Are these switches usable?

[sa230e]

So I'm working on a tube amp project and I bought some E-Ten 1222 DPST switches on eBay for the power and standby switches. According to the website, they're supposed to have a contact resistance of < 10 milliohm but when I measure my switches (I bought 4) with my multimeter I get much more. Most are in the ballpark of 10 - 30 ohms or so and one of them measured more than 100 ohms! That's going to make for some significant losses when I put 120v across them.

To make sure I wasn't going nuts I tested some other switches in my parts bin. None of them tested over 50 milliohms in the on position.

So are these switches junk or are they usable?

And yes, I know that you get what you pay for with cheap Chinese parts but switches are fairly simple devices and I figured it would be hard to screw up a switch. Maybe not...

[DrGonz78]

Perhaps there is a residue or something on the outside of the metal tabs that is increasing the resistance. I would solder a wire to all the ends of the switch and test it that way and also try scratching down on the metal a bit. What it sounds like is that they are junk though. I bought some mini clip multimeter probes from a China source a while back and was very disappointed. At first when I got the probes I would measure 300miliohms (0.3ohms) on the meter, which is normal. Then after a month or so I saw 3-6 ohms of resistance across the probes. I cleaned them and tried to see if that would fix the problem to no delight. If you have an AC power switch that is measuring 100 ohms of resistance then that is a big problem indeed.

Edit: I just had one more thought... Perhaps they are making these switches with the wrong or a bad type of lubricant. I mean all these switches will have lubrication inside the switch where the pole contacts flip and switch. Just got to wonder if the lubrication might be a factor in this case.

[sa230e]

Could be the lubricant. The resistance seems to change when I switch them on and off. Maybe the lubricant could cause that? In any case it looks like I have to buy new switches.

[DrGonz78]

Could be the lubricant. The resistance seems to change when I switch them on and off.

Seems like the whole batch has an issue but when you said one of them is over 100 ohms that raised the alarm even louder. So each time you toggle the switch you get different readings of resistance? I did assume that you have no issue with your meter and it is not a low battery. Personally I would take them apart and look inside the switch, just out of curiosity. Perhaps the lubricant has hardened/aged badly? 

[phatt]

Yes it's a common issue ,, If I recall the older equipment had silvered contact points which lasted many years. the cheaper ones at least had marine grade copper contacts but today all you get is a strip of metal with a bit of plating on it if you are lucky.  Darn hard to find quality switches now.
Phil.

[phatt]

Scroll down to see a detailed drawing of the internal parts of a switch.
http://www.alibaba.com/product-detail/RT-S6-14B-RT1121DF-SPDT-ON_241742656.html

Note the little button contacts,,, a lot of switches now do not have them.

All those 3 pole stomp switches are so badly made they are hardly worth the $10 you pay.
The old Carling type stuff had proper button contacts and lasted for years.
All you can get now is crap unless you dig hard to find the real thing.

Switching 6.5 sockets have also gone down the crap hole they also used to come with silvered contacts but now it's just a piece of shinny tin. No wonder the FX loops in the back of amplifiers give so much trouble.

Rant over,,, Phil.

[DrGonz78]

I have to share a story about finding an old toggle switch. There use to be "the old man" who once lived in my house and he did all sorts of Ham work in addition to restoring old Model T's. So one day I am doing some weed whacking and deep in the dirt comes this old toggle switch. Cleaned it up just a bit and took it in the shop. Also found some old tube socket adapters out there too. Anyhow I hook the thing up to an ohms meter and it measures just perfectly. Mind you the switch is not properly lubricated as it once was but the dang thing still showed perfect continuity readings. To me that is a testament to the way things use to be made.   

[Enzo]

And once more for the umpteenth time, buying parts on ebay is always a crap shoot.

before you throw them out, though, try wiring one into a real circuit and see if the contacts don't just burn themselves clean.  A couple of sparky on/off cycles might just clear away whatever film there is from the manufacturing process.

[sa230e]

And once more for the umpteenth time, buying parts on ebay is always a crap shoot.

before you throw them out, though, try wiring one into a real circuit and see if the contacts don't just burn themselves clean.  A couple of sparky on/off cycles might just clear away whatever film there is from the manufacturing process.

Good call. I just built a dim bulb tester which comes in handy for testing the switches at mains voltage (which is 120VAC 60Hz here). A 100 watt light bulb in series with the live wire serves to limit the current and is a fairly heavy load. The tube amp I'm building is a small one and I don't expect it to consume that much power so this should function as a good test case to see how well these switches hold up.

As I said before, the resistance would change every time you switched it on and off. Before testing it was anywhere from 1 to 20 ohms. For the first test I left the switch in the on position and used a power strip to switch the mains. After disconnecting the power I again measured the resistance in the on position and it came out to around 0.3 ohms. I toggled the switch on and off a few times disconnected and the resistance jumped around again between 1 and 8 ohms.

For the second test, I reconnected everything and toggled the switch itself while it was pulling current hoping, as Enzo suggested, that arcing inside the switch would burn the contacts clean. That seemed to be the case. When I disconnected the mains and measured the resistance it seemed to be a pretty consistent 0.3 - 0.4 ohms when toggling on and off.

That's still not a great value, about 30 times what was quoted in the datasheet, but not as bad as the readings I was getting suggested. The switch did not heat up at all. Maybe I can salvage these things after all but I think more testing is still in order before I trust these things in my amp circuit. It didn't occur to me to measure the voltage across the switch while it was on so I'll probably do that next. That way I can calculate the resistance of the contacts while it's in a live circuit.

[Enzo]

Measuring 0.3 ohms is difficult with a hand meter.  That is why lab gear uses four wire testing.  Did we ask if you shorted your meter probes together to see what your meter thought the probe resistance was?  If not, you would need to subtract that from any reading.  30 times 0.3 ohms is about 0.01 ohms, and I really doubt your meter can measure that.

And in the what is there to lose department, a drop of contact cleaner down into a switch might even help.  But it sounds like they are working now, and leave well enough alone.

[J M Fahey]

Cheap multimeters are VERY unreliable measuring low ohms, so forget them.

The test is very simple: wire a 25-100W bulb in series and plug it into the mains.

Does the bulb light properly?

I bet it does.

With the lamp on, measure AC volts across the switch (be careful, you have mains voltage there) ... I bet you will read 0 or a *very*  low voltage (less than 1 VAC).

So where did the "100 ohms" go?

On mains rated switches such as those, you will always have a tiny oxidation layer on surfaces ... any real voltage punches through and destroys it.

Just don't use those switches for low level audio and you'll be fine.

[sa230e]

Cheap multimeters are VERY unreliable measuring low ohms, so forget them.

The test is very simple: wire a 25-100W bulb in series and plug it into the mains.

Does the bulb light properly?

I bet it does.

With the lamp on, measure AC volts across the switch (be careful, you have mains voltage there) ... I bet you will read 0 or a *very*  low voltage (less than 1 VAC).

So where did the "100 ohms" go?

On mains rated switches such as those, you will always have a tiny oxidation layer on surfaces ... any real voltage punches through and destroys it.

Just don't use those switches for low level audio and you'll be fine.

I've done this and it works just as you said it would. I get less than 100mV across the switch.

Thanks guys.