From: firstname.lastname@example.org (N. Thornton)
Subject: Re: How to make/solder a circuit fast?
Date: 24 Sep 2002 08:59:32 -0700
References: <email@example.com> <firstname.lastname@example.org> <3D864834.email@example.com> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <email@example.com>
NNTP-Posting-Date: 24 Sep 2002 15:59:32 GMT
firstname.lastname@example.org (The little lost angel) wrote
> On 23 Sep 2002 04:56:22 -0700, email@example.com (N. Thornton) wrote:
> >Very useful nonetheless. And it'd only cost you a soundcard. They also
> >have no response at dc, they'll go down to around 20kHz.
> No response at DC? Does it mean if I plug in something that's stable
> at say 0.75V, I would see nothing at all?
Pretty much, if its 0.75v dc, the screen will show zero. Soundcard PC
scopes will show you the waveform correctly, at least upto their
frequency limit, but they dont show the dc offset. So a square wave of
0v and 5v would appear the same as a square wave of -2.5v and +2.5v.
They are far from ideal, but whats a soundcard cost, 6GBP for a new
one. Still pretty useful.
> >With a bit of thinking one can do a great deal with primitive tools. I
> >remember when I needed to fix something and had no multimeter with me,
> >so I just used lightbulbs instead. :)
> ???? How? Different voltage rating bulbs and see which's brightest? :P
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.
A 240v bulb extinguishes at about 120v, so brightness gives you a
rough idea of what V and i is going on. Choosing the right wattage
bulb that lights fully gives you a rough reading of what current is
flowing. A 12v bulb is useful for lower V circuits. No-one else
thought it was possible, but the job was indeed done. IIRC it was a
300v power circuit that was shorting its supply, and I had no
instruments with me.
This kind of lateral thinking is seen all the time in poorer
countries, and in electronic history, where equipment was either non
existent, very expensive, or of very poor performance. The first
oscilloscopes were mechanical, and had frequency response measured in
Hz rather than kHz, let alone MHz.
> >They do turn up, probably auctions. But there are reasons people put
> >things in auctions.
> I was worried about this, though I'm eyeing those sub $20 scopes
> onebay, I mean, for $20, even if only one trace is working, wouldn't
> really hurt my pocket.
I would check out the seller profiles carefully. I'm not that
impressed with online auctions.
> >A fan motor is wire wound, so its inductive. If its driven off a PWM
> >supply, it will produce an inductive kick voltage every time the
> >supply goes off, lots of times per second. And that kills LEDs. Thats
> >the possible problem.
> Hmm, but the fan's not directly tied to the LED no?
> The output from the driver is paralleled to my 5V regulator, and to
> the fan. Wouldn't the regulator buffer the LED from any backflow.
Maybe. The reg would be the first in line to die I guess. If it is a
> Would adding a diode or capacitor helped? Since diodes prevent current
> in the undesired direction.
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.
> I've also read that capacitors are used as
> buffers in passive PFC which mentions inductance?
OK you got me puzzled there. :)
BTW, did anyone mention the 1 transistor current regulator? It might
compare favourably to a little volt reg IC.