Diode on solenoid.

[derekliston]

I need the help of an electrical genius on this one. A few days ago, when I attempted to start my engine and I let the key return from the start position, the starter continued to run. With master switch and magneto switch off the starter continued to run although disengaged from the starter ring. My first thought was that it must be a stuck starter solenoid, however the easiest thing to try first was to remove the diode from the solenoid, so I did and the fault disappeared. To double-check I put the diode back and the problem returned. Removed it again and again the fault disappeared. Ok! Down to Jaycar for some new diodes, fit a new diode and the problem returned. I wondered if I might have mistakenly installed the diode the wrong way round so I reversed it but that didn't help. Currently I have no diode fitted and it is all working fine. What I need to know is what value diode I need to fit? I cannot find any info on that. The ones from Jaycar are marked 1000V 1A which I presume is 1000volts and 1 amp.

 

 

[Blueadventures]

I need the help of an electrical genius on this one. A few days ago, when I attempted to start my engine and I let the key return from the start position, the starter continued to run. With master switch and magneto switch off the starter continued to run although disengaged from the starter ring. My first thought was that it must be a stuck starter solenoid, however the easiest thing to try first was to remove the diode from the solenoid, so I did and the fault disappeared. To double-check I put the diode back and the problem returned. Removed it again and again the fault disappeared. Ok! Down to Jaycar for some new diodes, fit a new diode and the problem returned. I wondered if I might have mistakenly installed the diode the wrong way round so I reversed it but that didn't help. Currently I have no diode fitted and it is all working fine. What I need to know is what value diode I need to fit? I cannot find any info on that. The ones from Jaycar are marked 1000V 1A which I presume is 1000volts and 1 amp.

Hi Jakej gave me some great advice a couple of years ago about correct start circuits and solenoid brand and type for certian applications. Give him a message with your question and I'm sure he may be able to help. Re knowing the starter was engaged do you have a warning light? I fitted the warning light to alert the starter motor is running in my Nynja build; I recommed such. Tonight I'll look up the info Jakej sent me and forward to you in case he is not on the site at present. Cheers

 

 

[derekliston]

Hi Jakej gave me some great advice a couple of years ago about correct start circuits and solenoid brand and type for certian applications. Give him a message with your question and I'm sure he may be able to help. Re knowing the starter was engaged do you have a warning light? I fitted the warning light to alert the starter motor is running in my Nynja build; I recommed such. Tonight I'll look up the info Jakej sent me and forward to you in case he is not on the site at present. Cheers

Thanks for that. Don't really need a warning light, it is quite evident on my Skyview panel that the problem is there. I'd appreciate your info. Going out at the moment but I'll try Jake later, if he has not already seen this by then.

 

 

[cscotthendry]

Derek:

 

The diode is a "snubber" device used to shunt back EMF from the solenoid coil to ground when you release the start switch (or button). The actual value of the diode is not too important and if your diode has failed, try one with a higher PIV and current rating. The 1000V rating that you mentioned is the PIV (Peak Inverse Voltage) rating. All the diode really does is to stop arcing in the start switch (or button) after you release it by shunting the pulse generated by the collapsing magnetic field in the solenoid, back to ground.

 

 

[Ayecapt]

Derek:The diode is a "snubber" device used to shunt back EMF from the solenoid coil to ground when you release the start switch (or button). The actual value of the diode is not too important and if your diode has failed, try one with a higher PIV and current rating. The 1000V rating that you mentioned is the PIV (Peak Inverse Voltage) rating. All the diode really does is to stop arcing in the start switch (or button) after you release it by shunting the pulse generated by the collapsing magnetic field in the solenoid, back to ground.

I have no probs with the explanation above, but i think there is somthing else going on.. the solenoid wont operate if the diode is shorted or fitted the wrong way round. In fact if it were fitted backwards it will be destroyed.

So im sort of thinking that the diode might not have been wired in correctly in the first place. Snubber diods are connected across the solenoid coil terminals and as close as possible to those terminals. Is this how your diode is fitted?

