take a large capacitor, bend the leads to just hover over the body,
charge it up to a hundred volts and then set it on your desk on top of
things you don't want others to touch
On 2/7/24 08:38, Ken Hohhof wrote:
When I was in college we had something called cooperative education or
“coop jobs”, basically a semester in industry as a paid intern. At my
coop job you typically arrived at 8am, grabbed coffee from the
machine, and turned on the power strip at your lab bench.
They never tired of sticking an electrolytic capacitor into one of the
outlets on your power strip so it would explode.
Almost as much fun as a banana potato.
*From:*AF <[email protected]> *On Behalf Of *Bill Prince
*Sent:* Wednesday, February 7, 2024 10:16 AM
*To:* [email protected]
*Subject:* Re: [AFMUG] OT fun trick running a variac in reverse
I always used a potato.
bp
<part15sbs{at}gmail{dot}com>
On 2/7/2024 7:48 AM, Ken Hohhof wrote:
Variac is just an autotransformer with a variable tap. Not
surprising you can swap input and output. Watch out for voltage
ratings though. And wrong gender plugs.
I thought it was potato in the tailpipe.
---- Original Message ----
From: "Cameron Crum"
Sent: 2/7/2024 9:15:03 AM
To: "AnimalFarm Microwave Users Group"
Subject: Re: [AFMUG] OT fun trick running a variac in reverse
Ah the old 'variac in reverse' trick, similar to a banana in the
tailpipe.
On Tue, Feb 6, 2024 at 6:05?PM Chuck McCown via AF
<[email protected]> wrote:
Was testing a repair to a 480 volt induction heater today.
One of our employees decided to blow the dirt out of it, took
the cover off and got a copper tube across an inductor to case
ground. It was probably 800 VDC at that spot. Discharged the
capacitor. Sounded like a gunshot. Tripped a 125 amp 480
volt breaker at our power service panel. Turning it off at
the front switch just turns off the control circuitry.
Everything else is hot unless you kill the breaker on the back
of the unit. I think the kid is still shaking.
In any event, took the power supply to the lab. Used a variac
to put 0 to 130 volts across each leg with a clamp on volt
meter on it as I tested. Never got past 10 volts and was
drawing 3-5 amps. 3 phase bridge rectifier was totally
shorted out. Exactly as expected. These things take raw 480
VAC, rectifier, 800 VDC cap and then on to the IGBT
transistors that chop it into ac etc. I was hoping it was
just the rectifier.
So we got the replacement today. Put it in and started
testing. No current, all the way up to 130 volts. But the
cap was charging. So far looks good. Told my sons to take it
back and hook it up to 480. My son Frank said “just reverse
your variac and use it to step up”. I initially refused to
believe it would work. Then I thought through it a bit and
decided that it actually should work.
I started with the variac set at 130 volts output. Feeding
120 into the output gave us about 110 on the input (that was
connected across one phase of the induction unit). As I
turned the variac down the voltage went up. I got to 380
volts before we started smelling that wonderful “Allen
Bradley” wafting through the lab and the variac started
buzzing pretty bad. I think I got it down to about 60 volts.
But we got it high enough out (in?) that the control
transformer made enough juice to power the control circuitry.
It appears that the machine is fixed. Of course until we
actually try to use it we will not know for certain.
But the TL;DR is: You can run a variac backwards and make
higher voltages.
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