (some call it a snap switch) because it has caused problems like this before. Still would not work. Checked switch with meter, found that it would not conduct. Luckily I had a spare, installed it, still didn't work. Read with meter, it wouldn't conduct either. The diode across the solenoid coil was shorted allowing power to go directly through the switch to ground when acuated. Burned up 2 $40 switches.
Does anyone on the forum know the rating of the diode?

[Dave Shepard]

I have a 2008 LT40 Super. I had the solenoid in the fusebox go bad for my hydraulics last week. I had to do some sleuthing to figure what was good, what was bad, and what the heck everything was actually supposed to be doing. :D When I read that diode lead out with the continuity tester, it was 0 one way, and I got a reading of 630 or 640 the other way. I don't know what that is. Ohms, mili-ohms?

[Magicman]

The ironic thing is that the diode is there to protect the microswitch. 


 
You can Google and find replacement microswitches for about $2.50 each.  Just use the existing leads and crimp a terminal on the end.  Electronic parts stores have them too.

[StimW]

The ironic thing is that the diode is there to protect the microswitch. 


 
You can Google and find replacement microswitches for about $2.50 each.  Just use the existing leads and crimp a terminal on the end.  Electronic parts stores have them too.

I bought 25 micro switches on ebay for $13. a few years ago!
The seller agreed to send me half with the 1/4" spade terminals and half with the smaller (metric,DIN ???) terminals.
All a diode does is lets current flow in only one direction. It is basicly a check valve.
With AC it converts to DC (simply stated), with DC it stops back flow.

[DR Buck]

Ok  I hate to point out the mis information, but I will. 

The diode is there to protect the circuitry from voltage spikes caused by the inductance in the selinoid coil.   It has nothing to do with converting AC to DC in this use.   With an ohm meter it should read 0 ohms one direction and with the leads reversed it should be very high the other way.  Close to infinate.   If it is 0 or high  both ways , then it is bad.  It must be measured with one lead disconected from the coil.    You can replace it with almost any silicone diode.  1 amp 1000 volt diodes sell for about .02 each.

Just make sure if you replace it you get the polarity right.  There is a stripe around one end.  Make sure the new one goes in tne same direction.

The diode shorting or opening  will not do anything to the switch.  If the diode shorts, the selinoid will not close.    If the diode opens, you will probably not even know it.

[Magicman]

Ok  I hate to point out the mis information, but I will. 

The diode shorting or opening  will not do anything to the switch.  If the diode shorts, the selinoid will not close.

If the diode shorts, the solenoid will not close because the shorted diode has shunted the solenoid coil windings.  The resulting high current flow will either burn the shorted diode open, burn the microswitch contacts, or kick the breaker.
(*edit*  There is no circuit breaker in that microswitch circuit.)

[DR Buck]

The micro switch contact current rating is much higher than the current rating of the circuit breaker.   The circuit breaker will trip long before the micro switch burns out.

[Magicman]

Thanks Buck.  I am still wondering why/how the OP was trashing microswitches??

[Ga Mtn Man]

The micro switch contact current rating is much higher than the current rating of the circuit breaker.   The circuit breaker will trip long before the micro switch burns out.

What circuit breaker are you referring to? 

[pineywoods]

Since nobody has mentioned it, I'll add my 2 cents. Current through the windings of the solenoid sets up a fairly intense magnetic field. When the current is interupted by the microswitch, the magnetic field collaspes and in doing so produces a high voltage spike across the solenoid windings. Without the diode to absorb that spike, It will cause severe arcing of the switch contacts. Arc = heat, enough to destroy the microswitch contacts. If you regularly burn through microswitches, the diode is probably open and should be replaced...

[Magicman]

I kinda did in Reply #2, but I did not go into detail.  I have not replaced a microswitch since WM added the diode in the circuit several years ago.

Good point Ga Mtn Man.  The only protection in that circuit on my sawmill is the 450 amp fuse.

[LaneC]

I am in no way trying to be disrespectful or come across as being rude in any way, but the current rating on most micro-switch contacts are usually less than the main current rating on a control circuit, circuit breaker or fuse.

[Magicman]

Correct Lane.  The microswitch in this instance operates the one or two "solenoids" that supply power to the hydraulic pump(s).  The only 12 volt supply in the hydraulic control box comes through the power strip, which was my reference to the 450 amp fuse in my sawmill.

