Switches from hell

[jwax]

Has anybody witnessed this?
I bought 8 SPST toggle switches from Lowe's.
What could go wrong?
I have to switch low value resistors in and out of a circuit for testing, using resistor values from 1 ohm up to 20 ohms. Had trouble getting consistent results!
Problem was that the switches measure from 1-100 ohms when closed! GRRR! Lowe's took them back as defective.

[Janitor Tzap]

Problem was that the switches measure from 1-100 ohms when closed! GRRR! Lowe's took them back as defective.

That sound about right for AC Light Switches.

Thou, I would be some what worried about the one that measured around 100 ohms.
{That could be a fire hazard!}

But most wall light switches are used for just lights.
And are rated for a maximum 10 amps.
So, as long as you didn't go over the maximum amps.
There just fine for the job.

I have to switch low value resistors in and out of a circuit for testing, using resistor values from 1 ohm up to 20 ohms.

What are you trying to make?
A Resistance Decade Box?


Signed:Janitor Tzap

[jwax]

"That sound about right for AC Light Switches." ?
All this time I was thinking a switch was a closed contact- something like an ohm or less. Several ohms in a closed switch is a defective switch.
I'm building a selectable resistive load for loading a solar panel to plot IV characteristics.
These were 10 amp switches, and I'm running about 3 amps max.

[Robert Reed]

Assume the wall switch was running a light load such as a 120 watt light bulb and the switch had a low end value from your scale of say 10 ohms. That switch would have to dissipate 10 watts in heat and would probably burn up in short order. They were definately defective switchs and even a dirt cheap switch from china should have less than 1 ohm contact resistance. I mean how hard is that?

[Lenp]

Defective Switches? I'll bet not!

Because these were power rated swithes, for typical load switching, the first time they switched a load the oxidation on the contacts would be cleaned due to the small arcing that occurrs. This oxidation started the day they were made and continues during storage and the rest of their life.

If you would have 'flashed' the contacts by switching a small (maybe .5A) load a few times with the switches, you would have seen a dramatic downward change in the contact resistance.

These switches were not designed, and can not reliably be used for 'dry' circuits, which are classified as having minimal or no current flowing.
This is why there is precious metal plating on switch and relay contacts, to avoid the oxidation issue. Power rated switches, like Lowe's stocks, are generally not precious metal plated. Electrtonic suppliers carry relays and switches that are listed as 'signal' relays, which are suitable for 'dry' circuits. Your meter simply didn't have adequate current to break down the oxidation layer when you tested them. Had you hooked them to a load, I'll bet a Starbucks they would have worked, as intended.

We used to have problems with older commercial intercoms that used bays of station selector switches. The switches which were seldomly used would have lots of static. We built a 'flasher', small inductor and battery with an approporiate connector to, 'clean' the switches. It worked wonders, and often for years! And, these switches were plated...

The devil is in the details...

Len

[jwax]

Thanks, Lenp! I had ran them with a few amps repeatedly, and thus discovered the resistive anomaly. Perhaps not enough to dissolve the oxidation.
BTW, the next set, bought from an electronics supplier, worked fine out of the box. I measured those with an ohmeter first, and found a consistent resistance of less than one ohm.
Caveat Emptor!

[Robert Reed]

Lemp
What you say makes sense and for the most part I agree with you. However working with small signal relay/switch contacts in the past, I've noticed there is not much contact force applied here and although a certain amount of wiping action is mandatory, there still isn't a strong scrubbing action going on here - hence the precious metal contacts. I would think that for a manually activated wall switch, there would be enough force applied to make strong contact so as to overcome some oxidation - but maybe again not with some of the cheap products on the market today. Even with a well constructed one thogh, I would never use them in low level electronic circuits that demanded any accuracy.

[Janitor Tzap]

Thanks, Lenp....

Your explanation was far more precise than I could of ever said.

Oxidation on switch contacts is something I've run into a lot with older
Audio Equipment, Mechanical Tuners on TV's, and Relays in Older VCRs.

It is kind of strange how some switches, and relays are designed to work in a
certain voltage, and current range.

You go to high, and you burn the contacts.
You go to low, and you get oxidation forming on the contacts.

Live and learn.


Signed: Janitor Tzap