Solenoid protection diodes

[spikey]

I am aware that the life of the switches controlling my SEEP solenoids would be extended if I were to fit protection diodes to them, but my searching has thus far failed to turn up a circuit diagram showing me how to do that.  Can somebody please direct me to one, and also advise which diodes are suitable for this application?

[meil]

Your switches would have their life extended if you used a proper CDU.

[spikey]

1 hour ago, meil said:

Your switches would have their life extended if you used a proper CDU.

 

I do use a proper CDU.

[meil]

In that case fit a 1N4002 across each coil.

[cliff park]

You need to fit it such that it acts as a short circuit to the collapsing field !! The easy way to work this out is that if the positive of the supply is at say tag 'a' of the coil then the positive of the diode needs to go to that tag. 

[Crosland]

On 10/04/2019 at 19:19, spikey said:

 

I do use a proper CDU.

 

In that case you do not need the diodes.

 

The thing that kills the switches is the arcing when opening and trying to break the circuit (it burns the contacts). The current from a CDU dies away very rapidly and there will be very little current flowing and very little arcing when you release the switch.

 

This is why switches always have a higher current rating for AC than for DC. The AC current passes through 0 twice every cycle and any arc is extinguished.

 

[kevinlms]

52 minutes ago, Crosland said:

 

In that case you do not need the diodes.

 

The thing that kills the switches is the arcing when opening and trying to break the circuit (it burns the contacts). The current from a CDU dies away very rapidly and there will be very little current flowing and very little arcing when you release the switch.

 

This is why switches always have a higher current rating for AC than for DC. The AC current passes through 0 twice every cycle and any arc is extinguished.

 

If a CDU has large value capacitors, it may be slow discharging totally. If the switch contact is released to quick that will cause arcing too. In that keep contact a little longer, until the capacitor has discharged fully.

 

[spikey]

Thank you gentlemen.  The CDU does indeed deliver a fair old whack (obsolete technical term) and there are times when we're decidedly nifty on the switches.  I've also had three switch failures in the last year or so, I'm getting older, and it's worth my while doing whatever I can now to minimise extra demands on my deterity and eyesight in the future.

 

FETs are out on account of cost and complexity on a layout with 25 points, but fitting a pair of 1N4002 diodes to each of them isn't a big deal if I do it a few at a time, starting with the pairs of points that are fired from one switch, followed by the rest in order of use. 

[Chris M]

Could it be a bad batch of switches? In 40 odd years of switch driven points with a cdu I've never had a switch fail. I use simple push button switches from various suppliers. My current layout uses a big cdu which can switch five point motors at the same time. I've now done many full days of operation without any grief from the switches. One point has fallen apart and one motor has given up but the switches are ok, and they do get a bashing during a day at an exhibition.

Edited April 16, 2019 by Chris M

[spikey]

2 hours ago, Chris M said:

Could it be a bad batch of switches?

 

Possibly.  But I look at it this way; in theory, fitting a pair of diodes to each point motor reduces the odds of switch failure.  By how much, I have no idea.  At present I still have reasonable eyesight and dexterity, but the omens on that still being the case in a few years' time are not good.  Therefore it makes sense to me to fit diodes while the doing thereof is no great embuggerance - especially when it only costs me 4p a point.

 

It's all about planning for decrepitude

[wainwright1]

Using a diode in this way would divert some of the kick from the solenoid which would probably not affect a single one, but would it not reduce the effectiveness of throwing multiple points ?

We use Peco solenoids on our club layouts and my Hawkhurst layout, all 00 . I currently have three points on Hawkhurst,  (Peco code 100) , more than 30 years old, where a point blade has broken from the tie bar. These have been temporarily repaired, but the points will need replacing. We had a similar point on another club layout, also code 100, which broke and has been replaced.

Our Crystal Palace High Level layout (Peco code 75), which is approaching 30 years , originally had  Seep solenoids which we found fell to bits after a while. It now has Peco ones,  but no breakages to date. On this layout we have a route setting control panel which does throw up to five points in one go from one switch and we did in fact add extra capacitors to make sure all the points threw positively.

I can only remember one of the miniature toggle switches burning out in the last 30 years and that is with quite a few exhibition appearances for each layout. If you open up one of these switches to look inside, there is not much in there and you can see where they would arc and burn.

We have also used the small buttons particularly for route setting in fiddle yards and these do burn out, so we now avoid them. Where possible on replacement or for new layouts, we now use the large buttons which should  have enough meat in them for the lifetime of the layout.

Hope that this helps.

RB

[spikey]

2 hours ago, wainwright1 said:

Using a diode in this way would divert some of the kick from the solenoid ...

 

Really?  I don't understand how, but if that is indeed the case, it might not be a bad thing!

[AndyID]

On 13/04/2019 at 07:39, Crosland said:

 

In that case you do not need the diodes.

 

The thing that kills the switches is the arcing when opening and trying to break the circuit (it burns the contacts). The current from a CDU dies away very rapidly and there will be very little current flowing and very little arcing when you release the switch.

 

This is why switches always have a higher current rating for AC than for DC. The AC current passes through 0 twice every cycle and any arc is extinguished.

