Solenoid protection diodes

[kevinlms]

22 hours ago, Ian Morgan said:

The main problem with locating the diode by the switch is that the current spike caused by the coil back-emf after switch off will travel the length of the cable from the coil to the switch, which could radiate electro-magnetic noise and affect other, nearby electronics. Having the diode next to coil gives a very short path for this current spike, and less radiated noise.

 

All mechanical switches bounce, during switching on and switching off. The first bounces on switching on will be very short duration, not long enough to fully discharge the capacitor.

 

 

Not entirely sure that the BEMF is what is killing the switches.

 

IMO what is killing the switches is releasing them BEFORE the capacitor has discharged fully. That means that if its trying to switch off the current to the solenoid, or even part of, then its going to exceed its ratings.

 

Assuming the capacitor has discharged a portion of its charge, when the switch is released, its still attempting to switch perhaps a couple of Amps. That its going to way exceed the ratings, of any but a quite large switch on DC.

 

On a typical CDU like this one.

http://www.talkingelectronics.com/projects/CDU-2/CDU-2.html

The diode on the top right is there to prevent back feed to the capacitor. Since the switch(es) are in series with this diode, so therefore the BEMF CANNOT effect the switch.

 

I conclude that diodes across the coils OR switches, are a waste of effort. Making sure the CDU is properly discharged BEFORE releasing the switch is released, is going to be more effective. Holding down the button or probe for longer is the easiest solution. Two seconds?

 

Yes, contact bounce may be an issue, but its nothing to do with BEMF. It's the current breaking attempt, by what is usually a very puny switch.

[spikey]

5 hours ago, AndyID said:

I'm sorry that you seem to find it irritating ...

 

Irritating?  Far from it: I've learned a lot from this thread

[AndyID]

19 hours ago, kevinlms said:

Not entirely sure that the BEMF is what is killing the switches.

 

IMO what is killing the switches is releasing them BEFORE the capacitor has discharged fully. That means that if its trying to switch off the current to the solenoid, or even part of, then its going to exceed its ratings.

 

Assuming the capacitor has discharged a portion of its charge, when the switch is released, its still attempting to switch perhaps a couple of Amps. That its going to way exceed the ratings, of any but a quite large switch on DC.

 

On a typical CDU like this one.

http://www.talkingelectronics.com/projects/CDU-2/CDU-2.html

The diode on the top right is there to prevent back feed to the capacitor. Since the switch(es) are in series with this diode, so therefore the BEMF CANNOT effect the switch.

 

I conclude that diodes across the coils OR switches, are a waste of effort. Making sure the CDU is properly discharged BEFORE releasing the switch is released, is going to be more effective. Holding down the button or probe for longer is the easiest solution. Two seconds?

 

Yes, contact bounce may be an issue, but its nothing to do with BEMF. It's the current breaking attempt, by what is usually a very puny switch.

 

Yes, releasing the switch prematurely would not be good for the switch. I suppose there are two time constants to worry about. There's the cap and coil resistance and also a mechanical constant that determines the duration of the back EMF. There are probably others too.

 

We might have to drag out a storage scope and take some pics to get a better handle on this one. I have an ancient "Optascope" which might be equal to the task, but that's assuming I can still find the software. I know I put it somewhere safe.

[Crosland]

On 20/04/2019 at 05:06, AndyID said:

We might have to drag out a storage scope and take some pics to get a better handle on this one. I have an ancient "Optascope" which might be equal to the task, but that's assuming I can still find the software. I know I put it somewhere safe.

 

If someone can measure the resistance and inductance of a Peco (say) solenoid, then it would be easy to simulate.

 

I have seen scope traces (done by a MERG member, but I can't find a copy) and the CDU current dies away very quickly.

 

[SHMD]

I played about with SEEPs, PSUs, Oscilloscopes, with my own "designs", where I measured the inductance of the SEEP.

(Several transistors were hurt during the experiments!)

 

I'm sure that I posted it up on RMweb at the time, (with the DSO screen grabs), but can't find any trace (no pun...) of it at all now!

 

I did take notes though and the SEEP had an inductance of 2.79mH and a resistance of 2.79Ohms.

I used a 1.00 volt step rise and measured T to be exactly 1000uS, (not 980 or 1020 but 1000uS!), with the current quickly levelling off at 358mA.

 

 

Kev.

[DCB]

Sure fire way to stop switches burning is to not use them.

Edited May 18, 2019 by DavidCBroad

[Junctionmad]

On 18/05/2019 at 01:30, DavidCBroad said:

Sure fire way to stop switches burning is to not use them.

A bit like saying “ a sure fire way to ensure you don’t wear out your car , use a horse “

[Clive Mortimore]

On 18/05/2019 at 01:30, DavidCBroad said:

Sure fire way to stop switches burning is to not use them.

 

40 minutes ago, Junctionmad said:

A bit like saying “ a sure fire way to ensure you don’t wear out your car , use a horse “

Why do you always comment on anyone who has an alternative.

 

Like David I also use stud contact to switch my solenoid point motors. When building my layout I looked at various alternatives. I would have liked something like Tortoise motors  on all 68 points and with the combination of switches, the built in switches and relays had all the signalling interlocked. Sadly like most of us I was working to a budget. I did consider passing contact switches but my experience of them eventually failing lead me to use stud contact. It is cheap, reliable, easy for others to follow, and best of all it works.

 

Now Spikey might say, "That is not a route I want to take" or he might say "I hadn't thought of that and it could be a route I might take." Presenting an alternative is what discussion is about.

[Talltim]

Do you not potentially get the same issue using stud contact as with push to make buttons in that you make not make the connection for long enough?

[Clive Mortimore]

1 minute ago, Talltim said:

Do you not potentially get the same issue using stud contact as with push to make buttons in that you make not make the connection for long enough?

Never had a problem.

[kevinlms]

12 minutes ago, Talltim said:

Do you not potentially get the same issue using stud contact as with push to make buttons in that you make not make the connection for long enough?

Even if you do get a build up of carbon on the studs, it's very easy to scrape off,  than mess about with a soldering iron to replace a useless switch. 

[Junctionmad]

Quote

Why do you always comment on anyone who has an alternative.

 

 

you did notice the emoji !!!

 

I commented because the OP is using switches and looking  to fix the issue .  Its an say issue to fix and it doesnt need ripping out everything 

 

stud and contact is fine but its rather dated at this stage dont you  think. Switches can be got for a dime a dozen etc 

 

 

[Clive Mortimore]

1 hour ago, Junctionmad said:

 

 

you did notice the emoji !!!

 

I commented because the OP is using switches and looking  to fix the issue .  Its an say issue to fix and it doesnt need ripping out everything 

 

stud and contact is fine but its rather dated at this stage dont you  think. Switches can be got for a dime a dozen etc 

 

 

Yeah but it works.

 

[spikey]

On 21/05/2019 at 15:31, Clive Mortimore said:

Now Spikey might say, "That is not a route I want to take" ...

 

Correct!  That's exactly what he did say.

 

Can't be doing with studs and probe ...