>Date: Sat, 02 Jan 1999 17:22:57 -0700
>Subject: Re: Sam's High-Energy Double Car Ignition Coil Driver...
>From: Tesla List <tesla-at-pupman-dot-com>
>Original Poster: wwl-at-netcomuk.co.uk (Mike Harrison)=20
Thanks a lot for the reply, Mike!
>>>I would use a space heater or electric oven element instead of those
Hehe, done it! I got a 1.1kW oven heating element from a
combi-microwave oven and now I use it to limit the power into the
system. Arcs got louder and hotter, but, as expected, there was no
increase in spark length. The heater gets HOT!!! (somehow that sounds
like a stupid thing to say… I’ll let it trough anyway:-)…
>It varies enormously with filament temperature. At 'normal' operation
>you can calculate it easily. Assuming it's rated 240V, 500W :=20
>Current is 500/240 =3D 2.08A, R =3D V/I =3D 115R=20
>It may however be a quarter of this when cold.=20
Thanks, that’s useful stuff!
>> So, what rating should the resistances be for maximum performance?
>Just high enough to stop blowing the fuse/circuit breaker/user!
Hehe that’s true… Got any suggestions of what I could use?
>No difference - series or parallel will both distribute the power over
>several devices. For series, the resistance is the sum of the
>individual resistances, For parallel resistances of the same value,
>divide the value by the number of resistors.
>To increase the power rating without changing the resistance, you need
>to use a series-parallel combination - e.g. 2 pairs of paralleled R's
>in series quadruples the rating.
Yeah, but I want to DECREASE the resistance, remember? I want to have
the voltage drop across them as small as possible in order to obtain
maximum spark length… A good way to look at it is if I had a
low-resistance wire in place of the resistance. That way ALL power
would go into the system and the output and efficiency would be
optimal. But my system isn’t current limited so I need something that
will let trough as much voltage as possible, using up as little energy
on it as possible and still limit the power. And inductor would be
ideal but I can’t figure out how to make a suitable one. At 1.5kW
it’ll be damn big too, so I’m not sure I want it either.
>Remember that a 20W resistor will produce the same amount of heat as a
>100W one when carrying the same power
Yeah, but it is not carrying the same power. A 1.1kW oven element
will let trough 1.1kW (which is what it needs to operate at full
power) and a 100W lightbulb will only let trough 100W, in order to
light up full brightness (more on this later).
the only difference is how
>well the heat is removed from the resistance element, which determines
>the operating temperature.
True if the system limited the power by itself, which is not the case.
The easiest way to increase the power
>handling is to use fan cooling. The commonly available 'gold' coloured
>metal clad resistors are fairly cheap up to 100W, and can be bolted to
>a substantial heatsink and fan-cooled for high powers - you can
>probably at least double the rating with fan cooling. Be careful not
>to push them too far, though. internal thermal resistance means that
>if you push them too far, they can get too hot internally however much
>cooling is used - I've seen the ends violently blow off these when
Well, now that I run 1.1kW those become quite useless… And they cost
a lot too, so I’ll stick to the oven element, which, also gets hot,
regardless of the massive heatsinks its mounted on.
>Remember of course you don't need to buy ready-made resistors - a few
>feet of iron wire, fan cooled, should be able to handle a few hundred
>I tried this arrangement a while ago - I think it produced bright
>sparks about 1.5" long. The dimmer setting makes little difference - a
>variac is really needed to regulate output. I think the series cap was
Ummm… I wonder why mine didn’t work. I know the coil worked because I
took it off a working system. I know the dimmer worked because I used
it with a lamp and it did the job. I also know the cap worked because
it was new. And yet the coil would only produce a small spark when I
plugged it in. The dimmer I used had a resistance to overheat and blow
too :-( It was a 750W dimmer for normal lights. Nice because it had
filtering caps and chokes… Does the dimmer need to be for Halogen or
normal lights? They are different, you know.
>If you have 800W of lamps lighting brightly, you certainly DON'T have
>anything like 800W into your circuit - the lamps are taking most of
Not really, Mike. You see, as ANY coiler using oven elements to
limit power to his pig will tell you, those things get HOT when you
run them. The fact that the lamps are lighting full brightness means
that there is enough energy trough them to cause them to light up full
brightness, in their case, 800W. So, if they were just across the
mains they would draw 800W to themselves and use it to light, BUT I
have my driver in series with them and all the power that gets to them
has to go TROUGH the driver first, which implies that the full 800W
are going trough it. Of course this also means that the driver is very
inefficient, taking in much more than it outputs as most of the power
is wasted at the lamps. I am working on it, BUT I hope you get my
point that the driver is really running at 800W.
Oh, BTW, it actually runs 1.1kW now that I have an oven heating
element replacing the lamps. More on that later…
Any more help/comments would be greatly appreciated.
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