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Old 10th December 2005, 04:08 PM   #1
sklimek is offline sklimek  United States
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Default Mercury wetted relays...

Looking for a cool way for signal switching and amplifier selecting The Jones book ‘Valve Amplifiers’ suggested mercury wetted relays. “The only disadvantage to mercury wetted relays is their cost”. And, well their availability now due to mercury being a health hazard.

I found a source for the relays that is quite cheap, here is the description:

“This item consists of fifteen CP Clare Type HGSM5003 Mercury wetted high speed relays Unused PC Board mount Bistable dual coil Coil Resistance 1100 ohms Operates on 9 volts Contacts are SPDT 2 Amps Operates in approx 1 millisecond 2 1/16inches by 5/8 inches by 5/8 inches”

I was wondering if these values are good based on what I want to do.

This was an EBay item which I can post if there is interest, I believe he has more.

BTW - I did a search and found some great threads talking about these relays comparisons/applications here -

http://www.diyaudio.com/forums/searc...der=descending

Stan
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Old 10th December 2005, 04:16 PM   #2
poobah is offline poobah  United States
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Hey Stan,

What levels, voltage and current< would you like to switch with relays?

Mercury wetted have special strengths... mostly useful for low level, highly accurate stuff. They are not real good for much in the way of power though...

Cheers,
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Old 10th December 2005, 04:18 PM   #3
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Get them while you can!

Right now a lot of audiophiles frown on relays for switching audio or for use in attenuators, but when mercury wetted relays disappear from the market, they will suddenly become a hot commodity for which people will pay stupid-money, sort of like 3 gallon per flush toilets.

If I were you I'd buy as many or those thing as I could lay my hands on and stock pile them for the imminent ban. You'll make a fortune!

Carbon composition resistors used to be the same way- people were paying dumb prices for them because no one was making them and the supplies found in old warehouses were drying up. Some clever business guys saw the high prices and decided to take the carbon comp resistor machines out of mothballs. Now you can get newly manufactured carbon comp resistors again.

That won't happen with mercury wetted relays due to the toxicity of mercury.

I_F
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Old 10th December 2005, 04:43 PM   #4
sklimek is offline sklimek  United States
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Quote:
Originally posted by poobah
Hey Stan,

What levels, voltage and current< would you like to switch with relays?

Mercury wetted have special strengths... mostly useful for low level, highly accurate stuff. They are not real good for much in the way of power though...

Cheers,
Hey poobah, just preamplifier signal switching ~2V and thinking it might be used to switch signals to seperate preamps as well. Doable?

Also, this was quoted from the Jones book, I was wondering if this shunt and series switching is over-kill or just right?

"If we had a series relay on each source, we could precede it with a shunt relay to ground that would normally be closed, and this would ensure almost perfect attentuation of unwanted sources".
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File Type: jpg relay_inter.jpg (51.7 KB, 380 views)
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Old 10th December 2005, 09:46 PM   #5
poobah is offline poobah  United States
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Yep.

The mercuries will work very well for that and the circuit is good too... keeps the other inputs quiet.

The relays you are looking at may not the best choice though... they are dual coil... you have to pulse one coil to close the relay and another coil to open. Single coils would be simpler, but there is always a way!
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Old 11th December 2005, 06:15 PM   #6
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I used to use Mercury wetted relays in telecom equipment; you should read an applications note that describes the necessary contact protection. An amazingly small load voltage will cause arcing and pitting of the surface underneath the mercury, which will prevent mercury from wetting that area. The area then spreads, and failure can happen amazingly quickly; it's not the voltage per se, but the rate of change of the voltage, dv/dt.

I once built a breadboard and the load was a non-inductive resistor with fairly short lead wires. Too lazy to calculate and add the contact protection. At 10 pps, it failed within minutes.

http://relays.tycoelectronics.com/da...159-160_DS.pdf
An old fashioned app note, like Mom used to make.


