[Mini] [minElist] Shunts and ammeters & starter motor load

Richard Porter ricp at minijem.plus.com
Thu Apr 21 03:25:34 MDT 2016

On 21 Apr 2016 Dr John C Bullas john.bullas at gmail.com [minilist]

> OK Questions:

> 1) How many amps does an inertial mini starter motor draw

I don't know the answer but it will depend on the speed of the motor
and the voltage available to it. The faster the morot tunrs the
greater the back-emf so the lower the current. The greatest current
will be drawn when the motor is stalled, but it is most likely to
stall if the voltage drops so you can't say exactly.

It's best to start with the battery. An HB057 battery (Cooper S type)
has a CCA (cold cranking amps) rating of 350 amps but you're unlikely
to see that figure in practice because of the resistance of all the
connections, cables, solenoid, etc.

> 2) I have a 200a rated 75ma stunt, how do you read the current loading
> across the shunt? with a rescalled voltmeter or ammeter

The current is split between the ammeter and the shunt in inverse
proportion to the resistance of each. Most meters are ammeters which
measure milliamps and have low resistance. You make a voltmeter by
putting a high value resistor in series with the meter. You make an
ammeter to measure high currents by putting a shunt (low value)
resistor in parallel with the meter so only a fraction of the current
passes through the meter itself.

I say most meters because some automotive gauges use bimetal strips
which are heated by a current passing through a winding around the
strip. These have a higher resistance and by their nature are heavily
damped so are not suitable as ammeters. They do however damp out the
voltage fluctuations caused by some voltage regulators and by
switching equipment on and off (and using direction indicators).

> I have fitted ammeters to measure all load/charge other than the starter to
> give +charge and -discharge however the ammeters were direct read +/- 60amp
> say

A 130 or 160 ammeter will have a built-in shunt. You could still add
an external shunt to rescale the meter to, say, 1300 amps. I would add
a fuse in the meter circuit just in case the shunt fails. You don't
want to put 200 amps through the meter!


Richard Porter                        http://www.minijem.plus.com/
Skype: minijem2                       mailto:ricp at minijem.plus.com
I don't want a "user experience" - I just want stuff that works.

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