120V AC only/ 5V DC cir

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chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
I have a lighting control system with a 5V DC override cir. I used 15A momentary switches labeled 120V AC ONLY.
Comments besides 110.3(B)?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
150830-1559 EDT

chris kennedy:

I believe a 120 V switch is a bad choice for switching low current 5 V DC circuits.

A suitable switch at this voltage level and lower is a reed, a gold plated or cross-bar, a mercury, or solid-state switch.

Most copper based and silver switches develop non-conductive surfaces from atmospheric contamination. I have equipment that I built 55 years ago with 50 millionths gold over nickel contacts that work perfectly today yet HP, Tektronics, and other commericial equipment that used silver contacts from that same time period require much switch turning to make good contact, and sometimes that does not clean up the contacts. Nikel is not porous in thin layers, whereas gold is. Gold has to be 250 millionths or more to prevent base metal under it from migrating thru the gold.

.
 

chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
150830-1559 EDT



I believe a 120 V switch is a bad choice for switching low current 5 V DC circuits.

Problem is the install instructions said in one place to use a DC switch and a couple pages later said use standard wall switch.
 

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
150830-1559 EDTI believe a 120 V switch is a bad choice for switching low current 5 V DC circuits.

A suitable switch at this voltage level and lower is a reed, a gold plated or cross-bar, a mercury, or solid-state switch.

I agree IN SPADES!

I'm in the industrial hydraulics side of things; still today, customers want to use multiple potentiometers and relays to select the command. Input impedances are always 10k or higher, signals 10V maximum, so 1 mA maximum. Common plug-in relays work fine ... for weeks to months.

Although they are harder to find today, P&B makes suitable relays with 12 and 24 VDC and 24 and 120 VAC coils. They are typically crossbar construction. P&B's KHAU with contact code 6 was my usual recommendation; present literature shows it to still be available. I have not found that the industrial relays are always suitable with their "logic reed" contacts.

The key specification I've found is "suitable for dry circuit applications". While I'm sure you (gar) are aware, dry circuit and dry contact are not the same thing.
 
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