From: Kevin McMurtrie
Subject: Re: Ignition Coil High Voltage Power Supply
User-Agent: MT-NewsWatcher/3.2 (PPC Mac OS X)
Date: Sun, 12 Jan 2003 09:55:18 GMT
NNTP-Posting-Date: Sun, 12 Jan 2003 01:55:18 PST
In article ,
Kevin McMurtrie wrote:
>In article ,
> Thoremail@example.com (P!erCer) wrote:
>>John Larkin wrote in message
>>> >> A) Is the output of my coil AC or DC? How do I check? I want pure or
>>> >> pulsed DC.
>>> >If You use an automotive ignition-coild and pulse the current on the
>>> >primary, I'd say is is DC. you shut off the current, and the coild makes a
>>> >flyback-voltage, which is in one direction only. For it to be AC would
>>> >require some kind of capacitor it could swing/oscillate with (sp?)
>>> A regular points-and-coil ignition system (or even a fancy CD type)
>>> produces a damped sinusoid every shot. The DC component is nil. The
>>> output of any transformer averages 0 volts, ie, has no DC component.
>>Hmm...I don't really see why.
>>The transformer is driven by current oscillating from 0 to 12 volts.
>>From what I understand, the transformer's function is to amplify this
>>voltage by a certain number. No matter what the voltage is, I will
>>never get below 0. The output may indeed be sinusoidal, but the
>>offset will place the "average" point into a strong DC range.
>No, the average DC voltage output from a transformer is always zero.
>The primary voltage is hardly 12V too. It's -12V while it's charging,
>about +300V during flyback, some bouncing, and then eventually 0V. It's
>at -12V a lot longer than +300V so it all averages out to zero, not
>counting some wire resistance that can throw the average off a bit.
Forgot to mention -
If you use a MOSFET, you need to protect the gate with a diode that
prevents <0V on it. When the voltage on the primary increases,
capacitive coupling pulls the gate high. No big deal. The MOSFET will
turn itself on and self regulate the rise of voltage. When the primary
voltage falls, capacitive coupling pulls the gate low. There's no self
correction for that. It will blow the 555 chip and/or the MOSFET. The
negative current from the gate can be as high as 500mA.
If you're seeing random burnouts, it's because you need the diode.