[ARC5] Dynamotor Capacitor

[Fuqua, Bill L]

   You figure out what should work and then you add a safety margin. 
The more critical the application the greater the safety margin. 
As an example you don't put in just enough bypass capacitors to get buy on you 
memory boards. You over do it. In come cases more the better. 
  As Stroker McGerk's sign on his wall says, " The only substitute for cubic inches is more cubic inches".
A friend of mine many years translated that into engineering terms as " If some is good, more must be better".
Now my 1970 VW engine is no longer 1600cc but 2387cc, with dual Webers and all the trimmings. 
73
Bill wa4lav

________________________________________
From: arc5-bounces at mailman.qth.net [arc5-bounces at mailman.qth.net] on behalf of Kenneth G. Gordon [kgordon2006 at frontier.com]
Sent: Tuesday, March 25, 2014 8:14 PM
To: Bruce Long
Cc: ARC5 at mailman.QTH.net
Subject: Re: [ARC5] Dynamotor Capacitor

On 25 Mar 2014 at 16:12, Bruce Long wrote:

> It seems to me the original ARC designers did not select the dyno bypass
> capacitor voltage rating on the basis of the largest amplitude voltage spike
> that could be generated by the receiver dyno but rather from the largest spike
> that could be created, typically a load dump spike- that could be generated by
> any large 28vdc load on the aircraft 28vdc buss.

Yes. That is my thinking on it too. Although I am not very good at
communicating my ideas (all you have to do is ask my wife about that) that is
what I have been trying to say.


> I can't say I have a detailed knowledge of WW2 28vdc power systems but I think
> the load dump spike from a prop pitch motor or the aircraft engine starter
> motors coudl be really impressive.

Starter motor spikes are quite impressive. I speak from experience from
grabbing the hot lead of a Model "A" starter motor (6 VDC) as a friend
started the engine then got off the switch. I let go of that lead really quick. I
was also quite surprised.

> As a partial reality check I think I remember the design standard for 12vdc
> automotive electronics is to resist 600 volt load dump spikes. I think there is
> an old National applications sheet or data sheet for one of their 12vdc
> automotive service IC audio amplifiers that has a discussion of the largest
> voltage spikes that can be expected in a 12vdc automobile buss.
>
> If my memory is correct, then a 2kv rating for the arc-5 receiver 28v bus bypass
> is not out of line.

I wouldn't think it was either. All I was trying to say was that the dynamotor
alone could not possibly have enough inductance to cause a 2KV spike.

> We are talking about the 28vdc bypass?

Yes.

Ken W7EKB
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