Switches for AC/DC

Not generally interchangeable. DC arcs are harder to quench, so faster operation and larger clearances may be needed for DC switches.

what about when you're in Europe or another place with 50 HZ?

Same diff, execpt it's only 100 times/second.

Metric, you know.

There's an additional concern when switching what many people consider DC. Many AC rated switches do not have self wiping contacts. The switch designer expects the contact oxide layer to break down with the high voltage. The low voltage and currents of many DC circuits will not work with some AC switch contacts.

As has been stated, arc control is the main difference between AC and DC switches due to the tendency of DC to attempt to maintain its flow and so produce an arc upon contact break.

It is common practice when switching DC to use multiple poles of one switch in series to increase the arc gap and thus prolong contact life. You can often get by with an AC rated switch by utilising this method.

Yes perhaps you could get away with the use of an AC rated device by wiring the poles in series.

But... I must point out that doing so is using a device for a purpose it is not certified for and is not allowed by the codes (and therefore by the legislation) adopted in North America and would expose anyone who did so to liability should an accident (read failure) occur.

Most switches that I have seen for both, have the current/voltage values stamped or marked on the switch itself for AC and DC.

Just follow that advice!!

Nothing could be simpler.....

Some relays are similarly marked....

Quite! Different values for the 2 cases. GA

You are correct.

Most dual rated switches have a lower value for DC than for AC use because of the previously mentioned reasons.

Relays,however have an additional element that is required for AC usage:

It is a shading coil.

This consists of a single turn copper shunt in the armature or stator of the relay.

This generates an opposing current in the shunt,and holds the armature when the AC crosses zero and the magnetic field collapses.

This prevents the armature from "buzzing" or "Humming" on A/C.

I have used AC coils for DC operation in a pinch,but much less voltage is required to

activate the AC coil when used on DC,due to the lack of inductive reactance.

A 120 Volt AC coil will work safely in a 24 Volt DC application, but make sure the

contacts can handle the applied load.

The contacts are usually have a DC and AC rating,even on an AC coil.

but the reverse is not true for obvious reasons.

I would not specify or recommend such use,but in an emergency,it will work quite

reliably.

DC coils do not have or need this shading coil feature,so when an ac relay is used in

a DC situation, the shading coil is redundant.

This also makes it easy to determine if a relay is designed for AC or DC,in the absence of a label:

Simply look for the shading coil.It will be obvious to the trained eye.

Not needed on relay coils driven by DC, but switching AC on the contacts, as is often the case!!!

I had forgotten about how it was organised with AC coils, thanks.

Which is why the contacts on relays still need to be both AC and DC rated, but not the coil in most cases.

Maybe even less often today than years ago!!

I would expect AC or DC coils to be identifiable also in some clear way (but I am too lazy to look up at this time!).

I have personally always avoided using AC relay coils as they ALWAYS make some noise, especially as they get older. I hate them alone for that reason!!....

An extra diode bridge and a cap sorts that out for peanuts!! DC relay coils are often cheaper and easier to source as well.....

I do believe that the possible AC vibration can weaken the AC coil winding over long periods, especially where maybe not fully mechanically supported, so reliability is also a question/issue in my mind with AC relay coils on AC, from experience only.....

On older washing machines when they went wrong, it was either the motor or a relay 90% of the time in my small experience....probably both too cheaply made....

Have a great New Year!

As the farmer said about the mud hole"It is deeper than it is wide!" this subject can be very deep when you get into it.

Most digital controls of loads are now solid state,instead of dry contacts.

Even 2kw motors are switched by solid-state relays that have a digital switching input.

The older systems are still plentiful,and will be around for many more years.

Motor starters,for instance,are mostly A/C coils,and are very reliable.

I know of an instance where 2ea. size "0" reversing starter was energized from

forward to reverse about every second,and it was continuous duty,24/7.

It was replaced once a year,on routine maintenance and was not replaced because of failure.

That is over 3 million operations.

I inspected the armature on one of these relays,and the laminations around the

shading coil had been hammered down to flush,but it still worked flawlessly.

I eventually talked them into replacing the electric 3 phase motor with a hydraulic

unit and a solenoid.Much more efficient.

All of your larger motor sizes were controlled by dry contacts,with thermal overload

contacts in series with the coil.

They do hum a bit, but this does not harm them,and does not weaken the coil.

I look forward to the time when Piezo activated motor starters will dominate the

large motor controller market. This will be a tremendous electrical savings world wide.

Just think about how many motor contactors are energized at any one moment.

You missed my point, its the hum I don't like......

The real reason they hum is:

------------------------------------------------------------------------------------------------

They don't know the words.

LOL!!

Business as usual. I do not think the ugly corporates will be happy with this (replacing dry contacts with SSR) Hitekrednek. What's wrong with the world today?-Everything was not built to last and the marketers want it that way, in fact, they still did a sales forecast with quota monthly on a service maintenance company, which I think is dumb - managers get frustrated when your forecast sales is low, now tell me, are they maintaining or service inclined? O, you know what it is.

I really agree what old folks says(classic was all good stuff-music, appliances, morals), I got still the clothes iron my mom got as a gift on her wedding about 1960's and still works well.

SSR's are not the same as Piezo Relays.

SSR's are commonly used in hundreds of current applications today when digital

switching of motors is required.

A SSR uses Triacs for switching,a Piezo Relay uses Piezo-electric movement of

contacts,which is currently limited to low current-low voltage operations.

They are generally constructed with 2 parallel strips of piezo material,and when

voltage is applied,it either contracts or lengthens, depending on polarity of applied

voltage.

This makes the strips bend to the left or right,causing a contact to open or close.

The energy consumption is very low compared to electromagnetic coils,however they

are currently limited to low current and low voltage applications, but I expect that to

change as the emphasis on energy conservation becomes more focused.

As has been said, DC arcs are harder to quench, and artificial blow-out means are generally employed. Permanent magnets or blow-out coils are usually seen in heavy duty switching devices. With the advent of solar Pv technology, MCBs with permanent magnet blowouts are common nowadays.

Here is one depiction of the blowout coil, suitable for AC and DC. Obviously, one need not worry about polarity of the DC while connecting. (Many PM blowouts are polarised, in that one has to watch where +ve and -ve leads are connected)

Every conductor used in wires and contacts has power capacity.

What's the difference in the material used in DC or AC contacts irrespective of aspect ratio?- I say none, there all same material.

But think about the below

in DC (Power=Volt x Current in Watts) Service Voltage (3, 5, 12, 24V)

in AC (Power = Volt x Current in Volt-Ampere) Service Voltage (110, 220, 380, 440 Vac)

DC has lower service voltages compares to AC, right? For a given Power Rating say 1100 Watts, what were the corresponding Current Capacities with the contacts?

Of course, DC contact has larger contacts than AC, Yes? (Power Factor disregarded)

In relays, there are always specification, sometime AC/DC contact ratings like the pic attached. Therefore, the best practice is do not dare to AC/DC a contact without seeing the manufacturer's specification of it.

In our country, the Fire Department, whenever there is fire always has a report like "Fire is due to electrical faulty wiring".

Thank you all for your responses. It seems there is more to a simple looking switch (toggle, stab) than meets the eye.

Contact ratings rated for DC operations are much better and will last longer when used for AC operations...

From experience, In the 60's I used to series connect AC rated contactors in order prolong and extend their usable life. Contact arcing were greatly diminished doing such practices!