Mixing AC with DC - Good Idea?

That is usually against code and against other safety regs. There needs to be a minimum clearance and creepage distance between the two sides. You'd have to make sure those is maintained, plus whatever else the regulations in your market mandate.

--
Regards, Joerg

http://www.analogconsultants.com/

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** This is definitely not good practice.

First, you must use a switch that is purpose designed and rated for AC mains supply operation - plus if it has a metal lever and frame, the metal must be connected to the supply safety earth.

Second, the vast majority of such switches have insufficient insulation between the poles to prevent a short occurring from one to the other in the event of an internal mechanical failure.

I have seen a few push-on / push- off switches that had a combination of low voltage and AC supply switches on the one actuator. Very likely these were specially made for the user.

..... Phil

YES BOTH SIDES ARE ISOLATED NO PROBLEM I HAVE ONE WITH 220 AND 24 ON SAME SWITCH

Sounds OK to me.

John

"will" snipped-for-privacy@yahoo.com

** But not safety isolated in the event of a switch failure where parts come loose and float about. ** Massive safety problem - you donkey brain.

** Irresponsible to advise others to do the same dangerous and illegal things you do - f****it.

.... Phil

** Beware - this is the advice of a nutter.

.... Phil

Yes you can how ever, you may want to check the spec's on the switch.

DC does not do well with switches that depend on "wetting" to keep the contact surface electrically clean.

You may notice that many switches are derated for use when passing DC over being used as AC due to the difference in DC/AC and lack of Wetting.

In your case, I think you'll be fine because most AC switches are derated at ~ 50% at DC over AC.

"

"Jamie" = Maynard A. Philbrook KA1LPA

Radio ham, total f****it and wonky code scribbler by trade.

** WRONG.

** WRONG.

** WRONG !!

The *current capacity* de-rating is because of excessive arcing when switching DC of more than about 24 volts.

** BOLLOCKS - ignore this know nothing, PITA bloody TROLL !!

DC current de-rating is the LEAST of your worries when mixing up a low volts circuit and the AC supply on the same toggle switch.

..... Phil

Actually, the possibility of sustained arcing is the reason switches are often derated for DC, not wetting.

The whole wetting issue seems overblown to me, perhaps a leftover from olden days of crap contact alloys. I've never seen a power switch or a power relay fail to work at low level. Has anyone else?

John

To do it safely, you can use two separate switches, each in its own enclosure, with the enclosures close enough together so you can tie the toggles together. One throw action throws both switches, yet the two systems remain separate.

Ed

Generally, I would say this is bad engineering practice. The arrangement might work fine, but could become a problem down the road, if the switch were to fail, or if it required replacement. Depending on your application, there may also be code issues / or UL certification issues that would likely frown upon such use.

It is unusual to have both AC and DC on the same toggle switch, and for that reason alone, you can probably expect some repair staff down the road to get any replacement switch mis-wired. -mpm

This is what relays are for. Put a AC mains operated relay in the DC power line.

--
The Force is dark on one side, light on the other and holds the world
together.

Hmmm, just like Gaffer Tape then.

No. I have seen mains switches where a wire has come free over time. When the two meet you might have more than a blown fuse.

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
Remote Viewing classes in London

Not enough information.

If, for example, this is for use in house wiring, I believe that it would violate code. If you are an equipment manufacturer and want UL listing, it may not have sufficient clearance between the two. If you are bulding your own electric gutar amp, go ahead, but use heat shrink and cable ties so that if a wire conmes loose it will not short between 120V an 48V.

The easy answer is to use a relay instead of worrying about it.

Yes, they still make crap relay/switch contacts that are mostly intended for 120 V AC or higher use. They're switches and relay contacts that will not work very well after a period of time if they were strictly AC type contacts being used on DC even at very low currents. One of the common designs of AC power relays were to use materials and shapes that could handle an arc from things like inductive loads etc.. These type of contacts tend to have some resistance due to the material used.

When selecting relays and special switches, contact operation modes and ratings are very important in design. Material designed to perform better for DC in many cases will live a short life when passing inductive loads through it.

A practice we used in the past was to place a .1 cap across the contacts of a relay/switch for low current DC. This would allow the cap to charge during the open state from the connected circuit. When the contacts closed, the charge in the cap is dissipated through the contacts which causes a small arc needed to help keep the contacts normally designed for AC in better operation.

This practice was taught me 35 years ago back when they had real school teachers that knew more than the average teacher does now.

One can use what one believes to be fit, after some time, one usually ends up with a practice combind from experience, references and hand me down information.

"

Personally for safety reasons I would not use the same switch. I would use a low power SPST to switch on DC through an relay (or solid state relay) capable of switching on the AC with sufficient current ratings in the contacts.

shortT

Personally for safety reasons I would not use the same switch.

** Correct.

I would use a low power SPST to switch on DC through an relay (or solid state relay) capable of switching on the AC with sufficient current ratings in the contacts.

** A " solid state relay" has a small problem with DC - it cannot switch it off !!!

The OP has not supplied enough details with his dopey Q to say what sort of switch or relay he needs for the 48 volt DC circuit. If he needs to switch off more than a 2 or 3 amps, then he has a real problem.

A small (ie mini) toggle for example will simply not do - a DC arc will form at switch off and burn the contacts up.

Same goes for typical PCB mounting relays, even ones rated at 10 or 20 amps AC at mains voltages.

..... Phil

Geez, what a bunch of little-old-ladies.

John

What? please speak up Johnny I can not hear you :)

shortT