 

 

[Ayecapt]

I need the help of an electrical genius on this one. A few days ago, when I attempted to start my engine and I let the key return from the start position, the starter continued to run. With master switch and magneto switch off the starter continued to run although disengaged from the starter ring. My first thought was that it must be a stuck starter solenoid, however the easiest thing to try first was to remove the diode from the solenoid, so I did and the fault disappeared. To double-check I put the diode back and the problem returned. Removed it again and again the fault disappeared. Ok! Down to Jaycar for some new diodes, fit a new diode and the problem returned. I wondered if I might have mistakenly installed the diode the wrong way round so I reversed it but that didn't help. Currently I have no diode fitted and it is all working fine. What I need to know is what value diode I need to fit? I cannot find any info on that. The ones from Jaycar are marked 1000V 1A which I presume is 1000volts and 1 amp.

the 1000 volts is not so important. The diode is used to limit the size of what is called a back emf ( back voltage if you like) the forward voltage that the diode sees is your battery voltage ( 13 volts or so ) what is important is the speed that the diode can change its state and start conducting the back emf when the solenoid is switched off .Schottky diodes tend to be a better choice , somtimes. called high speed diodes and are available from jaycar aviation lol.

As a guess im thinking your diode has been wired up so that it is across the start button or switch contacts. If it is and if the diode is shorted the solenoid will engage all the time. But without further info its hard to say with certainty what is going on.

 

 

[derekliston]

the 1000 volts is not so important. The diode is used to limit the size of what is called a back emf ( back voltage if you like) the forward voltage that the diode sees is your battery voltage ( 13 volts or so ) what is important is the speed that the diode can change its state and start conducting the back emf when the solenoid is switched off .Schottky diodes tend to be a better choice , somtimes. called high speed diodes and are available from jaycar aviation lol.As a guess im thinking your diode has been wired up so that it is across the start button or switch contacts. If it is and if the diode is shorted the solenoid will engage all the time. But without further info its hard to say with certainty what is going on.

Thanks for all suggestions. The point is that the diode was on the solenoid trouble free for 40+ hrs without a problem then the fault developed. Without a diode the solenoid functions well, that is how I have it at the moment, not ideal long term but for the moment it will do. What I don't understand now is why, even with a new diode fitted, the problem returned!

 

 

[Ayecapt]

Thanks for all suggestions. The point is that the diode was on the solenoid trouble free for 40+ hrs without a problem then the fault developed. Without a diode the solenoid functions well, that is how I have it at the moment, not ideal long term but for the moment it will do. What I don't understand now is why, even with a new diode fitted, the problem returned!

And neither does anyone else... can you take a photo of the solenoid and diode that shows where it was connected, do you own a. Multimeter?

 

 

[derekliston]

And neither does anyone else... can you take a photo of the solenoid and diode that shows where it was connected, do you own a. Multimeter?

I will do that, but can't until tomorrow and yes I do have a multimeter.

 

 

[Ayecapt]

I will do that, but can't until tomorrow and yes I do have a multimeter.

Ok well get some photos . One of your multimeter as well And i can explain how to test your new diode , then we can take it from there. By the way im a retired electronics tech and engineer. So with some patience we should be able to sort it out. What airfield is the plane at ?

 

 

[derekliston]

Ok well get some photos . One of your multimeter as well And i can explain how to test your new diode , then we can take it from there. By the way im a retired electronics tech and engineer. So with some patience we should be able to sort it out. What airfield is the plane at ?

Warwick Qld. I am a retired LAME and did 100% of the airframe, electrical, radio and painting and it was all Ok but I am no electronic wiz and this has me stumped, especially since it was working previously.

 

 

[Ayecapt]

Warwick Qld. I am a retired LAME and did 100% of the airframe, electrical, radio and painting and it was all Ok but I am no electronic wiz and this has me stumped, especially since it was working previously.

Great stuff with your tech background we shouldnt have any probs sussing it out. It doesnt make sense right now but it will in hindsight We should go to private messaging i guess and not clutter up the forum?

 

 

[Pearo]

I need to ask a question here. Elec engineers used diodes on electromechanical devices such as solenoids to prevent back EMF destroying transistors and other sensitive electronics. If its just a plain old switch driving the solenoid then why the diode? You can't destroy a switch with back emf. If you are worried about switch arcing then a proper LC (inductor capacitor) snubber is far better solution.

 

 

[Ayecapt]

I need to ask a question here. Elec engineers used diodes on electromechanical devices such as solenoids to prevent back EMF destroying transistors and other sensitive electronics. If its just a plain old switch driving the solenoid then why the diode? You can't destroy a switch with back emf. If you are worried about switch arcing then a proper LC (inductor capacitor) snubber is far better solution.

