Switch current
FlyingFishFinger
Posts: 461
Hi again...if I have a switch that says 6A 125VAC, is that equivalent to putting 62A at 12V through?
My app reqires between 10A amd 15A at 12V, so I could use this switch (it's about 1 cubic cm or the size of a dice, looks small for that current rating), correct?
I'm worried because when I tried to solder to it (at least when I tried half a year ago, when I was still teaching myself to solder), the case started melting and one of the contacts came off (it was a DPDT). So I cracked that one and looked inside, and I found just a few small bits of metal. It was probably just my solder technique, but I thought I'd better check anyway before I melt the other one...
Rafael
My app reqires between 10A amd 15A at 12V, so I could use this switch (it's about 1 cubic cm or the size of a dice, looks small for that current rating), correct?
I'm worried because when I tried to solder to it (at least when I tried half a year ago, when I was still teaching myself to solder), the case started melting and one of the contacts came off (it was a DPDT). So I cracked that one and looked inside, and I found just a few small bits of metal. It was probably just my solder technique, but I thought I'd better check anyway before I melt the other one...
Rafael
Comments
6 Amps is six Amps, at 125 V or 2 Volts.
On a switch, the maximum Amperage is determined by how much internal resistance, and hence heat, will build up in the switch. This internal resistance creates a small voltage drop (normally not measurable), and the "loss" of electrical energy is converted to heat. Whether 12 Volts or 120 Volts is going through, you can still only take the current (Amps) that the switch is rated for.
The voltage rating (and AC vs DC) is determined by a combination of how well the swtich is insulated, and how far the internal contacts separate when opening. While the switch you have specifies 125 VAC, this doesn't mean that it can't be used for DC, but it's maximum voltage in a DC application is probably lower. One reason for this is because when the swtich "breaks" contact, DC will have more of a tendancy to arc than AC. (As the AC current reverses direction, the arcing "stops" as the sine wave passes through zero, and it takes more energy to start the arc back up than keep it going. DC doesn't cross back and forth over zero, so the arc can last longer, causing more pitting, or even welding of the contacts together.)
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John R.
8 + 8 = 10
Rafael
At lower voltages, there MAY be an increase in allowable current, but it does not following that the watt formula can be used over the entire voltage range from 0 to the rated voltage. It also does not follow that the switch in question would be good for 1.5 Amps at 500 VAC.
The current and voltage ratings may also be quite different for DC voltage.
Your best bet would be a manufacturers data sheet.
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John R.
8 + 8 = 10
Depending on the TYPE of load you have (inductive, capacitive or resistive), you might get away with that switch, but then maybe not.
As John R. indicated, inductive DC loads are very prone to arcing due to current wanting to continue to flow by means of the collapsing magnetic field after the switch is opened. This kind of acts like a generator, and the result is murder on small switches.
Cheers,
Peter (pjv)
Only thing is, it's so big I'll have trouble mounting it on my controller PCB...
Thanks everybody
Rafael
If you tried to use that switch, it would either;
a) weld the contacts together.
b) melt the switch housing.
c) probably both at same time....
That small (physical size) of a switch was not made to handle heavy amperage.
You are much better off switching a heavy duty relay ( automotive 30A )
or using a power rated mosfet with heat sink...
The automotive relay is the lowest cost solution, and the pysical size is around one cubic inch...
Remember 10-15A at 12v requires a large wire, or very heavy circuit board trace, and a strong connection to unit
doing the switching ( switch/relay )
I have done the same thing as you, but I set up an automatic power center with battery backup for Ham radio...
My 1st switch was not heavy enough, and it burnt up
Bob N9LVU
Rafael
If it is switching a DC voltage, it might work for a while, what does it say on the switch???
Typically a switch that is rated for 7A 120AC, is not rated for the same current 7A at DC
due to the caracteristics mentioned before "John R ".....
Seeing its only about 1 cubic cm, I will bet that it is NOT rated for high current at dc voltages...
Are you switching 12v dc, or 120v ac???
What is your power source? 120v ac to 12v dc supply???
Bob N9LVU
Rafael
Ok, just remember, even though the switch says 20A 125vAC / 15A 250vAC, it will NOT
have 20A at 12vDC, it will be more on the order of 10A 12vDC as explained before.......
Some automotive switches for external lights ARE rated 15-20A at 12vDC.....
Bob N9LVU
Rafael
Most automotive switches are panel mount (snap in rectangular/ round hole threaded + nut) and use .25 flat connector.
Some are also lighted which could come in handy, spiff up your project.
Bob N9LVU