 

 

That's true if the switches are perfect, but they are not. When a switch is closed the contacts bounce and that produces a short series of open/close cycles. When the contacts bounce open the solenoid's inductance tries to maintain the current that was established while they were closed by rapidly increasing the voltage across the open contacts resulting in an arc that erodes the contacts.

 

Quench diodes prevent arcing while the contacts are bouncing and that can extend the life of the switch.

[RobjUK]

21 hours ago, wainwright1 said:

Using a diode in this way would divert some of the kick from the solenoid

 

Having a reverse diode connected across the coil - aka a "flywheel diode" - should slightly increase the force of the solenoid, not decrease it.

The current will continue to circulate in the solenoid coil for a short time after the switch is opened or power disconnected.

 

You get a slightly longer duration of "pull" for the same duration input pulse; with a small solenoid the extra duration is likely in single digit milliseconds, but is there.

 

And of course it prevents any back EMF spikes damaging the switch contacts or external electronics.

 

If you want to reduce the switching spike without the extended pull, use a VDR (voltage dependant resistor) such as a "zenamic" instead, rated at the same DC voltage as whatever power supply is in use.

We use those in electromagnetic clutch controllers to give the fastest possible drop-out, without excess back EMF spikes.

 

[cliff park]

It always amazes me how much discussion can occur over such a simple question. For a few pence per coil this is a job well worth doing. Also note that physically the diode does not have to be fitted directly to the coil. The effect is exactly the same if wired across the switch terminals, and these are usually a lot easier to access. ( This may be obvious to some, but not all ) Just make sure to get the polarity right.

[AndyID]

3 hours ago, cliff park said:

It always amazes me how much discussion can occur over such a simple question. For a few pence per coil this is a job well worth doing. Also note that physically the diode does not have to be fitted directly to the coil. The effect is exactly the same if wired across the switch terminals, and these are usually a lot easier to access. ( This may be obvious to some, but not all ) Just make sure to get the polarity right.

 

Except that it's not all that simple. As Crosland pointed out, when using a CDU, there is no current flowing by the time the switch is released which makes the diodes pointless, but the diodes do reduce arcing if the contacts bounce when the switch is closed.

[Grovenor]

Quote

there is no current flowing by the time the switch is released

there may be no current flowing by the time the switch is released

Rgds

[AndyID]

1 hour ago, Grovenor said:

there may be no current flowing by the time the switch is released

Rgds

 

That depends on how quickly the switch is released. I imagine most people will close it long enough to easily discharge the cap, but I suppose there's always someone who tries to jump the lights

[RobjUK]

8 hours ago, cliff park said:

The effect is exactly the same if wired across the switch terminals, and these are usually a lot easier to access.

 

Sorry, that is incorrect. An R-C snubber or VDR can be used across the switch, but a normal diode would do nothing.

The polarity across the solenoid coil reverses when the switch is opened while current is flowing, so a diode there conducts. The polarity across the switch does not reverse, so a diode there is ineffective; it would never conduct or have any function.

[Grovenor]

RobjUK,

I suggest you think that through again.

Cliff park is correct, the diode serves to conduct the coil current during the collapse of the magnetic field, preventing a voltage build up across the open switch contact. For this purpose across the coil or across the switch have exactly the same effect. The advantage of having the diode close to the coil is that the current does not have to flow through the cabling between switch and coil and hence is less likely to cause inductive interference with other circuits, such as DCC cabling.

PS, RobjUK, seeing Cliff's subsequent message his 'across the switch' was unclear, I understood it as 'at the switch' just as he has drawn it, clearly you had a different understanding.

rgds

Edited April 19, 2019 by Grovenor
explain misunderstanding

[cliff park]

Sorry, that was misleading . What I meant of course was that the diode can be positioned at the switch, wired as shown here

 

 

[Ian Morgan]

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.

 

 

[AndyID]

18 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.

 

 

 

Except that, as I mentioned at the Brighton Conference above, it's not all that simple. There isn't a current spike. The inductance in the solenoid coil is "trying" to maintain a constant current, cos that's what inductors tend to do in these circumstances. If I'm not mistaken (which has been known) that means there will be a rapid reversal of the voltage across the solenoid until the diode starts to conduct to maintain the current, probably at around one volt.

 

So here's the thing. The current flowing around the loop between the switch and ground is about the same when the diode is mounted close to the switch, but the "loop" is essentially short-circuited by the diode when the diode is mounted close to the solenoid, and that might produce more interference.

 

I really do not know if it makes much difference either way but I'd be very interested to see the results if anyone cares to conduct all the tests.

[spikey]

35 minutes ago, AndyID said:

... I'd be very interested to see the results if anyone cares to conduct all the tests.

 

Well I for one suggest that you volunteer.  You'd be making a useful contribution to the fount of knowledge on here (or the font if you prefer, but don't let's go there), and your standing among your peers would be raised to even greater heights. 

 

I'd do it myself, but my inductance-fu is not strong ...

[AndyID]

56 minutes ago, spikey said:

 

 your standing among your peers would be raised to even greater heights.

 

Many thanks, but I'm not in the slightest bit interested in elevating my standing among my peers, most of whom, sadly, are now deceased.

 

I'm sorry that you seem to find it irritating, but it's a simple enough question. Is it better to put the diodes close to the solenoid, or the switch?