school around here is a joke. Starts after 8 and out by 11:30, 4 days
per week. Why are my taxes being wasted like this? Huge big busses,
bussing all these kids for fewer hours in class for a week than I had in
a day. I didn't learn shit then, how can these kids today be learning
anything worthwhile at all?
On 2/7/24 09:15, Cameron Crum wrote:
We had a lot of hijinks in the lab as well. Today though, if there was
a pop like that, half the class would have to go to trauma counseling
and need a safe space to do their work the rest of the year.
On Wed, Feb 7, 2024 at 10:39 AM Ken Hohhof <[email protected]> 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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