Sb-301

[Garey Barrell]

Dan -

The problem is that when that fuse opens, the full HV AC, PLUS whatever 
inductive kick the transformer may have left, appears ACROSS the little 
fuse that is rated at 125 VAC.  When that fuse vaporizes, as it will in 
a short-circuit condition,  the metal is deposited on the inside of the 
fuse cartridge, and what usually happens is that the whole thing 
explodes.  The circuit is also not as well protected, since the voltage 
continues to arc across any gap that appears in the fusible link.  HV 
fuses are much larger / longer.

A better HV fuse than the 3AG is an inch or so of #30 wire (or whatever 
size will fuse at the desired current) across a terminal strip.   A good 
source is a single strand of wire from an 18 gauge line cord.   This is 
usually around #34.

73, Garey - K4OAH
Atlanta

Drake C-Line Service Manual
<http://hr99.home.mindspring.com/R-4C_Servicez/>



Dan Merrick wrote:

>Chris:
>
>I'm just talking about High Voltage supplies in
>general and not specific to a particular piece of
>equipment. If the HV Secondary winding uses a grounded
>center tap to provide a return for a full wave
>rectifier circuit, the fuse may be placed between the
>center tap and ground. Then a 3AG fuse should suffice.
>You could also put a standby switch in series with the
>fuse to ground if the circuit is not equipped with
>one.
>73,  Dan Merrick -- K8TX
>
>Don't know about the quick and easy but one thing you
>will have to 
>watch 
>is the type of fuse you use if on the HV secondary.
>The normal 3AG 
>glass 
>fuses are only rated for a (?) 125 VAC. There are
>ceramic cased 
>versions 
>which I think were intended for higher voltage
>service. To set  the 
>fuse 
>rating, you would want to:
>1.    Find out the maximum RMS current that you will
>be pulling
>2.    Look on the Internet or some othe source for
>some manufacturer's 
>data sheets and, most particularily, the current/time
>curves.  Then you 
>find a fuse which will 'hold' the current you want for
>infinite time 
>with some safety fudge (say 50%) but not too much!
>
>The light bulb - hummm.  Don't know about that.  Same
>problem with 
>voltage rating.
>
>73
>Bob, WA2CKY
>  
>

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