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. > > > > -- > AF mailing list > [email protected] > http://af.afmug.com/mailman/listinfo/af_af.afmug.com > > > > -- > AF mailing list > [email protected] > http://af.afmug.com/mailman/listinfo/af_af.afmug.com >
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