SU petrol pump

A better way might be to consider the magnetic field collapses to maintain the current?

Um no. The magnetic field isn?t sentient. It doesn?t do anything for a reason. It just is.

Tim

It may not have feeling, but it has a relationship with Amp-Turns, hence current.

The magnetic field represents energy, a function of Amp-Turns, much in the same way you might relate energy of a capacitor with voltage.

I think most would say when trying to discharge a capacitor, it would endeavour to maintain voltage. It is just so.

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:

Cheers, T i m

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

It is quite ridiculous to suggest that electrical components don't have feelings and aspirations.

Quite ... and spirits, unlike with humans you can often see their spirits leaving as they die! ;-(

Unfortunately, 'some people' are very literal and so if you use the word 'try' ('to attempt to do or accomplish') in regard to an inanimate object they get confused. However, inanimate things do exist and they therefore can be described as having a reaction that can quite legitimately be described as (say) 'trying' to do something, if only from an observational POV.

Like, it would be perfectly legitimate for someone to say that 'if you bring a magnet close to some iron filings it will try to pick them up' in contrast a piece of would wouldn't been seen to be trying to do the same thing. We know the magnet isn't 'trying' in a human sense, but the effect can legitimately explained as such, just as a collapsing magnetic field could be descried as 'trying to maintain itself' (especially when it can't, otherwise it would be described as 'maintaining itself').

Of course, from a science POV we know these things aren't 'trying' as in a human effort POV, but we as humans are perfectly correct to describe such effects as such. ;-)

'The pressure in the coolant system was trying to push the hose off ... '

Cheers, T i m

In message , T i m writes

You need to remind it that the function of the pressure is for the coolant to operate an elevated temperature (even > 100C) - and that it's not supposed to "blow the bloody hose off".

Quite. ;-)

When that happened on my old Rover 218SD I initially looked at fixing it to get home and then I just drove it home gently, empty of coolant. ;-)

Those old 1.9 Pug lumps certainly had an 'iron will'. ;-)

Cheers, T i m

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

(snip)

Given that there was a reference upthread to North America, I take this opportunity to throw in this connection to this mention of Sawtooth:

One of the best fairly widely available beers produced in the US, in my view. And I like my British bitter.

While on the subject of beer, this is the menu for the place I used to drink at:

Cheers, back to the SU Pump, (I have a dead one waiting for attention)

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.

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?

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

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

"across the load side", which side is this?

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?

Um, the olde worlde SU pump had points.

Tim

I've seen both plain diodes and zeners used for this purpose.

Yep, makes a change for armpitsfarm to get something right.

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?