Zanzibar effect

Almost a Rheostat.

Someone in this group gave me two enormous rheostats years ago. They are really useful for acting as dummy loads when testing PSUs.

Bill

Whoever wrote that was a technical writer who didn't have a clue.

I suspect the intention was to prevent anyone from applying 10V to the potentiometer but somehow lost in the translation.

My take.

A typical DVM will be 3.5 digits.

On the 10V scale, the display has the possibility of showing between

So you can see the allowed limits low - 0.300 high - 0.350

On a higher scale, say 30V

the display will show just 0.00 to 30.0. You lose a digit of display, they can't do anything with the leading 1 digit.

So you can see the allowed limits displayed as low - 0.30 high - 0.35

If your component is set above 0.35 (say 0.358), you won't be aware of it until you see the smoke.

Erm. Rereading that, I think I may have written some bollocks there.

Somebody correct it.

(By a similar logic(?) the 0-3V range would not work)

It only has a very limited range of adjustment - fixing holes are slotted.

But just how would any setting cause smoke? The track is wired across 5v. The output is 0-5v, depending on throttle position.

Perhaps it was written for the average garage mechanic. Who doesn't know what a decimal point is. And tries to force it round to get 35 volts out of it. ;-)

I must admit to not quite following it. ;-)

I did wonder if it was to do with the current taken by a 'mechanical' volt meter. The vast ancient units from the 1920s.

Dave Plowman (News) submitted this idea :

The higher the set voltage range, the lower the current absorbed by the meter.

The higher the voltagee range, the same is the current absorbed by te meter for the same percentage of full scale deflection

Quite. Although a massive old mechanical voltmeter might draw more current than a newer one of the same.