Relay Circuit

I have a PCB mounted relay that is driven from a transistor. So the
collector of the transistor is connected to the anode of a diode and to
one side of the relay coil. 5 VDC is connected to the cathode of the
diode and to the other side of the relay coil. This relay switches L1
which then goes to an off board relay coil. The other side of the off
board relay coil goes to L2. The off board relay has an RC (snubber)
across the contacts and drives an inductive load (motor). My concern
is that although I have some protection for the PCB relay on the coil
side? Am I at risk for RF noise and/or contact wear by not supressing
the line side of the PCB relay? I thought of using another RC across
the PCB relay contacts but this would allow L1 to bleed through an
activate the off board relay...no good! What about an MOV or diode?
Would it be better suited here? Other ideas?

Also, this transistor is driven from a PIC via a 1k ohm resistor to the
transistor base. Now, shouldn't there be a pulldown between the 1k
resistor and the base of the transistor? I'm pretty certain that is
the case; but what are the reasons and guidelines for doing this?

Thanks.

 

[Dan Hollands]

The answer depends on the specifications of the relays involved.

Relays will have a contact rating based on voltage and current being
switched.

In general you can probably switch the coil of another relay from the
contacts of the first relay.

There is possibility that when the on board contact opens, the inductive
kick from the off board relay will get into the PCB and affect it. A
tranzorb or MOV would normally be used to take care of that.

For the trasistor it all depends on low voltage from the PIC. Now adays with
most devices built from CMOS the output will go to zero and there is no need
for a pull down on the base. When TTL logic was common the output would not
go to zero and a pull down was needed to make sure the transistor turned
OFF.

Dan

--

Dan Hollands
1120 S Creek Dr
Webster NY 14580
585-872-2606
QuickScore@USSailing.net
www.QuickScoreRace.com


<richard.bair@revco.spx.com> wrote in message
news:1122323090.006376.23870@g47g2000cwa.googlegroups.com...

I have a PCB mounted relay that is driven from a transistor. So the
collector of the transistor is connected to the anode of a diode and to
one side of the relay coil. 5 VDC is connected to the cathode of the
diode and to the other side of the relay coil. This relay switches L1
which then goes to an off board relay coil. The other side of the off
board relay coil goes to L2. The off board relay has an RC (snubber)
across the contacts and drives an inductive load (motor). My concern
is that although I have some protection for the PCB relay on the coil
side? Am I at risk for RF noise and/or contact wear by not supressing
the line side of the PCB relay? I thought of using another RC across
the PCB relay contacts but this would allow L1 to bleed through an
activate the off board relay...no good! What about an MOV or diode?
Would it be better suited here? Other ideas?

Also, this transistor is driven from a PIC via a 1k ohm resistor to the
transistor base. Now, shouldn't there be a pulldown between the 1k
resistor and the base of the transistor? I'm pretty certain that is
the case; but what are the reasons and guidelines for doing this?

Thanks.

 

[Spehro Pefhany]

On 25 Jul 2005 13:24:50 -0700, the renowned richard.bair@revco.spx.com
wrote:

I have a PCB mounted relay that is driven from a transistor. So the
collector of the transistor is connected to the anode of a diode and to
one side of the relay coil. 5 VDC is connected to the cathode of the
diode and to the other side of the relay coil. This relay switches L1
which then goes to an off board relay coil. The other side of the off
board relay coil goes to L2. The off board relay has an RC (snubber)
across the contacts and drives an inductive load (motor). My concern
is that although I have some protection for the PCB relay on the coil
side? Am I at risk for RF noise and/or contact wear by not supressing
the line side of the PCB relay?

The remote relay has an AC coil or DC?

I thought of using another RC across
the PCB relay contacts but this would allow L1 to bleed through an
activate the off board relay...no good!

You can put the RC across the relay coil rather than the contacts.
That means a C short won't turn the relay on, and will probably
self-heal a film C.

What about an MOV or diode?
Would it be better suited here? Other ideas?

MOV is of dubious benefit, IMHO, and could be a problem when it fails
shorted. A diode would be okay if it's a DC coil (across the coil).

Also, this transistor is driven from a PIC via a 1k ohm resistor to the
transistor base. Now, shouldn't there be a pulldown between the 1k
resistor and the base of the transistor?

No, that would just be silly. The PIC output is push-pull. Assuming
it's not a safety-critical system.

I'm pretty certain that is
the case; but what are the reasons and guidelines for doing this?

Thanks.

Watch your layout, especially around the drive transistor. You may
want to kill the dv/dt at the collector with a small cap to E.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com