AVR-Chat

Yeah, I stupidly blundered into that one on the capacitor, all right.   But,
I'll bite.   I don't have any diodes.   The power source is not an AC power
supply but a 12 VDC battery (a large 700 amp-hr boat system) and I don't
bother with reverse polarity protection (I prefer being careful to suffering
the voltage drop).  It's just battery to fuse to switch to regulator, with
cap on the input to regulator.    Would a current-limiting resistor in
series with the cap be a good idea to help protect the switch?   Steve

 

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of David VanHorn
Sent: Tuesday, February 10, 2009 10:18 AM
To: AVR-Chat@yahoogroups.com
Subject: Re: [AVR-Chat] switch rating

 

> In my case the switch is input to a regulator which is supplying DC
voltage
> to a microcontroller and its peripheral chips. The regulator has the usual
> input capacitor on it. Does this qualify as a resistive load? It would
> seem to me that it does.

definitely not!

Calculate your inrush current, assuming the cap is at 0V, and using
its ESR as it's impedance, assuming that closure happens at the peak
input voltage, with the transformer DC resistance in series with it as
a voltage source. (Worst-casing it)

The first few uS will greatly exceed the switch ratings.

In practice will this matter? Maybe not, but these effects are real,
and should be looked at when designing a system.
The inrush current may pit the switch contacts and cause premature
failure after a few thousand cycles.

Also, you might look at your diode current ratings vs the size of the
regulator filter capacitor.
When the cap is mostly charged, and the system is under full load, the
diodes only conduct part of the time. (conduction angle)
This means that the peak current is much higher than the average, if
too large a filter cap is used!
You can blow your diodes if their repetitive peak rating isn't up to
what is actually happening.

In the hobby world, a given solution "works", if you have a loose
enough definition of "works".
:)





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