[Mgb-v8] Voltage stabiliser

[Smit, Theo]

I've built a couple of these using the LM2940 and you absolutely MUST
have at least the recommended capacitance on the output, or else the
output voltage will be incorrect. Low-dropout regulators (unlike the
old-school 78xx regulators) are not unconditionally stable so you need
the external capacitors - it's the price you pay for the low-dropout
function. The older regulators such as the 7810 require about two to
three volts of headroom to regulate properly (i.e. the ignition voltage
has to be at least 12 and preferably over 13 volts), and that's close
enough to the normal range of the car's electrical system that it's
better to use the 2940. A good feature of the LM2940 is that these parts
were developed for the automotive industry and they are designed to be
proof against most known electrical system issues in cars: Doubled
battery voltage (up to +26 volts), reverse-polarity (-12 volts),
alternator load dump (a 100 volt transient) and other inductive-load
transients.

Barrie, I have a couple of those instrument stabilizer cans at home...
I'll have to measure one up.

I actually have a PCB layout finished for a 'proper' replacement of the
stabilizer that includes all required components plus a low-voltage
alert LED, which could be used to replace the charge light. This is
primarily aimed at the Tiger market, where the OEM generator gets
replaced with an alternator that may or may not have a charge-light
indication output. So the LED is 'on' unless the ignition voltage is
over about 12.7 volts.

I just have to get around to ordering the boards and building some up.
The electronics fit in a can that is larger than original, but mounts
and connects using the OEM hardware.

Cheers,
Theo

> -----Original Message-----
> From: mgb-v8-bounces at autox.team.net
> [mailto:mgb-v8-bounces at autox.team.net] On Behalf Of Barrie Robinson
> Sent: November 28, 2008 2:26 PM
> To: mgb-v8 at autox.team.net; mgs at autox.team.net
> Subject: [Mgb-v8] Voltage stabiliser
>
<snip>
>
> It would seem that the circuit as described in the article is
> a bit short and would be subject to erratic behaviour under
> certain circumstances.  My circuit has double bypass filters
> using both electrolytic and ceramic capacitors.  The
> components are so damn
> cheap so overkill is not a problem.   However, I have to make a unit
> to fit inside the original casing and I do not have a spare
> close by.  Can anyone give me the inside dimensions of the
> casing please?.  Or, if anyone has a dud one I would gladly
> give you a couple of dollars, plus postage, for it.
>
> Regards
> Barrie
>
> Barrie Robinson
> (705) 721-9060
> http://www.britishv8.org/MG/BarrieRobinson.htm
> http://www.britcot.com
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