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Topic Title: normally open switch - monitoring ?
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Created On: 13 November 2013 10:55 AM
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 13 November 2013 10:55 AM
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DTSFA

Posts: 1
Joined: 13 November 2013

I have a normally open switch that temporarily shorts out part of a control circuit for machine to continue operation,

from a safety perspective I wish to monitor this switch in case it 'welds shut'


volts = 24v dc
current = 1A max

one switch is reed type, other is plunger type Link

apart from bespoke electronics to do this, does anyone know of safety type normally open switches
?

or some off the shelf solution for this problem

thanks
 13 November 2013 01:57 PM
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jarathoon

Posts: 1041
Joined: 05 September 2004

Originally posted by: DTSFA

I have a normally open switch that temporarily shorts out part of a control circuit for machine to continue operation,

from a safety perspective I wish to monitor this switch in case it 'welds shut'


volts = 24v dc

current = 1A max


one switch is reed type, other is plunger type Link

apart from bespoke electronics to do this, does anyone know of safety type normally open switches

?

or some off the shelf solution for this problem

thanks


Which switch is in danger of welding shut? Why have you described one switch in the first part of your post then listed two types of switch in the second part? Is one switching or powering the other?

What do you want to do if your critical switch fails?

Presumably you want to make your switch fail detect completely hardwired and independent of a more complex control system (via a high impedance digital input for example).

I would ask questions like:

- Can you reduce the current in your problem switch and drive the current through a more reliable relay or solid-state relay?
- Is there inductance in the circuit causing sparking problems?
- Do you really want to monitor the switch (which implies a timeout on the problem), or do you need the hardware to detect a error conditon imediately to stop the machine proceeding in an unsafe state (e.g. with a guard open)?
- Do you want a dual reduntant switching arrangement with monitoring?



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James Arathoon
 22 November 2013 01:08 PM
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kengreen

Posts: 400
Joined: 15 April 2013

there are several ways of coping with your problem.

The first is a regular testing and examining schedule - nothing can beat the "eyeball test".

Failing that the best way is to render the circuit as a "constant-resistance network". Essentially this is a critically-damped resonant circuit; i.e. it has the nature of a pure resistance and so operation of the switch does not cause arcing - which is of cause of switches welding.

If you want more details send me either a PM or an e-mail address.

In this context it is worth noting that all electronic circuitry suffers of necessity from stray inductance and stray capacitance.

Ken Green
 02 December 2013 01:50 PM
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iie63674

Posts: 78
Joined: 17 May 2006

Originally posted by: DTSFA

I have a normally open switch that temporarily shorts out part of a control circuit for machine to continue operation,



from a safety perspective I wish to monitor this switch in case it 'welds shut'





volts = 24v dc

current = 1A max



one switch is reed type, other is plunger type Link



apart from bespoke electronics to do this, does anyone know of safety type normally open switches

?



or some off the shelf solution for this problem



thanks


If there will be an increased risk if the switch welds closed, then you cannot use a normally open switch. Safety circuits must use a normally-closed switch with positive opening action.
What exactly are you trying to achieve?
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