Zanzibar effect

[Dave Plowman (News]

Interesting one about multi-meters in a BL service instruction booklet.

To set the TPS (throttle position sensor) as fitted to early (1980) Rover V8 Lucas injection.

It is a large carbon pot sitting across 5v (nearly) and provides a variable voltage out by throttle position. Quite a chunky device with twin wipers, so a lot more butch than the average volume control.

It is adjustable, and the original way to set was to use a BL special tool. You set it to null a couple of LEDs. No doubt cost a fortune.

The later Land Rover guide gives a method using a volt meter. Pic shows a DVM - rather than an AVO 8, etc. You connect to output and ground and set it to 0.325v +/-0.025v.

It's the note below which is interesting:- CAUTION: The following adjustment must be made using a voltmeter set to a maximum scale of not more than 10v. If a voltmeter of more than 10 volt scale is used the potentiometer will be irreparably damaged.

Try as I might, I ain't got a fooking clue what they're on about. ;-)

[jon]

Almost a Rheostat.

[williamwright]

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

Bill

[Fredxx]

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.

[Adrian Caspersz]

My take.

A typical DVM will be 3.5 digits.

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

0.00 to 10.00

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.

[Adrian Caspersz]

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)

[Dave Plowman (News]

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. ;-)

[Dave Plowman (News]

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.

[Harry Bloomfield, Esq.]

Dave Plowman (News) submitted this idea :

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

[The Natural Philosopher]

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

[Dave Plowman (News]

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