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NNTP-Posting-Date: Mon, 28 Oct 2002 23:29:44 -0600
From: "John Fields"
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Subject: Re: V regulator input cap size?
Date: Mon, 28 Oct 2002 23:25:05 -0800
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> As to the zener - I don't see it being worthwhile.
> You are quite right that if you disconnect the load
> from the regulator the thing will be exposed to >35
> volts. But the OP intends to build a single circuit
> and power it from a permanently installed xformer.
> The probability is very low that he will build the
> regulator separable from the load. As to current
> lower than expected - it would have to drop below
> 12 ma to expose the 7812 to >35 volts. Again,
> extremely low probability. Finally, why a zener?
A Zener will look like a very high impedance and will dissipate no power
until the voltage impressed across it is >= its knee voltage.
> A 1.5K resistor to ground after the voltage dropping
> resistor will keep the voltage under 33 at the
> input to the 7812 with no load. At full load,
> the 220 resistor would drop about 21 volts, still
> leaving the 7812 with a bit over 16. If there was
> any chance of load disconnect, I'd use the second
> resistor instead of the zener.
Unfortunately, using a resistor will _always_ incur a power penalty. Using a
Zener will not, as long as the Zener voltage is not exceeded.
> The nice thing about your circuit with the cap in place
> of the first resistor is that it avoids all of the above
> paragraph - no need for the zener or a second resistor.
Thank you. I agree with your assessment of the lack of need for the Zener,
but only as long as the load current remains high enough to keep the voltage
dropped across the cap high enough to keep the input voltage to the
regulator below its maximum safe spec.
There is something else to consider, however, and that is line transients.
For the price of the Zener, I'd go ahead and put one in just to be safe.
Just to throw some shit into the game, there _is_ the case where the cap
could be placed in series with the primary of the transformer. Because of
the transformation ratio and the resultant lowering of the current needing
to be flowing through the cap, a cap with a larger Xc could be used which
would result in a smaller capacitance. However, the voltage rating of the
cap might have to be raised. It's late and I'm toasted, so I don't really
feel like doing the numbers right now. Maybe tomorrow...
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