[Amps] SB220 step start blows fuses

Bill Turner dezrat at outlook.com
Thu Jul 6 21:07:36 EDT 2017

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On Thu, 6 Jul 2017 19:14:06 -0400, Mark B. wrote:

>Long story short, light bulbs in series with the main voltage act as perfect 
>current limiters.  


No, a light bulb is NOT perfect and I'll tell you why.  

The cold resistance of a light bulb is much lower than when it is hot.
Thus the 100 watt light bulb which draws only .83 amps when hot will
draw MUCH more than that when cold and therefore may very well blow
your fuse anyway. 

If you are going to use a light bulb, first measure its cold
resistance and calculate the initial current draw from that. Keep the
initial current flow below the rating of your fuse (assuming it is a
line fuse) and you'll be OK.  Basic Ohm's law. 

The only problem with using a low wattage bulb like that is that it
will likely light to full brightness whether there is a short or not. 

Because of all the above, IMO, a better way is to use a Variac to
slowly bring up the voltage while monitoring the current. Variacs are
widely available on eBay. Every ham shack should have one. 

73, Bill W6WRT

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