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AC switch vs DC switch

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SJP0tato

Senior member
Aug 19, 2004
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Hi guys,

Looking up switches for a DC application I'm planning on, I've found several that are rated for various voltage/wattage for AC (say 10amps at 120vAC), but I was curious if there's a good, or even an approximation of what this would be for DC. My voltage is going to be small for the DC (in the range of 10-14 volts), but amperage could vary from 0 up to around 15 max.

I found an artice explaining that AC vs DC amperage ratings are very different, and that a switch that can handle 10amps at 120vAC might only handle 0.5amps at 120vDC. But what I'm trying to figure out for my greatly reduced DC voltage range, would a 10amp/120vAC switch be enough to handle the current?

If anyone has any insight it would be appreciated,

Ryan P.
 

bruceb

Diamond Member
Aug 20, 2004
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Question: How do I figure the DC rating of a switch if I know the AC amperage rating?

Answer: Multiply the AC amp rating by 1.8. A switch that has a 10-amp rating at 125 volts AC would be rated at 18 amps at 12 volts DC. Use a 20-amp fuse or circuit breaker to allow for voltage surge. A good rule to remember is "when voltage goes down, amperage goes up."
 

bobsmith1492

Diamond Member
Feb 21, 2004
3,881
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Where did the 1.8 rule come from, bruceb?

One to think about is the release of the switch; at high voltages, when a switch is released, it may pull a small arc which damages the switch a lot. That becomes a non-issue in your case.

However, since your switch is only rated for 10A AC, and you want up to 15 A DC, I wouldn't recommend it. DC is more stressful (more power) for the raw amount of current, unless the AC is rated RMS, in which case the switch should be good for 10A - NOT 15, though. If the AC current is maximum or something, its RMS would be even lower, like 7.1A.

The voltage drop across a switch (a very low resistance device) will be very small regardless of the voltage of the supply that is being run through it. The current is the most important thing; more current => more voltage drop => more power dissipated in the contact => higher chance of the switch heating up and melting.

'A good rule to remember is "when voltage goes down, amperage goes up." '
This wouldn't really apply to a switch, except in the breaking situation (high voltages pull an arc); the voltage across the switch WILL NOT go down if the current through it is the same.
 

DrPizza

Administrator Elite Member Goat Whisperer
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Mar 5, 2001
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Wouldn't the heat generated in the switch be caused by I^2R types of losses? As long as the resistance in the circuit is >> the resistance in the switch, couldn't the voltage simply be ignored? (I could be quite wrong.) I really haven't much experience in going back and forth between AC and DC using the same components.
 

PowerEngineer

Diamond Member
Oct 22, 2001
3,293
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When it comes to switches, the big difference between AC and DC is that AC naturally has "current zeros" twice a cycle. As you open the contacts, you'll draw an arc until there's a current zero which obviously means it extinquishes. Assuming the current and voltage are somewhat in phase, the voltage across the contacts at the time will also be relatively low. This gives the ionized air formed by the arc some time to dissapate before the voltage rises to a maximum, and so the air gap should hold. With DC, there are no current zeros. You have to break the current by separating the contacts across so much air that the voltage isn't enough to maintain the ionized path. This means that the AC rating of a switch should be higher than its DC rating. I'm guessing that a 120 VAC switch might handle 14 VDC, but (as others have said) I'd look for one with at least a 20 amp rating. My advice is to consider using automotive parts rated for similar volatges and currents; maybe a starter relay

Here's more information
 

MrDudeMan

Lifer
Jan 15, 2001
15,063
90
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Originally posted by: PowerEngineer

When it comes to switches, the big difference between AC and DC is that AC naturally has "current zeros" twice a cycle. As you open the contacts, you'll draw an arc until there's a current zero which obviously means it extinquishes. Assuming the current and voltage are somewhat in phase, the voltage across the contacts at the time will also be relatively low. This gives the ionized air formed by the arc some time to dissapate before the voltage rises to a maximum, and so the air gap should hold. With DC, there are no current zeros. You have to break the current by separating the contacts across so much air that the voltage isn't enough to maintain the ionized path. This means that the AC rating of a switch should be higher than its DC rating. I'm guessing that a 120 VAC switch might handle 14 VDC, but (as others have said) I'd look for one with at least a 20 amp rating. My advice is to consider using automotive parts rated for similar volatges and currents; maybe a starter relay

Here's more information
my advice is to listen to PowerEngineer since he definitely knows what he is talking about.
 

Calin

Diamond Member
Apr 9, 2001
3,112
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0
Also, you should take into consideration that the air gap needed to separate a certain voltage is on the order of the kilovolts per centimeter. However, once a spark is there and running, the air gap needed to close it must be much larger (hundreds of times). Industrial designs use forced flow to "break" the spark (fresh air is forced along the airgap).
The spark in the air gap is dependent (much more) on the current capacity of the circuit than on the voltage it can produce. Anyway, in the end, listen to PowerEngineer
 

Jimmah

Golden Member
Mar 18, 2005
1,243
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That starter relay seems overkill for such small power requirements. Go to your hardware store or automotive store and buy one of those standard 12v/30amp DC relays, I had one gerry-rigged into my old comp to turn on another PSU and WC pump. For only 4.99$ its worth experimenting.
 

SJP0tato

Senior member
Aug 19, 2004
267
0
76
Thanks for the suggestions/help guys. I'll keep looking for a more heavy-duty switch or relay to get what I need accomplished.

