I’d be grateful if someone could be kind enough to explain why the mica sheet is required in this design of circuit. [I’m an engineering graduate although that was 20 yrs ago and i’m a bit rusty on electronics these days 
]
From what i can see
a] the mica sheet provides insulation between T2 and the heatsink [surely if the heatsink is taking heat away it’s not required].
or
b] the mica sheet provides insulation between T2 and the heatsink AND the plastic enclosure [i.e. an exact hole is cut for the T2 and the mica sheet should be placed on top of the enclosure and between the heatsink - it’s difficult to see from the instructions diagram the exact location of the mica sheet]. Wouldn’t the plastic melt otherwise… but having said that folk say the heat sink barely gets warm - it would seem thus to be over engineered.
I know the instructions say the dizzy capacitor has to be removed. Why is this as most TAC units say don’t remove the capacitor as it’s not used and can be left in place if there is a malfunction and one has to revert to points. Sorry to be irritating and curious but being an engineer i’d just like to know. Most TAC units don’t even have a heat sink!
I’m returning to ‘old fashioned ignition’ having had problems with several full electronic systems. I’ve bought four of the kits for my motors and am grateful this kit is still on the market.
thanks
Hi,
Just a guess on my part.
I think the back of T2 is metal.
Without the mica this would make a connection from T2 to the heat sink.
If the heat sink touches the frame or body of the car this might cause a short.
Like I said this is just a guess.
As i see it the mica is defeating the purpose of the heat sink. Thanks for the comment but if that was the case then surely a guard should be put round the heat sink e.g. a plastic grid/frame. Instructions do say don’t let the heat sink come into contact with any vehicle metal - why say that if the mica is there to prevent short circuit… or it’s just to be on the safe side?
If the T2 runs hot surely it would be close to melting the connection to the pcb. I can’t see how the mica can even out temp before the heat sink as that’s what the heat sink will do.
I noticed an earlier post which states
vel417
“Most likely, you did not use any heat conducting paste between the mica.
The heatsink must be isolated to avoid a short circuit with the chassis of the car.”
I’m now lost as this supports 'wrong way’s post.
If one puts heat conducting paste on in addition to the mica then one is adding more insulation to the kit. The Hall Effect modules have heat conducting paste as some of them have suffered from heat.
I didn’t want to seem to be asking an irritating question but this doesn’t make sense. IF the mica is there as a safety purpose to stop a short circuit - if one mounts the box which i’m intending to do in the cab under the dash then one can leave off the mica altogether? The heat sink can then do its job - after all the heat sink is almost larger than a PC cpu heatsink… they do run hot!
I’ll just have to try a case of ‘with’ and ‘without’.
The tab of T2 is common with its collector, which is the connection to the negative going lead of the ignition coil. Should that point (the tab/collector of T2) become directly connected to ground the car will not run, and the coil would likely overheat and self-destruct.
Therefore as sound engineering practice dictates, for a commercial product, it is good design to isolate the tab of T2 from the heat sink; and as long as the conductive properties of the mica and heat sink compound¹ were considered as part of the design exercise–which I am certain they were given my experience with Velleman’s products–there are no issues.
As you have described your intentions, and making 110% certain the heat sink is isolated from the vehicle ground, there should be no issues with not using the mica insulator.
¹ - A thin layer of which will greatly improve the thermal conductivity of the assembly–[color=#0000FF]read more here)[/color]
Thank you very much for the reply. The link is a long read but is very helpful and interesting. The use of heat sink compound should perhaps be mentioned in the instructions. For the auto enthusiast there is a good alternative that has high thermal conductivity - Moly grease.
I’m still a little unclear as to the mica insulation reducing the effectiveness of the heat sink. [I follow the safety aspect against a ground]. Is this an engineering compromise? Reading through the other posts it seems that one can barely detect any warmth on the heat sink… without the mica i’m hoping to feel some warmth there?
[quote=“jessejazza”]Thank you very much for the reply. The link is a long read but is very helpful and interesting. The use of heat sink compound should perhaps be mentioned in the instructions. For the auto enthusiast there is a good alternative that has high thermal conductivity - Moly grease.
I’m still a little unclear as to the mica insulation reducing the effectiveness of the heat sink. [I follow the safety aspect against a ground]. Is this an engineering compromise? Reading through the other posts it seems that one can barely detect any warmth on the heat sink… without the mica i’m hoping to feel some warmth there?[/quote]
Engineering compromise is just what it is, and the mica is so thin that it’s thermal insulative properties are relatively small. Put one on a hot plate set to 200°F (~90°C), then press on it with your finger–you will find that it can transfer a great deal of heat, quite rapidly.
I have not run a K2543 on an automobile so I cannot speak to it’s behaviour in that environment. I have however built a coil tester around it ([color=#0000BF]click here for more about that[/color]), and have had the heaksink quite warm at times when using repeatedly long coil “charge” times and very wide gap discharges. T2 remains functional still…
many thanks for your help.
Mustang V8 4.6L… i’m jealous
I’ve got 2 x landrovers, 2 x Reliant Regal 1955 (aluminium bodied), 2 x Heralds. [I don’t have two of everything it’s just how it happened].
[quote=“jessejazza”]many thanks for your help.
Mustang V8 4.6L… i’m jealous 
I’ve got 2 x landrovers, 2 x Reliant Regal 1955 (aluminium bodied), 2 x Heralds. [I don’t have two of everything it’s just how it happened].[/quote]
My first car was a '57 Hillman Minx, followed by a number of Triumph Spitfires and a GT6; then in 1973 a Plymouth Cricket (Hillman Avenger) the last British car I owned–though I would like to have a '65 or so XKE someday.
BTW, here’s a photo of the completed tester:
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