Bernd Felsche > wrote:
>Paul Saccani > wrote:
>>Bernd Felsche wrote:

>>>I also have a hand-held, digital 'scope. One of the clamp meters has
>>>peak-hold -- I'd forgotten about that. It might do the trick.

>>>Well, it'd tell me the current through the circuit. Which isn't
>>>necessarily the same current that'll be drawn when the long wire is
>>>replaced with a short one and a transistor switch.

>>...and now we get to what you are really up to. No doubt with a
>>MOSFET in mind. Don't neglect the back EMF, and the transient inrush
>>current could be significant.

>That's my main concern. The inrush and other transients.

>>You could always buy a COTS solid state relay - for you to engineer as
>>good a solution from parts for the hostile environment it can expect
>>to live in, is something that will cost you a good deal more than the
>>$23 for a COTS 80 A 0-100VDC solid state relay.

>BTS50055-1TMB is designed for the environment, provides useful
>feedback and therefore diagnostics (at about $6). I need to
>determine the solenoid current to allow for an adequate "overkill".

The clamp meter shows just 5A peak in the solenoid. I suspect that's
the average over about 0.1 seconds; which is about how long it takes
to close the contacts that pull in the starter fully.

The 5A is the highest peak out of about 20 (*) successive starts. I
suspect that that's an average over the sampling period. Figures
are scattered all over the place; from 0.3A up to 5A. Most in the 3
to 5A range.

I'll have to put the digital scope with linear Hall-effect sensor on
the wire to find the peak inrush current.

(*) Don't want to cook the starter motor with too many successive starts
without a reasonable cooling off period.
--
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