Quite correct, not all aircraft systems have flyback protection. This aircraft does and it seems its not working , or at best doesnt behave correctly when its connected. In general though its good practice to clean the electrical system of unwanted electrical noise.

Voltage spikes from the solenoid can destroy sensitive devices , and many such devices exist in our aircraft from voltage regulators to intercomms and radios, a big list of things.

 

Setting aside the technical question, if as a maintainer or a pilot you get a feeling that somthing is just not quite right eg that little flick of a gauge that you have never seen before and it does it again or more often. You gotta at least consider is it a developing fault . Do not ignore the early signs your life depends on it!

 

So i guess chasing a diode issue that may not be completly essential to operating the AC might be thought of as time wasting, its not , in this case i think we will find that the diode was never originally wired correctly and when it failed it engaged the starter solenoid. That is what we are sorting out as to why a replacement diode is not operating in the same way as the old one did.

 

Here is a guess ( no facts yet to suppoert it ) original diode was wired across the switch in reverse, when it should have been across the coil terminals . the back emf from the starter coil eventually zapped it thus engaging the coil full time and starter motor runs. Remove the old diode and all ok. Fitt a new diode but due to inexperience with diodes , its fitted the wrong way round , coil current is carried through the diode and damages the diode, which in turn pulls in the solenoid and runs the motor. We are back to where we started ( no pun intended lol) But lets see what further investigation turns up without facts its all a guess.

 

 

[Ayecapt]

I need to ask a question here. Elec engineers used diodes on electromechanical devices such as solenoids to prevent back EMF destroying transistors and other sensitive electronics. If its just a plain old switch driving the solenoid then why the diode? You can't destroy a switch with back emf. If you are worried about switch arcing then a proper LC (inductor capacitor) snubber is far better solution.

Forgot to mention , back emf actually can contain a lot of enegy ( lenz's law ) this energy eventually disappates as heat eg forms a arc at a switch contact thus repeated concentrated heating at the contact melts the contact surface ..... hence burnt contacts. We have all seen them The back emf is formed due to the system attemping to maintain a steady state , ie keep current flowing in the solenoid coil . Additional LC snubbers were often used before solid state devices were rugged enough to do the job . Diodes do a good job but they must be fitted as close as physically possible to the solenoid which is the source generating the back emf. , any wiring to the diode must be as short a possible .

 

 

[IBob]

All good stuff. Long time ago I was involved in developing dedicated controllers for apple packhouse equipment.

 

Packhouses tend to be electrically noisy (rural, lots of stuff switching on and off) so we built in mains filtering and brownout/powerfail detection shutdown/recovery.

 

We ran simple bench tests on this stuff............and some of the dirtiest power we were able to make was by simply switching solenoids on and off.

 

 

[derekliston]

 

 

 

[derekliston]

[ATTACH=full]51928[/ATTACH] [ATTACH=full]51929[/ATTACH] Tried to pm a photo or two but couldn't for some reason, so here they are!

Sent a pm to Ayecapt explaining solenoid pictures but for other interested parties this is the story. The diode was fitted purely for illustrative purposes and is now removed again. Diode is covered in heat shrink and the red end is the end of the diode with the line on it. Ayecapt's suggestion that the diode may have been wrongly fitted from the start may be correct, I am no electronic ace, but I would have thought that the problem would then have been there from the start.

 

 

[Ayecapt]

[ATTACH=full]51928[/ATTACH] [ATTACH=full]51929[/ATTACH] Tried to pm a photo or two but couldn't for some reason, so here they are!

Which of the 2 bigger cables has 12 volts on it all the time . And the red end of the diode has it got 12 V on it all the time ( both above might be after the master is on )

 

 

[derekliston]

Which of the 2 bigger cables has 12 volts on it all the time . And the red end of the diode has it got 12 V on it all the time ( both above might be after the master is on )

Cable on left is battery to solenoid and so has 12volts, likewise I presume (haven't put a voltmeter on it) that the red end of diode will have 12volts on it. I have a master solenoid for all of the electrics but the starter solenoid has 12V permanently at the left terminal, direct battery feed to starter which is why I figure that if the diode breaks down the current bypasses the solenoid and hence with all switches off, starter continues to turn.

