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Re: [Amps] SB220 step start blows fuses

Subject: Re: [Amps] SB220 step start blows fuses
Date: Fri, 07 Jul 2017 04:45:48 -0500
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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 <>:

------------ 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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