Jon Elson wrote:

Jeff Wisnia wrote:



snipped




I pulled the trigger and (as Claude Rains put it) I was shocked, just shocked, to
see it displaying 460 amps.


I have some doubts about that, as the transformers of these guns radiate
HUGE
fields, and may have affected the clamp-on pickup coil. You might hold
the clamp-on
next to the soldering gun, but without the probe clamped around the tip
wire, and
see if you still get a reading. My guess is you will get a substantial
reading from it.

You know, I thought the same thing when I was taking the current reading, so I moved
the closed amprobe jaws around the outside of the secondary "U" and the tip and read
zippo on the 1000 amp range I was on. Then I ran off work and wrote my post about the
460 amp measurement.

I thought about it during the day, and when I got back home, the first thing I did was
measure the voltage on the tip clamp bolts securing the tip to the "U". It was 0.25
vac as soon as I pulled the trigger, rising and stabilizing at 0.32 vac when the tip
heated up. I measured it with a digital voltmeter and double checked it with my scope,
which confirmed the expected sinusoidal waveform

I took a look at the Weller's nameplate and realized my memory was off a bit. It is
rated as a 200 watt iron, not 250 watts as I said in my post.

So, 460 amps and a third of a volt makes 150 watts or so, close enough for me. I
suppose I could have measured the power draw from the 120 vac line, but I think it's
time to quit playing mad scientist. I'm sure the "remaining" 50 watts is heating the
primary winding, not to mention all the power being gobbled up by that little
prefocused light bulb. G




I'd be willing to believe currents up to maybe 100 A for a 250 W gun,
but not much above
that.

I'm open to a wager (for our favorite charities) on that statement....Wanna play?

The problem with the blade welder is the steel is not such a
great electrical conductor,
especially when heated red hot. So, the blade welder needs to deliver a
lot more voltage
across the blade to work.

I certainly will buy that!

The soldering gun is heating a very short
length of copper
bar to a much lower temperature, so the voltage drop must be quite a bit
less, a fraction
of a volt. The blade welder probably needs several volts open circuit,
and at least a
Volt or two when doing the weld. Two volts at 460 A would require
almost a KW.
460 A at half a volt would only require 250 W, so maybe your numbers are
OK, if
the complete circuit of the gun, including the one turn coil in the
transformer
secondary only drops half a volt. But, then it would need a resistance
of 1 milli-ohm,
which is AWFULLY low. I can't believe the resistance of the tip, with
two pressure
joints, is that low.

I think it could well be...The resistance (cold) of No.12 wire is 1.6 ohms/1000 feet,
so the four inches or so of copper tip on my Weller would have a resistance of only
about half a milliohm. (And I bet if I actually measured and calculated the tip would
be closer to a piece of No.10 wire, so even lower.) The pressure contacts are brass on
copper, socked down damn hard. Consider that...'eh?

Jeff
--

Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"If you can smile when things are going wrong, you've thought of someone to blame it
on."