BTW, solid carbon resistors, the old Allen Bradleys, had the best short term surge characteristics by far. If you were designing something that interfaced with, say, telephone lines, the cabon comps were best for absorbing surges. Under similar stress, wirewounds were apt to send pieces flying. They were discontinued because the manufacturing process was very "dirty" and pollution laws made the manufacture in the first world very expensive.
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Old 11th December 2005, 07:09 PM   #7
sklimek is offline sklimek  United States
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Quote:
Originally posted by Curmudgeon
I used to use Mercury wetted relays in telecom equipment; you should read an applications note that describes the necessary contact protection. An amazingly small load voltage will cause arcing and pitting of the surface underneath the mercury, which will prevent mercury from wetting that area. The area then spreads, and failure can happen amazingly quickly; it's not the voltage per se, but the rate of change of the voltage, dv/dt.

I once built a breadboard and the load was a non-inductive resistor with fairly short lead wires. Too lazy to calculate and add the contact protection. At 10 pps, it failed within minutes.

http://relays.tycoelectronics.com/da...159-160_DS.pdf
An old fashioned app note, like Mom used to make.


BTW, solid carbon resistors, the old Allen Bradleys, had the best short term surge characteristics by far. If you were designing something that interfaced with, say, telephone lines, the cabon comps were best for absorbing surges. Under similar stress, wirewounds were apt to send pieces flying. They were discontinued because the manufacturing process was very "dirty" and pollution laws made the manufacture in the first world very expensive.
Thank curmudgeon, It looks like a great read. Love that illustration showing the 'wetting' process. I am actually getting some help putting this together and will send this on. Ultimately I hope to end up w/ momentary action switching on the front panel.
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Old 11th December 2005, 10:49 PM   #8
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Mercury wetted relays still don't seem to be too hard to get on the surplus market, and distributors still carry them. I can score as many as I want in the local surplus market (Silicon Valley). I think C & H Sales still has some as well (search for C & H on Google). One thing to remember is that mercury wetted relays have a preferred orientation, so you have to mount them facing the right way or they won't work right.
On another note, if you are switching nasty loads with high inrush current, you can use what's called a mercury displacement relay. This uses a pool of mercury, a pair of contacts, and a solenoid-driven plunger that forces the mercury up around the contacts to close the relay. These are quiet (unlike normal high current contacters that scare the daylights out of you), and they can take a lot of electrical abuse. These guys are environmentally incorrect because they use a lot of mercury, but they are still available if you know where to look.
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Old 11th December 2005, 11:20 PM   #9
sklimek is offline sklimek  United States
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Quote:
Originally posted by wrenchone
Mercury wetted relays still don't seem to be too hard to get on the surplus market, and distributors still carry them. I can score as many as I want in the local surplus market (Silicon Valley). I think C & H Sales still has some as well (search for C & H on Google). One thing to remember is that mercury wetted relays have a preferred orientation, so you have to mount them facing the right way or they won't work right.
On another note, if you are switching nasty loads with high inrush current, you can use what's called a mercury displacement relay. This uses a pool of mercury, a pair of contacts, and a solenoid-driven plunger that forces the mercury up around the contacts to close the relay. These are quiet (unlike normal high current contacters that scare the daylights out of you), and they can take a lot of electrical abuse. These guys are environmentally incorrect because they use a lot of mercury, but they are still available if you know where to look.
Hi wrenchone, yes, every time I came across some information they discussed orientation for the mercury. Does anyone know what the 'bistable' refers to? I did not do an extensive search but finding multiple matching mercuries at an affordable price was a bit difficult. If you are going to be using say 6-8 of these guys w/ its matching relay shunt to ground your up to 12-16 matching, this can get quite pricey. Here is the link where I got mine:

http://search.ebay.com/mercury-wette...s_W0QQfromZR40

I noticed he already has increased his price. I purchased 15 for 22.00. If you scrool down a bit farther you will see some cool vintage octal ones for 30 to 80 bucks apiece.
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Old 11th December 2005, 11:39 PM   #10
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You should be able to get the octal case relays for about 5 dollars apiece if you know where to look. The bistable relay sounds like a variation of the old latching relay, which takes a pulse to close it and another to spring it open again. Iwould go with a plain jane vanilla NO relay myself...
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