[Amps] SB220 step start blows fuses

donroden at hiwaay.net donroden at hiwaay.net
Fri Jul 7 05:45:48 EDT 2017

A series low wattage light bulb may not be "perfect", but it IS a valuable
troubleshooting tool IF the user is comfortable working with the  
amplifier's High Voltage interlocks bypassed or even better, all  
safety interlocks and covers still intact.

I have built higher power three phase 480vac "shock absorbers" using  
six 750 watt twistlock bulbs mounted on a 3/4 inch plywood base and  
mounted near the Broadcast TV Transmitter's three phase 24,000 volt  
transformer contactor.

In that instance, trying to find a three phase 480 vac variac would  
have been difficult.

Bill is correct; every hamshack should have a medium amperage variable  
transformer ( variac ) and I would add..... a digital clamp-on amp meter
that can be purchased at Home Depot or Lowes for between $50 and $100.
The ones I saw measure amps, voltage, frequency, ohms, and  
capacitance, plus a "non-contact" AC "circuit hot light to let you  
know if a circuit is powered .
Quite the versatile tool to have around the house for lots of projects  
beside radio.

The series light bulb ( used alone without a variac ) is best when you  
first try it on a WORKING SB220 so that you would have a reference  
current or brightness to compare to.


Quoting Bill Turner <dezrat at outlook.com>:

> ------------ ORIGINAL MESSAGE ------------(may be snipped)

> 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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