 

pyro53

Junior Member
Apr 2, 2013
1
0
0
Misuse of switches can be bonehead stunt. Gain a little real application information. Here is an old article, by an old gentleman, note the info source,
as there is an incredible wealth of "real" & proven information available in this aviation community.



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[PDF] SWITCHESAC VS DC
... in load carrying capability are dramatically non-linear and are best appreciated
by carefully inspecting ahigh-quality switch carrying both AC and DC ratings. ...
www.eaa.org/sportaviationmag/2005/0502_switches.pdf - 2010-01-27




SWITCHES AC VS DC
by Art Bianconi (EAA 92330)
Technical Counselsor 1216
99 Dover Green
Staten Island, NY 10312
Some years ago I w».s fortunate to be able to work alongside engineers from
Underwriters Laboratories (UL) during destructive testing of electrical devices.
It was part of my apprenticeship as a designer for a major electrical manufacturer and it was during this period that I acquired an appreciation for the vital
differences between AC and DC ratings for switches.
I share this with you because I am growing increasingly concerned at the
widespread lack of appropriateness most aircraft builders demonstrate
when selecting switches for the cockpit environment. Each time a builder asks
me to perform a pre-FAA inspection of the aircraft, I carefully inspect the
switches and to date over three-fourths of the projects inspected have turned
up AC rated or non-rated switches in DC circuits.
There is a large scale misconception that any switch can be used so long as
its current rating exceeds the maximum load in the circuit. "Current is current;
what difference does it make whether it's DC or AC? Besides, I'm using a 125
volt AC switch in a circuit with only 12 volts!" The differences in load carrying
capability are dramatically non-linear and are best appreciated by carefully
inspecting a high-quality switch carrying both AC and DC ratings. Typical of this
is the roller and bar micro switch made by MICRO Corporation (Part No. DT-
2RV23-A7). Rated at 10 amps at 125 or 250 volts AC, the same switch can
only carry .3 (that's three-tenths!) of an amp at 125 volts DC. If DC voltage is
increased to 250 volts, the current rating drops even further to .15 amps! In
real terms, this represents less than 1/60 of the original load carrying ability
and all we did was go from 250 volts AC to 250 volts DC!
Those of you who can still remember the old Kettering coil ignition systems
will recall that when the condenser in the distributor went bad, the points generally turned blue and melted down in just a few minutes. Cockpit switches
don't have the benefit of condensers to absorb the electrical inertia present in a
DC circuit and, as a result, the gap temperatures get hot enough to weld contacts,
even those made with exotic high temperature alloys.
The reason for this is simple enough to appreciate: because AC current
changes directions 120 times a second in a 60 cycle circuit; there are 120 times
when there is no current flowing at all. The current actually helps turn itself off
the moment it sees a gap and switch desrgners use this phenomenon to help
reduce the cost of manufacturing AC switches. In DC circuits, however, the
"push" is constant even when the points begin to open and the resulting arc is
DC current's way of demonstrating its resistance to termination.
"But won't my circuit breakers protect me?" No, they won't. Fuses and CB's
provide overload protection and a welded set of contacts will not, by themselves,
cause an increase in circuit load. Furthermore, what often happened during UL testing
was that the points welded shut making it impossibleto open the circuit. Cycling the switch
to the open position was often misleading — yes, the lever moved but inside
the switch, the cam had separated from the welded points and while it appeared
to have interrupted the circuit, the circuit was, in fact, still hot. If the circuit involved was your fuel transfer pump or fuel boost pump and you thought it
turned off when in fact it was still running, what would the consequences
be? If it were a flap or elevator trim motor or a gear retraction device, how
would a tripped circuit breaker save you if the activating switch was welded
closed and in a mode other than what is required for a safe landing?
A DC rated switch will cost you about 3 times more than an AC rated switch
of identical current capacity. If your panel sports 10 switches (which is not
likely) the difference will be less than $35 (in 1986 dollars). You've gotten this
far. Is it worth jeopardizing your investment or your safety by cutting corners
with cheap or improperly rated switches.
 

Red Squirrel

No Lifer
May 24, 2003
59,336
8,442
126
www.uovalor.com
When it comes to switches, the big difference between AC and DC is that AC naturally has "current zeros" twice a cycle. As you open the contacts, you'll draw an arc until there's a current zero which obviously means it extinquishes. Assuming the current and voltage are somewhat in phase, the voltage across the contacts at the time will also be relatively low. This gives the ionized air formed by the arc some time to dissapate before the voltage rises to a maximum, and so the air gap should hold. With DC, there are no current zeros. You have to break the current by separating the contacts across so much air that the voltage isn't enough to maintain the ionized path. This means that the AC rating of a switch should be higher than its DC rating. I'm guessing that a 120 VAC switch might handle 14 VDC, but (as others have said) I'd look for one with at least a 20 amp rating. My advice is to consider using automotive parts rated for similar volatges and currents; maybe a starter relay

Here's more information
Interesting, that makes lot of sense. I always wondered why DC tends to arc more, but now I know.

Also while on the subject of a DC application, I assume there is the possibility that batteries are also involved. Make sure that the switch is not close to the batteries if there are any unless they are sealed. The spark could potentially trigger a hydrogen explosion. Use a sealed contactor if it must be near batteries. This is a low risk mind you, but still something to keep in mind.
 
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