dr_alinaeem

Hi

I was wondering if anyone could give me guidance or tips on how to design a system which will measure the contact switiching time of electromechanical relays.

I would be very grateful if someone could reply to this topic and help me out.


Thanks

Nigel Goodwin

I would suggest the best way is going to be a Digital Storage Oscilloscope (DSO), that way you can see (and measure) what's going on. Due to contact bounce it's not going to be easy to measure it otherwise, and even with the scope you will have to make a decision as to when the contacts are 'closed', probably after all the contact bounce has stopped. Also bear in mind that contact bounce is likely to increase as the relay ages, wear and sparking on the contacts can only make things worse.

ljcox

As Nigel said, a Storage scope is the best option if you can obtain one.

If not, another option is to drive the relay from a pulse generator (via a driver transistor) and look at the contacts (suitably biased) on a normal scope. Contact bounce makes it difficult, but not impossible.

Set the pulse generator frequency to say 50 Hz.

Also note that it is usual to put a diode across the relay coil (in the reverse direction) to prevent damage to the driving transistor due to the back EMF. This will increase the release time. This effect can be reduced by using a high voltage transistor and a resistor in series with the diode.

If the coil current is I amp, the voltage across the resistor (and therefore across the transistor) when the transistor turns off will be about I*R where R is the resistance in Ohm.

The Vceo of the transistor should be greater than this voltage.

len

john1

Hi dr_alinaeem,

I think there is a hand held piece of test gear,
which is intended to indicate fuse-break times.

I think it came about because of the legislation
about bonding. (earthing)

In certain situations the fuse-blow-time dictates
the current carrying capacity of the bonding.
It is sometimes necessary to improve the bonding
until the fuse blows within the time specified
under the legislation,
as a legal requirement.

If the requirement is not met, the 'responsible
person' would not 'sign-off' the installation,
this led i think to these small testers which
would indicate the time taken for the fuse to blow.

Although i have seen them, i have never used one
nor taken any notice of them.

I would imagine such an item could also be used on
a 'normally-closed' contact set, but you would
have to check out its method yourself.

Maybe others have more knowledge of this item
than i do.

Regards, John

dr_alinaeem

Thank you for the quick reply to my question.
What I intended to ask really, was can you give any ideas on how to connect a relay to a digital circuit of some kind which can then be connected by serial or parrallel port to a computerand also which can measure the relay switching time.

Thank you in advance!!

Nigel Goodwin

The same problems with contact bounce apply - how do you decide when the contacts are closed?. Generally with a push button switch, waiting for 20mS or so gives plenty of time for bouncing to stop, presumably a relay will perform in a similar fashion?.

Also, why do you need to know? - if speed of closure is important, a relay probably isn't the best device to be considering.

ljcox

another way to eliminate contact bounce and thus interface a computer would be to use a set/reset flip flop.

Use a relay with a change over contact (SPDT) and connect one to the set and the other to the reset. Pull up or pull down resistors are also needed.

Thus the FF is set when the relay operates (at the first bounce) and reset when it releases (at the first bounce).

Len