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Relay chatter & voltage threshold

?

.

Jan 1, 1970
0
Hi
I have a 12V relay that is fed from the mains via a capacitor power supply,
(cheap but dirty). If I get a brown out my relay chatters, due to the soft
nature of the capacitor supply.

I need to hold the power (12VDC) off whenever the voltage drops to a level
where the relay chatters. I don't have much room, and hope somebody can
suggest a single component solution to my problem. Something like a DS1810
Econo Reset, except with a threshold of about 10V would be ideal.

Any suggestions greatly appreciated

Kez
 
?

.

Jan 1, 1970
0
Ian
The relay is driven from a 2N3094 transistor which is driven from a 16F628
PIC.
the whole thing is a power up one shot timer with some fancy features. It
operates a motor, which can be on the same phase. If a long cable run is
used or there are power fluctuations this aggravates the chatter problem.
Basically power comes on, the PIC closes the relay, the relay starts the
motor, the voltage pulls down and the relay releases because the PIC goes
into reset due to the low voltage on it.

This is not really the main problem, as it is normally sorted out during
installation by running the PIC from a different phase for example. The
problem comes in when there is a brown out. Under this condition the
PIC/relay hunts and the motor pulses on and off and ends up overheating.

Regards
Kez




Ian Stirling said:
Assuming you were driving it from a NPN transistor.

Drive the relay with a NPN darlington, but hook the collector of the first
transistor to a 9V zener to the rail. (you may or may not need a resistor
to ground, to cope with leakage currents)

That's 2 extra parts.


A resistor the same resistance of the coil in series with a NPN transistor
connected to ground, with the base being driven through a 10K resistor to
the coil completes the circuit. (this relies on the first transistor
being well into saturation) As the supply voltage drops through ~10.2 volts,
or the relay is turned off, this adds a little hyster??? to stop chatter
as the load comes off.


For a total of 5.

If you lower the 10K resistor to 500 ohms or so, then you may be able to
omit the snubber diode for the relay.


What are you driving this from?
Brin
 
I

Ian Stirling

Jan 1, 1970
0
.. said:
Ian
The relay is driven from a 2N3094 transistor which is driven from a 16F628
PIC.
the whole thing is a power up one shot timer with some fancy features. It
operates a motor, which can be on the same phase. If a long cable run is
used or there are power fluctuations this aggravates the chatter problem.
Basically power comes on, the PIC closes the relay, the relay starts the
motor, the voltage pulls down and the relay releases because the PIC goes
into reset due to the low voltage on it.

This is not really the main problem, as it is normally sorted out during
installation by running the PIC from a different phase for example. The
problem comes in when there is a brown out. Under this condition the
PIC/relay hunts and the motor pulses on and off and ends up overheating.

Ok, then the circuit I posted should work.

However, now that you've explained a bit more, other solutions occur.
Perhaps the simplest, if you've got spare pins on the PIC is to
tie a resistive divider between ground and (unregulated) VCC, and take
it to an input. (assuming you are running the PIC off a regulated rail)

When you turn on the motor, poll for low supply voltage every few milliseconds
and if you see low voltage twice, turn the motor off.

The right fix is probably to add a slightly larger capacitor to the power
supply.
 
F

Fred Bloggs

Jan 1, 1970
0
Ian said:
Ok, then the circuit I posted should work.

However, now that you've explained a bit more, other solutions occur.
Perhaps the simplest, if you've got spare pins on the PIC is to
tie a resistive divider between ground and (unregulated) VCC, and take
it to an input. (assuming you are running the PIC off a regulated rail)

When you turn on the motor, poll for low supply voltage every few milliseconds
and if you see low voltage twice, turn the motor off.

The right fix is probably to add a slightly larger capacitor to the power
supply.

You're missing the point- the motor LRA pulls the line down to brownout-
so it goes on then off then on then off ad infinitum. The OP is
Australian, so maybe the answer has to do with scrap beer cans and tires
or something.
 
S

Spehro Pefhany

Jan 1, 1970
0
Hi
I have a 12V relay that is fed from the mains via a capacitor power supply,
(cheap but dirty). If I get a brown out my relay chatters, due to the soft
nature of the capacitor supply.

I need to hold the power (12VDC) off whenever the voltage drops to a level
where the relay chatters. I don't have much room, and hope somebody can
suggest a single component solution to my problem. Something like a DS1810
Econo Reset, except with a threshold of about 10V would be ideal.

Any suggestions greatly appreciated


Well, I had a similar problem with a domestic (invalid) elevator
installation, supposedly a production product and professionally
installed by a factory approved company. The line voltage was fine,
but the contactor/relay would sometimes (after it got older) chatter
and the breaker would blow, trapping a person in the elevator until
the cab could be lowered manually by releasing a clutch allowing
access to the breaker. The service people only knew to make expensive
service calls and replace the contactor so it would be okay for a
while then start again.

Solved the problem by adding a small surplus low-voltage transformer
in boost configuration to increase the voltage from the control
transformer, and thus at the relay coil by about 10-15% which was a
100% cure. Probably running heavier wires in the circuit they
installed back to the breaker panel would have cured it too, had the
numbskulls been able to identify the problem in the first place.

Didn't occur to me to use any extra electronics, the solution lay in
increasing (or stabilizing) the "on" coil voltage.

Best regards,
Spehro Pefhany
 
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