 

 

[Ayecapt]

Cable on left is battery to solenoid and so has 12volts, likewise I presume (haven't put a voltmeter on it) that the red end of diode will have 12volts on it. I have a master solenoid for all of the electrics but the starter solenoid has 12V permanently at the left terminal, direct battery feed to starter which is why I figure that if the diode breaks down the current bypasses the solenoid and hence with all switches off, starter continues to turn.

I doubt the diode would carry starter motor current without melting.

 

The diode should only be connected to the 12v input and to the start switch , it should not be connected in anyway to the starter motor positive cable . I have a diagram here that i cant seem to upload .

 

how do you post jpg pics from a ipad ....grrrrr....

 

 

[Pearo]

Forgot to mention , back emf actually can contain a lot of enegy ( lenz's law ) this energy eventually disappates as heat eg forms a arc at a switch contact thus repeated concentrated heating at the contact melts the contact surface ..... hence burnt contacts. We have all seen them The back emf is formed due to the system attemping to maintain a steady state , ie keep current flowing in the solenoid coil . Additional LC snubbers were often used before solid state devices were rugged enough to do the job . Diodes do a good job but they must be fitted as close as physically possible to the solenoid which is the source generating the back emf. , any wiring to the diode must be as short a possible .

I understand the implications, but there are better are far more effective ways to suppress arcing on switch contacts than a diode.

 

Today I installed a very similar solenoid for an anchor winch in the yacht club boat. Also installed a solar regulator. Everytime I switch the solenoid on it resets the solar controller. Perhaps this is what is/was happening on the aircraft in question? Eitherway, a diode is not the correct fix, a proper snubber is. The snubber can include a diode, but the diode on its own serves no useful purpose unless is driven by a transistor.

 

Oh, and BTW snubbers are still well utilised in modern electronics, especially where these is large instructors!

 

Back to OP, when a diode fails, it almost always fails short circuit, so this explains a lot. IF you want to install a new one, then go for the highest voltage and decent current rating. From jacycar, a 1N5048 is what you want. When installing it, the band on the diode, the stripe or band goes to the positive side of the solenoid. Photo below shows silver band,

 

 

 

 

[derekliston]

I understand the implications, but there are better are far more effective ways to suppress arcing on switch contacts than a diode.Today I installed a very similar solenoid for an anchor winch in the yacht club boat. Also installed a solar regulator. Everytime I switch the solenoid on it resets the solar controller. Perhaps this is what is/was happening on the aircraft in question? Eitherway, a diode is not the correct fix, a proper snubber is. The snubber can include a diode, but the diode on its own serves no useful purpose unless is driven by a transistor.

 

Oh, and BTW snubbers are still well utilised in modern electronics, especially where these is large instructors!

 

Back to OP, when a diode fails, it almost always fails short circuit, so this explains a lot. IF you want to install a new one, then go for the highest voltage and decent current rating. From jacycar, a 1N5048 is what you want. When installing it, the band on the diode, the stripe or band goes to the positive side of the solenoid. Photo below shows silver band,

 

 

That is worth a try so long as my local Jaycar has the requisite part. As I said previously, I am no electronic ace so I really don't know how one acquires a 'snubber' or what it comprises.

 

 

[IBob]

I'm not sure you're right there, Paero. Go here and you'll see any number of circuits with diodes fitted in parallel to solenoid coils:

 

diode on solenoid - Google Search

 

The difficulty with snubbers (which were usually as simple as a resistor in series with a capacitor back when) is that they are a sort of tuned circuit, so you are trying to select the correct component values. We did design them into AC solid state relays to help protect the triacs, but it was always a bit of a guess.

 

I would be going with the diode in this case, unless someone could offer proven component values.

 

 

[Pearo]

I'm not sure you're right there, Paero. Go here and you'll see any number of circuits with diodes fitted in parallel to solenoid coils:diode on solenoid - Google Search

I don't need to do a google search, I have made a living out of this stuff.

 

The difficulty with snubbers (which were usually as simple as a resistor in series with a capacitor back when) is that they are a sort of tuned circuit, so you are trying to select the correct component values. We did design them into AC solid state relays to help protect the triacs, but it was always a bit of a guess.

I would be going with the diode in this case, unless someone could offer proven component values.

I never suggested it was simple, I suggested the current approach is wrong. This is why engineers get paid to do this stuff, and why people on forums don't. And to calculate the correct values does not need to be a guess at all.

 

Anyway, I have said my part, I will hand off to the forum experts.