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Registered Member #3883
Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87
I've noticed that all coil driver designs out there are based on NPN transistors, leaving the coil "hanging" on the positive. Wouldn't it be better to use a PNP so that the coil gets clamped to ground?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
For semiconductor physics reasons, NPN transistors and N-channel MOSFETs perform better than their complements. (Electrons have a higher mobility than holes.)
Nowadays you can buy P-channel devices that are supposedly exact complements (like the IRFP9240 vs. the IRFP240) but the performance difference just appears in another way: the P-type ones need a bigger silicon die, so they are more expensive and slower.
Back in the old days, only PNP transistors were available, so the circuits were all built "upside down".
Registered Member #3883
Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87
I've noticed that PNPs and PFETs of the same rating are more expensive, so that might be part of the reason. This might also make everybody think NPN from the start rather than think of what they should do and choose components based on that.
But from a strictly theoretical point, am I right? Wouldn't it be better and safer to keep the coil clamped to ground?
Registered Member #3883
Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87
I'm not sure what you mean by bypassed here. My concern was the lack of common ground when hooking this up to further circuitry (like a Tesla), you're saying one could use a common positive with the same result?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I'm saying that if the power supply is properly bypassed (google it
then positive is the same as ground, only 12V further up, or 24 or 48 or whatever. Any nasty RF that gets dumped into the positive will flow harmlessly to ground through the bypass capacitors.
Anyway, I'm not sure what you mean by "coil". Do you mean an ignition coil where the high voltage secondary has one end connected to the positive end of the primary? Not even all ignition coils are like that, some have a separate ground terminal for the HV winding.
Registered Member #3883
Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87
Yeah, I see what you're getting at. Plus and ground are not absolutes so a circuit can be built "upside down" without any ill effects. Clever. It's easy to get stuck in a mindset, I was thinking of ground as an absolute. And since I am powering a tesla coil I wanted both connected to the same ground. But the tesla doesn't need to be grounded, and probably shouldn't either, right? You just need a ground plane for the top load to create the proper capacitance.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You'll need two wires connecting the HV power supply to the Tesla coil tank circuit. One of them obviously comes from the ignition coil's high voltage terminal. The other one can come from either the positive supply or the ground rail of your circuit. It should make no difference which.
The bottom of the Tesla coil secondary will need to be connected to a good RF ground.
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PNP (or P-FET) better for coil driver?
