[Amps] SB220 Meter blown

[Adrian]

The diodes(s) should be direct across the meter, and enough in series as 
needed to excedd the full scale deflection

voltage required before forward bias is achieved in the diodes. It's all 
very simple, as stated here a few times now. re ;

I would rather follow Rich's advice on the subject as per my previous 
link. contained withjin ;


"It may take more than one diode to protect a meter shunt resistor. A 
silicon diode begins to conduct at a forward voltage of about 0.5V. To 
avoid affecting meter accuracy, the operating voltage per glitch 
protection diode should not exceed 0.5V. For example, a 1 ohm shunt, at 
a reading of 1A full-scale, has 1V across it. Thus, two protection 
diodes in series would be needed to preserve meter accuracy. Similarly, 
if the shunt resistor for a 1A full-scale meter is 1.5 ohm, the maximum 
shunt voltage is 1.5V--so three diodes are needed.

Glitch protection diodes should not be petite. Big, ugly diodes with a 
peak current rating of 200a or more are best. Smaller diodes--and the 
meter they were supposed to be protecting--can be destroyed during a 
glitch. Suitable glitch protection diodes are 1N5400 (50PIV) to 1N5408 
(1000PIV). In this application, PIV is not important. The 1N5400 family 
of diodes is rated at 200a for 8.3mS.

During an extremely high current surge, a glitch protection diode may 
short out--and by so doing protect the precious parts. Replacing a 
shorted protection diode instead of a kaput meter is almost fun."

ref; http://www.somis.org/D-amplifiers2.html

Once a petite signal diode blows apart it is no longer protecting the meter.





On 17/5/20 4:57 am, Gary Schafer wrote:
> Jim is correct. It depends on the meter circuit that is used. You need to
> calculate how much voltage will appear at the meter.
> I have a drake L7 that I added protection diodes to. I was going to just
> hang diodes across the meter shunt but in looking at it I found that it was
> a very low value and that the meter did not have much series resistance.
> Putting the diode directly across the meter or across the meter shunt would
> allow too much current in the meter before the diodes would conduct.
>
> I ended up lowering the series meter resistor and placing another resistor
> in series where I placed the diodes. In case of a glitch this would allow
> the voltage across the diodes to rise high enough to conduct before the
> meter voltage gets to a dangerous level but not conduct at full amp
> operating current.
>
> 73
> Gary  K4FMX
>
>> -----Original Message-----
>> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Jim
>> Sent: Friday, May 15, 2020 10:20 PM
>> To: amps at contesting.com
>> Subject: Re: [Amps] SB220 Meter blown
>>
>> WRONG.
>>
>> .6 or .7 or slightly more during an "event" volts is still
>> enough to blow up the meter.
>>
>>
>> Thanks
>> 73
>> Jim W7RY
>>
>> On 5/15/2020 4:08 PM, Adrian wrote:
>>> from ; http://www.somis.org/sb-220ci.html
>>>
>>> "6. Meter damage can be avoided by connecting two, ordinary
>> = or >1A,
>>> any PIV, silicon rectifiers across the terminals on each meter. The
>>> two diode arrows should point in opposite directions."
>>>
>>>
>>> On 16/5/20 1:10 am, Artek Manuals wrote:
>>>> I am resurrecting a SB220 that had an apparent classic Grid/plate
>>>> short event. One of casualties appears to be the right hand meter
>>>> movement. This is the one for Grid Current and HV. I am pretty far
>>>> into it and the meter movement measures open , the internal shunt
>>>> resistor is blackened ( and open?)
>>>>
>>>> Anyone got a surplus movement from a parted out unit?
>> Nothing found
>>>> on evil-bay at the moment
>>>>
>>>> Dave NR1DX
>>>>
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