Re: Capacitor size
- From: w_tom <w_tom1@xxxxxxx>
- Date: Thu, 30 Aug 2007 16:04:13 -0700
On Aug 30, 8:49 am, Jan Nielsen
<nos...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
I have solved some of the problem with capacitors, its not perfect, but
alot better than before.
I added ....
It can now switch just about everything I have tried, but a rare few
times it still resets.
...
I would be guessing what your schematic is. Below are some
techniques and a concept that others have not discussed.
The relay contact is not isolated from relay coil. Electrically
there is still a connection (high impedance) between both. You must
consider that leakage in your design.
One technique means somehow gets circuits on both sides of the relay
to share a common ground. Yes, I know that may cause safety problems
which is why the common grounding must be designed carefully.
Second, how does the transistor drive that relay coil? For example,
if coil is connected to transistor emitter, then failure is an
option. Better designs connect emitter to the power source or ground;
collector to relay coil. Now the high voltage base to collector helps
isolate the controller from relay wiper. Again, appreciate that the
230 volts on that wiper still connects to relay coil and controller.
That circuit is not obvious - often denied by those who assume relay
coil and wiper are isolated.
Because of how relays work, we often would make dry connections with
other relays, then only power the connection with a special relay that
make a non-bounce connection. Sometimes the connection is made via a
thyristor - that can turn on DC power but cannot make the disconnect.
Disconnect performed mechanically (this assumed DC power). Or even
better, use thyristors that hold off their turn on until voltage is
zero crossing (this assumes AC power).
Another trick is to put a snubber across energizing relay wiper
contacts. A 'softer' make or break means less energy travels through
that circuit that connect relay wiper to coil.
And finally, the driver transistor is driven by a buffer transistor
so that any currents from emitter or collector of relay coil driver
would be connected to power or ground via the buffer transistors - not
enter controller via base (or gate) of buffer transistor. Sometimes
that buffer transistor is an IGFET so that reverse (isolation) voltage
gate to drain and gate to source is more than 20 volts. And or we add
limiting resistors to the controller to base circuit to further
isolate that controller from the relay coil.
Only then are we ready to add capacitors (or inductors) as high
frequency filters to control circuits between controller and relay.
All this because the relay wiper - especially at 230 volts - connects
current to relay coil - especially higher frequency currents.
Just a few solutions that I have used previously off the top of my
head.
.
- References:
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- From: Jan Nielsen
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- Re: Capacitor size
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- Re: Capacitor size
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