[ROVERNET - UK] Re: Electrical Question

David Read defender110 at ozemail.com.au
Mon May 14 04:22:21 BST 2007

Measured across the battery ...
My Fluke reads 14.2 on DC volts and 0.0016v on the AC volts, but it is 
hard to tell what the effect of having the battery in circuit has on the 

WARNING:- *DO NOT* disconnect battery from alt whilst engine is running!!

At 4000 engine revs the approx output ripple frequency would be 800hz
(assuming 12 pole field and 2:1 engine/alt pulley ratio)

 From the theory of 3 phase full wave rectifiers ....
Vdc out= Vac(rms)in x 1.35
Ripple freq out = 6 x generated frequency
Ripple out 4.6% of output

I don't call 4.6% substantial.

South Oz

Paul Smith wrote:
> Disagree there...
> you will get substantial AC on a multimeter when the diodes are ok too.
> The mm just puts a capacitor in line, so any variation in DC will be seen as AC (it is AC with DC shift).
> I think he has a generator anyway.  I have sent him the next stuff to check out.
> -----Original Message-----
> From: rovernet-bounces at lyris.ccdata.com
> [mailto:rovernet-bounces at lyris.ccdata.com]On Behalf Of David Read
> Sent: Monday, 14 May 2007 10:53 am
> To: rovernet at lyris.ccdata.com
> Subject: Re: [ROVERNET - UK] Re: Electrical Question
> H Walter
> Walter Reynolds wrote:
>> Thanks for the quick responses.
>> In answer to Paul:  IGN light goes out when ignition turned off.
> That's ok
>> In answer to David:  Battery at rest shows 12.5 volts on my volt meter. 
>> This drops to 12.0 v at engine start and drops further to 11.75 v after 
>> 3 minutes running.  After 5 minutes is's still at 11.75v.
> EEEK!!
> Doesn't sound good at all. Suspect regulator or a blown diode or two.
> If the diode/s are US, the average (DC) voltage of the output waveform 
> may well be below 12v.
> Try putting your multimeter on *AC* volts with the engine running. If it 
> reads anything substantial, then the diodes are definately suspect as 
> raw AC is getting to the system.
> Your battery will be suffering in the long term as well.
> Cheers
> Dave
> South Oz

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