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.
They were much more reliable, and could be fixed, or just tapped or kicked to get them going. I was lucky enough to find two mint ones recently, thrown out with someone's scrap metal !!!!!
I don't know about a diode, but a condenser across the points should help reduce arcing.
Remember seeing conversion kits to change an SU to solid state switching. Which did seem like a good idea. But one made out of discrete components where you could change the switching transistor (or whatever) if it failed would make even more sense. As such things cost pennies.
It's a big snag with modern replacements. They often aren't made to the same quality as the originals. Very much the case with those cheap 'Lucas' new distributors you find on Ebay, etc. A quick look at them compared to an original Lucas shows they are built very much down to a price.
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.
They were problemsome, especially with a car that wasn't often used, the points could glaze over and that's when they needed a tap. It isn't a big job to clean them but a faff jacking the car up so the fuel all goes to the other side, else it continues to syphon when you undo the banjos. Once out it's only one nut holds the end cap on.
I could use a spare!
Condenser is standard between points and earth but zener is a bit extra to take the surge as the points open.
Did it use a hall effect transistor? I can see how one might work by sensing the proximity of the diaphragm plunger and cutting the current but that would assume the shaft retained some magnetism for it to self start.
This is just the problem, had the solid state components been used in the end cap of the original pump replacement would have been trivial. however they are sealed inside the plastic housing and then the core is fixed in with a barbed washer with the coil then assembled over the coil. Plus the valves are all pressed in place in the plastic body. At the time I bought it is was 60% the cost of the SU contact breaker one but the profit margin must have been much greater.
So is it worth fitting the zener to increase the life of the points?
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.
Plenty cars with SU pumps had them in the boot. The beauty of the design is that it can lift fuel as well as pump it onwards. So doesn't have to be gravity fed.
IIRC, my wife's Mini had it outside, attached to the rear subframe. [Or was it my Austin A40 that had it under the boot (also outside).]
I know that with one of them, there was once a sudden mysterious rapid ticking (almost a buzz) coming from somewhere at the rear. It turned out to be the SU pump - and a firm (but gentle) whack fixed it. When operating correctly, it ticked around every 10 seconds.
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.
I've always called it back EMF, but the term seems to be unknown in north America when I've used it in discussion groups. Can't recall off-hand what they do actually call it. - no tell a lie I remember now, they call it "flyback" and dealing with it they call "snubbing" or "freewheeling."
An equivalent circuit is used in old televisions to generate the line scan current and the high voltage (upwards of 15kV - called EHT) for the anode of the cathode ray tube.
For each scan line, current increases (more or less) linearly in the line output transformer primary winding (driving the electron beam across the screen from L to R); and then is suddenly cut off. The magenetic energy in the transformer tries to maintain that current flowing in the same direction and to achieve that the voltage across the primary reverses. The duration of this reverse voltage is controlled by the value of the capacitor across the switch - the energy from the transformer primary is transferred to the capacitor - then back to the transformer. This in TV terminology is the "flyback" - and results in the electron beam being deflected across the screen from R to L - hence the name. You don't see this because the beam current is blanked.
The value of the "flyback" voltage depends on the capacitor, but it will generally be quite high. This is multiplied by the turns ratio of primary to secondary in the transformer then rectified to give the required EHT voltage.
See:
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A diode across the switching device passes current after the first half-cycle of flyback, improving linearity and reducing losses, so it's often called the "efficiency" diode". See also:
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Much the same technique is used in a modern switching power supply - in general energy is stored in an inductor then transferred to a capacitor as a switch opens. The energy is extracted at a different voltage - so in the TV case the purpose is in part to generate the EHT whereas for a laptop power supply the purpose will be to convert 250ac mains into something around 12v to 24v dc to charge the battery.
Much of the original design was by Alan Blumlein, see:
Good to see that it described the output valve as a switch, in the text it called it a pentode and then put a tetrode in a diagram. This is the circuit I would have seen in TV's with additional windings to control linearity.
The output valve is a switch, and each diagram shows it being driven with a sawtooth. In practice it would have been an on-off like drive as per the top link.
Given this is a car maintenance group, I might have thought an article on coils be more prudent? :-)
Thanks for the links, nice to reminisce of a bygone age.
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. ;-)
Yes - that's the Mini, bolted to the rear subframe, just where it could pick up enough muck. On a Mini I had it (and the fuel line) had been mounted inside, under the rear seat IIRC.
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