[Heathkit] meter protection

[Jeff Blaine AC0C]

Dennis,

These are 2 different things.

The meter spec refers to something that is a proxy for "how much of a 
magnetic field is needed to give a full scale deflection."  That turns out 
to be around 200uA.  And with a 1200 ohm coil, the voltage comes from ohms 
law as you suggest.

Now on the damage side, the question is "what level of resistive heating 
will lead to damage."  The more voltage across an essentially fixed 
resistance means a growing current.  More current; more heat.  And on and on 
until so much heat is generated the wire vaporizes at it's weakest link and 
the coil becomes open.

The amount of current needed to fry the coil is pretty high.  Depends on the 
current and the time exposed.  But the point is, you can run 1V across that 
meter all day (probably - don't try it as these sb200 meters are rare 
beasts) and it would not cause permanent damage.

If you want to be ultra safe, then you can use some shottkey power diodes. 
But for me, I like those huge bad boys that run 3A at 1000V.

Hope it helps.

73/jeff/ac0c


--------------------------------------------------
From: "Dennis Berry" <dennisberry at worldnet.att.net>
Sent: Wednesday, October 14, 2009 11:11 AM
To: <heathkit at mailman.qth.net>
Subject: [Heathkit] meter protection

> I have seen on several websites that advocate putting the back to back 
> diodes on a meter, which I am not rejecting as wrong, but I do have a 
> question.
>
> As I recall most of the Heath meters are about 1200 ohms and 200 uA 
> movements.  Wouldn't this indicate full meter movement is at 0.24V?  Do 
> diodes that conduct up to say 0.5V or 0.6V still protect the meter?  I 
> know it's better than a flashover voltage, but is 0.5V enough to still 
> damage the meter?
>
> Just trying to learn and understand, not pick on the method or Cal.
>
> Thanks in advance,
>
> Dennis, NU8S
>
>>  Message: 1
>>  Date: Tue, 13 Oct 2009 16:25:16 +0000 (GMT)
>>  From: C E <catman351 at yahoo.com>
>>  Subject: [Heathkit] SB-200 meter flashover/diode protection
>>  To: heathkit at mailman.qth.net
>>  Message-ID: <190179.86540.qm at web37103.mail.mud.yahoo.com>
>>  Content-Type: text/plain; charset=iso-8859-1
>>
>>  All:
>>  I noted Bill's comment on his friend's SB-200 meter flashover: "A friend 
>> of mine is looking for the meter (working) for his SB-200 - > apparently 
>> he had a flashover while tuning it up that took out his meter."
>>
>>  It would be cheap insurance to install a set of back-to-back diodes on 
>> your SB-200?multi-meter to mitigate the damage caused by such
>>  problems. One can replace diodes all day...meters, unfortunately, are 
>> another matter.? FWIW.
>>  Cal, N6KYR/4
>
>
>
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