Based on this scenario, I think you are going about powering your circuit t=
he wrong way.

You should be filtering the voltage from the magneto so that high speed spi=
kes don't pass through the filter, and then use a voltage regulator like so=
me of the ones designed for automotive use that can withstand hefty input n=
oise pulses to get your desired output voltage.

The design you have shown IS going to supply overvoltage to the load at som=
e point, destroying the ongoing circuit.


> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of Electron
> Sent: 28 February 2015 14:38
> To: Microcontroller discussion list - Public.; Microcontroller discussion=
 list -
> Public.
> Subject: Re: [EE] is base-emitter resistor really necessary?
>=20
>=20
> PS: now I recall that R 2K was not chosen just "empirically" of course, b=
ut
> (although I'm a hobbyst) I made some engineering efforts on it.
>=20
> Even in the worst case (400V) with 2K we will have 200mA flowing through
> the base of the PNP transistor. This is well within its capabilities, as =
it is
> within the capabilities of the NPN transistor (300mA max collector curren=
t).
>=20
> The Icbo of the NPN transistor was sheeted as being just 0.1uA max (with
> Ices 0.5uA), so I thought that a base resistor in the PNP one wasn't nece=
ssary
> (however, I left pads of it on the PCB). But as Vasile suggests, I have p=
robably
> underestimated EM coupling or miller effects. Let away that I'm now afrai=
d
> that those values of Icbo and Ices that datasheet gives will be very diff=
erent
> in case of higher ambient temperature.
>=20
> DC-gain of the PNP transistor is ~25 and of the NPN is ~100.
>=20
> Now I think that those pads must be really populated.. but how do I calcu=
late
> the value, given the "dynamic" nature of use of the circuit?
>=20
> Also, base-emitter resistor on the NPN transistor wouldn't harm, but it's
> probably really unnecessary as the PIC output forces it to logic "0" or "=
1"
> (sure, there's the 200K pull-up, but if I put a base resistor into the ba=
se-
> emitter of the NPN transistor, then I can say goodbye to the nice
> "bootstrapping" feature that powers my MPU even when starting from total
> inactivity).
>=20
> With kind regards,
> Mario
>=20
> --
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> View/change your membership options at
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--=20
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