SU petrol pump

I wonder if anyone here remembers anything about these old pumps which used to sit by the tank and pulsate a diaphragm whenever the contacts
met and once resistance built up from the carburetor float bowl the fuel in the diaphragm chamber held the contacts open?
I replaced mine when the contacts went high resistance, with a solid state one, which has now packed up after ten years and less than 10k miles. The original is much better built, did SU ever produce a solid state version on the original build components?
I cleaned the contacts and readjusted the toggle point and the old pump runs fine but I wondered about putting a zener diode across the coil as this is supposed to prevent the forward current, as the magnetic field breaks down around the coil, burning the contacts by reducing sparking.
https://uk.rs-online.com/web/p/zener-diodes/6256439/
Thoughts
AJH
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On 02/02/2018 11:02, snipped-for-privacy@loampitsfarm.co.uk wrote:

I would suggest a standard diode across the coil[1] would be a better solution. Its a technique often used to limit emf when operating relays.
Just about any power diode would do, even the 1N4001.
[1] Cathode on the +ve side, anode on contact side.
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Isn't it 'back emf', the voltage generated by the coil itself as you disconnect the power to it and it tries to maintain the magnetic field?

Yes, assuming a -ve earth vehicle (and no other circuitry, like immobiliser's etc), diode stripe end in parallel to the pump +ve feed and the other end to earth (both as close as possible to the pump connections).
Any 'back emf' generated by the coil as the contacts open are sent back round though the diode rather than turning into an arc to erode the contacts.
Cheers, T i m
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No it's a forward current as the field collapses it releases the energy stored during its on period the snubber diode would never come into effect. So the zener looks like an open circuit to the normal 12V supply and only dumps the forward current produced by the collapse in the inductors field, which causes a voltage spike and hence the sparking.
AJH
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On 02/02/2018 17:24, snipped-for-privacy@loampitsfarm.co.uk wrote:

You need to define "forward current". I also don't understand why you mention a "snubber diode" in one sentence and a "zener" in the next.
What do you mean by "dumps"? The whole point of using a diode is to stop any voltage spike.
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I've seen zeners specified across an inductive device. But more common just to use a plain diode.
You might want to measure the spike with a scope. A 1N4001 might be close to its limit, voltage wise. But a 1N4004 costs about the same.
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On 03/02/2018 00:27, Dave Plowman (News) wrote:

The whole point of the diode is to stop any spike. It should be limited to the diode drop at the current flowing through the solenoid at the point the contacts open. I would suspect a little over 1V.
A 1N4001 has a minimum working reverse voltage of 50V. The 1N4004 has a reverse working voltage of 400V which is rather overkill but probably costs the same. I'd wager alternator diodes and radios would give way well before a 1N4001 would fail.
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A common trick to drive a rev counter originally fed from a coil negative when changing to a more modern multi-coil system is to use a transistor driving a relay coil (as the inductor) from a low volt tach signal. This can develop a spike of some 80v.
Of course just what peak voltage a snubber diode would see for an instant is debatable. ;-)
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<snip>

Yes ... I think he means this auto-generated voltage is safely re-directed (dumped) back round into the inductor rather than being allowed to build up as an arc across the driving switch / device etc?
Cheers, T i m
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On Fri, 02 Feb 2018 17:24:28 +0000, snipped-for-privacy@loampitsfarm.co.uk wrote:

I'm not sure where the zener came into this scenario but yes, your description is better (even with electron rather than conventional current flow). ;-)
Like this one:
https://progeny.co.uk/wp-content/uploads/2014/01/image003.png
Cheers, T i m
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Yes that is what I think of as a snubber diode circuit, it dissipates the current continuing through the coil damped by the resistance of the coil. The zener is between the coil and earth and dumps the current straight to earth as soon as the voltage rises to 28V, which is low enough to prevent the arcing. I don't know if there is a significant difference in their effect but there is some reference online to SU actually having used zener diodes like this.
AJH
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A snubber diode can go to the power supply side too - doesn't have to go to ground.
Indeed this is quite common on relays with built in diodes. The diode is simply across the coil.
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On Sun, 04 Feb 2018 19:12:28 +0000 (GMT), "Dave Plowman (News)"

Yes
The thing is the Zener diode is across the load side of the contact breaker points to earth, not across the coil, so whereas the energy is dissipated in the coil resistance and (as someone kindly pointed out) in the voltage drop across the diode in the case of the diode snubber the zener simply conducts any current directly to earth all the time the voltage from the current generated by the collapsing magnetic field of the coil exceeds the specified 28V break down voltage of the zener diode.
As I said I don't know what the significant difference between the two methods would be except the zener diode does the job faster, this is unlikely to be a factor with the cycle time being in the order of a second. Cost of the bits is insignificant, the aim after all is to extend the service interval of the points.
AJH
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On 04/02/2018 21:45, snipped-for-privacy@loampitsfarm.co.uk wrote:

What zener diode? You seem to have an infatuation with this zener diode.

"across the load side", which side is this?
> not across the coil,
Which is normally where snubbers are placed. You wouldn't want all that inductive energy to go into the supply when the supply is switched off, would you?

That depends if the load is supply side or ground side of the switch.

What zener diode? Where is this magic 28V come from? Do all zeners "break down" at 28V?

What job are you talking about?

They are, that's why the original SU pump had no diode or any other method to save the contacts. One cheap way was to use a suppression capacitor and resistor in series.

Are we talking ignition coil now?
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Um, the olde worlde SU pump had points.
Tim
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On 04/02/2018 23:17, Tim+ wrote:

Yep, makes a change for armpitsfarm to get something right.
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On 04/02/2018 16:41, snipped-for-privacy@loampitsfarm.co.uk wrote:

For the avoidance of any doubt, it is a diode.

No, The voltage in image003.png goes to a diode drop below the -ve battery terminal potential.
Perhaps you can explain where you get your 28V from?

Arcing from the breakdown of air requires a specific voltage according to Paschen's law, the minimum breakdown voltage in air is 330V over a gap of 7.5um
It is easily confused with the creation of hot metal sparks.

What reference?
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I've seen both plain diodes and zeners used for this purpose.
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On 05/02/2018 00:18, Dave Plowman (News) wrote:

https://progeny.co.uk/wp-content/uploads/2014/01/image003.png
Shows a diode.
I have seen zeners, but normally across a drive transistor to limit the voltage of any back-emf, but I confess never across an inductor. I have seen a R and C in series, and I have seen a diode in series with a resistor, and a diode in series with a parallel resistor-capacitor combination.
More specifically, the reason why you would never use a zener across an inductor, is that the zener would conduct and act like a diode when the coil is energised.
I'm therefore intrigued when you might have seen one connected directly across the coil?
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The relay drivers on a Megasquirt ECU use a (22v) zener between driver transistor collector and ground. The board runs mainly on regulated 5v, so saves having to have the 12v supply for the relay brought close to the driver.
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