Switch Contacts at low voltages

[Noway2]

During a design review yesterday, a question was posed to me about
whether or not the customers will need to use gold contacts on the
switches that connect to our system. In response, I have been
searching for some guidelines as to when gold contacts are required. I
have found a lot of information discussing mechanical operation of
contacts, wetting current, oxide build up, etc, but have not found
anything providing some "rule of thumb" recommendations.

In this instance, the design is as follows:

Our design contains (user) inputs implemented with opto isolators. The
circuit design is such that we provide a 12V signal and ground and the
user provides a switch closure to flow current from the 12V supply
through the opto's diode and a current limiting resistor. I can't
control how far away the switch will be located or what type of switch
the customer will use. I can control the size of current limiting
resistor, which I present have set as 1.2K for 10mA of current @ 12V
through the opto.

My question is does anyone have any rule of thumb that says, generally,
under X volts or X amps, gold contacts are required. I understand that
this is a vague question and an exact answer depends on the
application, environment, etc, and information I don't (and won't) have.

 

[PeteS]

During a design review yesterday, a question was posed to me about
whether or not the customers will need to use gold contacts on the
switches that connect to our system. In response, I have been
searching for some guidelines as to when gold contacts are required. I
have found a lot of information discussing mechanical operation of
contacts, wetting current, oxide build up, etc, but have not found
anything providing some "rule of thumb" recommendations.

In this instance, the design is as follows:

Our design contains (user) inputs implemented with opto isolators. The
circuit design is such that we provide a 12V signal and ground and the
user provides a switch closure to flow current from the 12V supply
through the opto's diode and a current limiting resistor. I can't
control how far away the switch will be located or what type of switch
the customer will use. I can control the size of current limiting
resistor, which I present have set as 1.2K for 10mA of current @ 12V
through the opto.

My question is does anyone have any rule of thumb that says, generally,
under X volts or X amps, gold contacts are required. I understand that
this is a vague question and an exact answer depends on the
application, environment, etc, and information I don't (and won't) have.

My general rule of thumb for gold contacts is based on

1. Current - under 1mA and I start to consider them.

2. Signal speed. Once I get sub 10nsec edge rates, gold might be the way
to go. At < 100 psec edge rates, it's the _only_ way to go [YMMVG]

3. Low level analog signalling. I virtually always use gold plated
contacts in these situations.

4. The environmental situation, but even then the signal would have to
be somewhat critical.

For low speed, 12V applications at 10mA I wouldn't bother unless you
have some other analog signal riding on it. Keep in mind that gold will
still get some deposits (used to clean gold fingers with an eraser years
ago), so they're not a panacea.

Cheers

PeteS

 

[Mark]

if it is a slow mechaical switch I would add 0.1uF across each switch.
This will give a small surge of current each time the switch is closed
that will keep it clean and also filter noise and bounce.
Mark

 

[Noway2]

if it is a slow mechaical switch I would add 0.1uF across each switch.
This will give a small surge of current each time the switch is closed
that will keep it clean and also filter noise and bounce.
Mark

First, thank you to both of you for your responses.

Yes, it would be a slow mechanical switch. I had heard from talking to
a few people that they thought that 12V is generally fine as far as
breaking through any contact oxide, but I want to be sure.

I already had placed a small capacitor on the circuit for bounce. It
would appear, though, that it would also have a positive unintended
side effect. I was considering a larger cap with the thought of
providing a bleed-off path for any induced voltages as I can't control
how Joe Electrician will run the wires in the installation, along the
lines of a 4.7uF ceramic (non polarized with low ESR). Based on the
capacitor equation, Ic = C*dV/dt, the current could be fairly decent in
magnitude, although it would be fairly short in duration. I also found
some schematics where the designer placed a relay in parallel with the
switch so that when it closes extra current will flow through the
contacts that doesn't have to flow through the opto's LED.

 

[PeteS]

First, thank you to both of you for your responses.

Yes, it would be a slow mechanical switch. I had heard from talking to
a few people that they thought that 12V is generally fine as far as
breaking through any contact oxide, but I want to be sure.

I already had placed a small capacitor on the circuit for bounce. It
would appear, though, that it would also have a positive unintended
side effect. I was considering a larger cap with the thought of
providing a bleed-off path for any induced voltages as I can't control
how Joe Electrician will run the wires in the installation, along the
lines of a 4.7uF ceramic (non polarized with low ESR). Based on the
capacitor equation, Ic = C*dV/dt, the current could be fairly decent in
magnitude, although it would be fairly short in duration. I also found
some schematics where the designer placed a relay in parallel with the
switch so that when it closes extra current will flow through the
contacts that doesn't have to flow through the opto's LED.

Along with the capacitor (as this is a slow signal anyway), I would put
a small (< 100 ohm) resistor in series with the cap. That'll still give
you the filtering you need but it will significantly reduce the initial
surge.

Ceramics are great, but a 4.7uF ceramic driven from 12V will suck over
20A on the initial surge (I know, I've tested it) and I'm not sure you
want to blow fuses or otherwise cause distress to the source

Cheers

PeteS

 

[Jon]

Cheers

PeteS- Hide quoted text -- Show quoted text -

Leach International has a nice articel on this topic @
http://www.leachintl2.com/english/english2/vol6/properties/how4.htm
Regards,
Jon

 

[Noway2]

Leach International has a nice articel on this topic @
http://www.leachintl2.com/english/english2/vol6/properties/how4.htm
Regards,
Jon

Thank you to you both!

Pete, you have a very good point about the surge currents that would
skyrocket and a limiting resistor would be prudent.

Jon, yes that article does explain a lot about why a contact rated for
one load level is not suitable for another. Of course, in my
application I need to handle the worst case scenario, that being I need
low level of steady state current for the opto, with a high surge of
current to clean a silver contact switch.

Based on some simulations I ran this morning, yes a small capacitor
with a current limiting resistor will supply a surge of current through
the contacts while providing a low level load through the opto.

 

[joseph2k]

if it is a slow mechaical switch I would add 0.1uF across each switch.
This will give a small surge of current each time the switch is closed
that will keep it clean and also filter noise and bounce.
Mark

I would put about 5 to 15 ohms in series with that cap. I have seen bare
cap across the switch contacts cause erosion and microwelding of the
contacts. It was 22 uF tantalum at 28 V(dc), but the excess wear showed up
by 10,000 cycles.