Shorted turn detector

[tbryanh]

It sounds like the series diode is needed.

It may be that the neon bulb ultimately will never flash because the internal resistance of the pickup is too high.

Internal resistance is probably one of the big items that distinguishes pickups from transformers.

Replacing the neon bulb with an analog ammeter might work. The needle movement of the ammeter probably requires less curent to give an indication than the neon bulb.

Other indicating devices are probably possible, but none seem to pop up right now, but I bet an ammeter will work real good.

[tbryanh]

It sounds like you need to connect a zener diode in series with the ammeter. The zener needs to have an avalanche voltage higher than the DC supply voltage, and it needs to be connected in a direction that will block the current from the DC supply from going through the ammeter.

[tbryanh]

Probably the lower the avalanche voltage the better; e.g. use a 10v zener for a 9v supply.

[tbryanh]

It seems that a series diode is not needed between the supply and the pickup. Isn't there a switch there already?

The zener diode will still be needed if an ammeter is used.

[R.G.]

OK, well I tried more or less what R.G. suggested. I took a cheap potted humbucker that I removed from my old guitar, connected a neon bulb across it, (leaving out the 100k resistor because I reckoned the pickup DCR would be enough) and tried prodding it with a 9V battery.

Nothing happened.

So I decided to use some brute force: I put a 2-turn coil of heavy wire on the poleface of the pickup, and touched this on the terminals of a 7 amp bench power supply set to 12V. (Which I happened to know had a 1000uF capacitor on the terminals internally, to give it even more wallop.)

That made the neon flash all right! Unfortunately, when I introduced a simulated "shorted turn", the bulb would still flash anyway.

I guess this makes sense: look at pickups with nickel silver covers. Isn't that cover just one big shorted turn? And yet they still work with just some damping of the resonant peak.

I wouldn't recommend trying this experiment with any pickups that have value: it might screw up the magnets. (I could feel them vibrating when I did it.)

Bummer. I guess my caution about number of turns and resistance was justified. Thanks for trying that out for me, Steve.

[tbryanh]

[tbryanh]

Here is a link to a diagram I drew of the shorted turn detector that replaces the neon bulb with an ammeter and zener diode.

http://subdomain.freeservers.com/schematics/Shorted Turn Detector.jpg

[tbryanh]

Here is a better link.

http://subdomain.freeservers.com/sch...rnDetector.jpg

[tbryanh]

It looks like you need to copy and paste this link into your browser.

http://subdomain.freeservers.com/schematics/ShortedTurnDetector.jpg

[Joe Gwinn]

Other indicating devices are probably possible, but none seem to pop up right now, but I bet an ammeter will work real good.

What jumps to mind from the ancient analog domain is a ballistic galvanometer. These are hard to find outside of museums these days, so one uses an op-amp integrator.

You will need some diodes to steer the current. The key observation is that the voltage across the coil reverses when power from the battery is cut off (the switch opens). The easiest way to think of it is to imagine the current in the coil trying to continue in the same direction (driven by the collapsing magnetic field).

[tbryanh]

The key observation is that the voltage across the coil reverses when power from the battery is cut off (the switch opens). The easiest way to think of it is to imagine the current in the coil trying to continue in the same direction (driven by the collapsing magnetic field).

Yeah, that sound right. If that is the case, then the zener diode can be peplaced by an ordinary diode in the circuit I drew.

[Steve Conner]

You guys are all missing the point. It's not that we can't devise some circuit to measure the Q factor of a pickup as accurately as we like. It's that a shorted turn or two won't have enough of an effect on the Q factor to give an unambiguous indication, no matter what method you use to measure it.

Based on the results of this thread, I'd wager that pickups function just fine with dozens of shorted turns and nobody ever notices. What about a Strat with an aluminium pickguard: isn't that just another shorted turn (and a pretty big and beefy one at that)

[R.G.]

You guys are all missing the point. It's not that we can't devise some circuit to measure the Q factor of a pickup as accurately as we like. It's that a shorted turn or two won't have enough of an effect on the Q factor to give an unambiguous indication, no matter what method you use to measure it.

Based on the results of this thread, I'd wager that pickups function just fine with dozens of shorted turns and nobody ever notices. What about a Strat with an aluminium pickguard: isn't that just another shorted turn (and a pretty big and beefy one at that)

Good point, but probably one that needs some more experimentation. I did a little thinking about this setup based on my transformers experience.

#42 wire is about one ohm per foot, and a bit less than one ohm per turn for what my mind's eye sees as "typical" pickups. If this were a transformer,
a shorted turn would transform that 1 ohm up into a load on the rest of the coil.

If you have a 5K turn coil, the ratio is 5K^2, or 25M - not a significant load to the whole coil's "transformer". I'm doing a bit of forehead slapping over my not doing that calculation before, by the way. And I think that's the reason it does not flash the neon - the resistances are just too high.

On the other hand, if you short several turns, the load rises linearly while the transformer ratio goes down by the square. It gets significant pretty quickly, I think.

But I think that the leakage compared to a real transformer might cause a shorted turn to "shade" a portion of the coil, forcing the flux out of more than just its own loop. I wonder if the effect of a buried shorted turn is between what I thought and what you've just said - it may just decrease output or dull it by equivalent loading.

[Joe Gwinn]

If you have a 5K turn coil, the ratio is 5K^2, or 25M - not a significant load to the whole coil's "transformer". I'm doing a bit of forehead slapping over my not doing that calculation before, by the way. And I think that's the reason it does not flash the neon - the resistances are just too high.

On the other hand, if you short several turns, the load rises linearly while the transformer ratio goes down by the square. It gets significant pretty quickly, I think.

But I think that the leakage compared to a real transformer might cause a shorted turn to "shade" a portion of the coil, forcing the flux out of more than just its own loop. I wonder if the effect of a buried shorted turn is between what I thought and what you've just said - it may just decrease output or dull it by equivalent loading.

I think a direct test might be useful. Build a pickup with a isolated small coil wound first, using #36 wire or the like, and short this coil with a low-resistance pot. One can then test the effect as the pot is turned.

[tbryanh]

Based on the results of this thread, I'd wager that pickups function just fine with dozens of shorted turns and nobody ever notices.

Your probably right. Just as the internal resistance of the coil prevented the neon from firing, so would the internal resistance prevent a few shorted turns from loading down the pickup.