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[Amps] Re: diode and glitch resistor

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Subject: [Amps] Re: diode and glitch resistor
From: (2)
Date: Sun, 25 Aug 2002 07:21:22 -0700
>2 wrote:
>>2, 10-ohm, 10Wers in series would be enough for glitch protection,
>In the Europe the "best buy" are the 14W Welwyn series, as shown in 
The critical thing for a WW R in glitch service is V-gradient.  10w,  14w 
and 25w units are c. 50mm long.  Thus they have similar glitch protection 
ability.   However, glass-coating appears to be superior to molded 
[ceramic] in glitch service.  

>10-ohm resistors of this type are very comfortable at <1A intermittent 
>service - but they do run hot! Two or three in series make a nice glitch 
>resistor for amps of the 1-1.5kW class.
>These resistors have a nominal voltage rating of 750V, but that's the 
>steady-state voltage limit for the higher-resistance values. In practice 
>the 10-ohm types will withstand their share of 3-3.5kV in a "crowbar" 
>glitch test, for as long as it takes to turn the power supply off.

The number of Joules being discharged is another factor.  
>> but a huskier, 3A diode would still be needed.
>This diode only functions during a glitch - in other words, any severe 
>current surge (and let's not ask why :-)   The maximum possible surge 
>current is limited by the glitch resistor, but it's still about 10x the 
>normal current rating of a 3A diode, so we're relying on the surge 
>current rating of the diode.
Yes, a 200A-pk rating is more desireable for glitch service than a 50A-pk 
rating.  A glitch diode or a glitch resistor is too big if it wont fit in 
the box.  

>What makes me slightly uneasy is that the surge current rating comes 
>with a specified time limit. If the surge is larger and/or longer, the 
>diode will fail completely. And then we can only hope that diode will 
>fail short-circuit... which it usually will, but obviously this isn't a 
>property that the manufacturer guarantees.

I have seen 0.5A diodes disintegrate.  When in doubt, spend the extra 10 
cents for the next larger size.   Glitch diodes are like seat belts in 
automobiles -- cheaper is not good.
cheerz, Ian

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,  

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