Re: Electric circuit question

On Wed, 24 Oct 2007 01:39:44 GMT, Brian Whatcott
<betwys1@xxxxxxxxxxxxx> wrote:

On Tue, 23 Oct 2007 21:24:24 +0100, Goofball_star_dot_etal
<who@xxxxxxxxxxxxxxxxxx> wrote:

On Sun, 21 Oct 2007 14:58:03 GMT, Brian Whatcott
<betwys1@xxxxxxxxxxxxx> wrote:


Do you think I need supression capacitors to protect the
solid state control circuits of the autopilot?

Yes, I say again, yes. I say again, yes.

"Listen Very Carefully, I Shall Say This Only Once"!

Or no.. for the obvious reason that the circuit is so designed to
switch the motor on and off indefinitely and handle the transients.
Typically such a circuit might use a H-bridge of hexfets with integral
"freewheeling" diodes that clamp the voltage close to the supply
rails. The microswitch and diodes is a separate issue. If space is an
issue for a cap. I propose shunting the diode with two zeners in
series so that combined they drop more than the supply voltage in one
direction and more than the 'main' diode in the other.

Well, I listened carefully. What you seemed to say is, "You don't need
transient suppression" "And by the way, two zeners back to back is
the way to get suppression."


No, I clearly said the *autohelm* did not need additional protection
with the addition of a microswitch.

The microswitch and diode might benefit from transient suppression and
it would be good practice. However it would not be an *expensive*
disaster if the microswitch wore out or the diode failed. My gut
feeling is that a small 12v dc permanent magnet motor has low
inductance and therefore low energy stored in the magnetic field so an
unprotected microswitch/diode might last a lifetime in this particular
application. I have a motor from an old autohelm that I will measure,
since I think they could draw a lot more current than the estimated
value.

And I say in response: if you don't want to suppress a fast switch
breaking a motor drive, fine by me.
But if you want to use just a single zener, that might be quite a
useful way to go, if you size it right.


Yes a single zener would do the job but sizing it right might be
result in an expensive zener. Shunting a cheap ordinary diode with a
small zener means the zener anly needs to snub the stored energy in a
very short time, not pass the maximum motor current for perhaps a
second or so in the forward direction. Using two zeners avoids the
possibility of a small zener sharing some of the main forward diode
current and the two zeners, could in fact be placed either back to
back or in the same direction.. or you could use a shottky diode for
the main diode and a single zener. In any case most combinations are
small and quite cheap so why not do it.

.

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