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Old 08-06-2018, 23:16   #1
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Will a 24V alternator drive a 12V panel Tacho??

We are commissioning a Halberg Rassy with a Volvo Penta engine with 12V starter, panel and alternator. The alternator works but doesn't give a usable tachometer signal.
It also has a 24V alternator.
I am wondering if I can use the 24V alternator "W" tacho output as input to the tacho on the 12V panel without producing smelly blue smoke.

I suspect the tacho indicator is the same on the 12V and 24V panels but I am not sure!

Perhaps I should connect it through a capacitor. If so what size?












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Old 09-06-2018, 00:34   #2
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Re: Will a 24V alternator drive a 12V panel Tacho??

Quote:
Originally Posted by OndinePLC View Post
We are commissioning a Halberg Rassy with a Volvo Penta engine with 12V starter, panel and alternator. The alternator works but doesn't give a usable tachometer signal.
It also has a 24V alternator.
I am wondering if I can use the 24V alternator "W" tacho output as input to the tacho on the 12V panel without producing smelly blue smoke.

I suspect the tacho indicator is the same on the 12V and 24V panels but I am not sure!

Perhaps I should connect it through a capacitor. If so what size?












---------------------------------------------
"A Small Slip" and "Desperate Deliveries", true stories by Cris Robinson are avalable on Kindle ebooks.
Probably no problem at all, there is no real current draw on the sense wire (W terminal) and the voltage shouldn't be enough to damage the tach BUT if you want to be safe, you could use take a wire from the W terminal to two resistors (same value say 1Kohm, 1/4 watt)) in series to ground. Then take a wire from the junction of the two resistors to the tach.

This arrangement is a voltage divider and will halve the voltage on the sense wire from the 24 V alternator going to the tach i.e. making it the same as a 12V alternator sense wire.
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Old 10-06-2018, 23:09   #3
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Re: Will a 24V alternator drive a 12V panel Tacho??

Thankyou for that quick and useful reply.
Of course a potential divider is the solution, and I should have thought of it.


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Old 10-06-2018, 23:39   #4
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Re: Will a 24V alternator drive a 12V panel Tacho??

Well it is a easy and workable solution,

While it is probably not necessary, it is so simple that one might as well do it and remove all doubt.

I trust it all works out OK for you!
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