From: firstname.lastname@example.org (N. Thornton)
Subject: Re: How to make/solder a circuit fast?
Date: 25 Sep 2002 07:48:21 -0700
References: <email@example.com> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
NNTP-Posting-Date: 25 Sep 2002 14:48:22 GMT
email@example.com (The little lost angel) wrote
> On 24 Sep 2002 08:59:32 -0700, firstname.lastname@example.org (N. Thornton) wrote:
> >One of my classic stories :) A light bulb will indicate voltage and/or
> >current, and can be used in series with something to prevent it
> >shorting its supply, which very much helps to find out where the fault
> >is when all that happens is the fuse pops.
> How does it do that?
> I've seen a strange setup that the tech guys rigged up and used in my
> previous company which dealt with tech stuff. There's a light bulb
> connected to the AC main, a normal household wall switch box with two
> They connect this to a equipment reported as faulty, switch it on,
> flips the two switches in some sequence. And announces if the
> equipment was shorted internally or not by looking if the bulb lights
> or not.
Yup. Put a mains bulb in series with a shorting mains thing, and the
greatest current that can flow is when the bulb lights up fully. Same
with what I did, the 240v bulb limited the current flow to the 300v
circuit, avoiding fuse popping, and allowing the circuit to partially
function so I could test it enough to fix it.
I've even seen faulty equipment kept working using that method. Crude,
but if your compressor develops a shorted section you dont want to let
your food stocks thaw while waiting for a replacement unit. A big bulb
will keep it running temporarily and save all the food.
The bulb doesnt drop enough voltage to stall it on good winding
sections, but when the short comes along once every revolution the
bulb prevents anything bad happening, so it keeps going.
The bulb reduces the the motor starting power, so the sompressor has
to be run continuously, as it can't restart on its own. A good kick
gets it started :)
> >I would check out the seller profiles carefully. I'm not that
> >impressed with online auctions.
> Generally MOST of them have excellent profiles.
> Unfortunately I also heard from my friends that often if a seller gets
> more than what he expects, or a small problem crops up, they "buy"
> good rating from the buyer by offering each other good ratings and a
> compromised solution.
I've had quite a few problems with online auctions, so I only buy if
its so cheap its worth the risk.
> I took a Fluke and connected to the driver output. of course it's a
> DVM but it also had a Hz function. 0 Hz, does that means it's not a
It all sounds encouraging. If the fluke will accept a wave with 0v and
12v levels, which it almost certainly would, then I would think it
does. Sounds like youve got an easy life :)
> >Mmm, looks like you're right. Of course it isn't quite that
> >simple....groan... a diode will have leakage curent, and only if that
> >is significantly smaller than the leakage of the other bits will the
> >reg & LED see very little reverse voltage. But that is most likely so,
> >so probably would do it yes.
> How do I choose a diode? I wouldn't want to interfer with the forward
> flow at all, so I have to pick a low forward drop. The fan is expected
> to draw a nominal 0.3A but I expect it to peak at double that during
> startup, so 600mA.
> Assuming I pick a 1V forward drop, the diode would need to dissipate
> 0.6W rite? So a 1W diode should do no? And I should pick as low a
> forward drop diode as possible since higher forward drop means more
> power dissipated no?
You're dropping more than 1v anyway in the R and reg, so a 1v diode
drop is no problem. Any diode will do, as long as it can do the 600mA.
If its in the airflow I would think a 500mA diode would be happy. As
we can be confident-ish that youve got a linear fan supply, I guess
you dont need a diode after all.
Diodes are forward current and reverse voltage rated, power figures
arent usually quoted for rectifier diodes.
> >> I've also read that capacitors are used as
> >> buffers in passive PFC which mentions inductance?
> >OK you got me puzzled there. :)
> When I was reading up on Power Factor Correction, one of the things
> that came up was that capacitors can be used to provide PFC. Since PFC
> was explained with things doing with inductance and what not, I
> thought maybe the capacitors would buffer against inductance?
OK thats true when the inductive load is driven by sinusoidal ac, eg
mains. But in a PWM things run differently, youve got a roughly square
wave drive. Putting a capacitor on the load would just draw a large
current spike every cycle the supply switched on, and that would
liekly kill the supply.