On a slightly different subject, the term "solenoid" is not exactly right according to my definition.  :P  A solenoid uses an electrical current to develop a mechanical motion (electromagnet) such as a starter solenoid.  Here, I would define the "solenoid" as a power relay.  It is taking a small current through the microswitch and primary winding to operate a contact and supply the voltage and high current to the hydraulic motor.

I guess that the term really does not matter as long as we understand what is happening.   ;D

Back to the OP.  If the diode was shorted, then the microswitch contacts would suffer.  My Reply #5 regarding a circuit breaker was in error, because there is not one in that circuit.  *I will edit*

[TimRB]

When I read that diode lead out with the continuity tester, it was 0 one way, and I got a reading of 630 or 640 the other way. I don't know what that is. Ohms, mili-ohms?

If your meter has a diode check position on the selector switch, then a good diode will read open in one direction, and in the other direction will read the forward voltage across the diode when it is on.  600-800 millivolts would be typical for a silicon diode. 

Tim

[36 coupe]

I have been in electronics since 1950 .A diode should read about 10 ohms in one direction and infinity in the other.Now a diode that has reverse leakage is trouble.My Ford truck has a diode across the starter solenoid coil that protects the trucks computer from voltage spikes that happen when solenoid coil opens circuit.Diodes are used across relay coils for the same reason.I buy diodes by the hundred ,they do fail.

[Ga Mtn Man]

I have been in electronics since 1950 .A diode should read about 10 ohms in one direction and infinity in the other...

That is true if you're using an analog meter but not for a digital.  If you have a digital meter you need to set it to the diode test mode. 

[kenlt30]

In my case the diode was shorted allowing 12 volts on the blue wire from the micro switch. When the switch was made (closed) it created a direct path to ground through the red wire.. ruined the switch.
Thanks Gail tn man for the pm.

[Ga Mtn Man]

Letting one of the small wire terminal lugs on the solenoid rotate around and touch the +12V terminal will also cause this.  :-[

[rufus204]

This my first post, and I will get around to an intro later.
For two days I have worked on a WM40 super.  With issue of no hydraulics.  Mill was just serviced last week, and running fine, hydraulics just stopped yesterday.  Back and forth with WM tec (great folks) and long story short, the fuse had fried behind the nut terminal post (was hard to see) on the fuse and getting only partial current.
 

[TimRB]

In my case the diode was shorted allowing 12 volts on the blue wire from the micro switch. When the switch was made (closed) it created a direct path to ground through the red wire.. ruined the switch.

So is there really no fuse in the circuit to protect against this problem?  Presuming there's a battery involved here, you're lucky the switch goes before the wiring causes a fire.  Batteries can pump current like there's no tomorrow.

Tim

[Ga Mtn Man]

There is a 225 amp fuse (450 amp for Super Hydro mills) that feeds the power to the hydraulic box.  The microswitch will burn out long before the 225 amp fuse blows.  Perhaps a low current fuse in line with the microswitch wouldn't be a bad idea.

[DR Buck]

You guys are correct about the high current fuse on the hydraulic pump.   I only glanced at the schematic diagram and got confused with the solenoid on the 'head' side of the mill.  That one is on a 15 amp circuit breaker.   I agree, if the diode shorts, all the smoke will come out of the switch.  And, we in the electronics field are taught that once all the smoke comes out, you can't get it back in and will need to replace the part.    :D

The suggestion to put a fuse in line with the switch is a great idea.   The way it is wired, it may be easier to fuse the ground side of the switch.

[LaneC]

I agree tim-rb, it sounds as though they are using the switch for a fuse :D

[Magicman]

Welcome to the Forestry Forum, rufus204. 

Yes Tim, there is the strap fuses in line with the hydraulic power strip and as rufus204 experienced, the open fuse can be very hard to see.  I have actually seen them remake contact enough to measure 12 volts, but would open back up when the hydraulic pump motor was energized.

I have gotten the updated fuse assembly that I will install before I saw again.  It still uses fuses, but not the metallic strap fuses.

Buck, I love your illustration about putting the smoke back in.   

[Ga Mtn Man]

FYI, you can get the Ford brand of those "mega-fuses" on ebay for pretty cheap.