need help calculating 12 vdc switch rating

[mrwrstory]

I have these nifty little switch assemblies that raise and lower my windows electrically.  The switches are activated by the window crank.  They are known as "micro switches".  The switches are rated at 15 amps at 125 vac.  My question is, "what would be their amp rating at 12 vdc?"

[TodFitch]

15 amps, same as for the 125v application.

 

Might want to de-rate them a bit as a good DC switch has an anti-arc insulator to break any spark on high current applications while an AC switch doesn't need one as the arc will stop when the phase change occurs. On the other hand, I'd be a little surprised if it made much difference for something like you are using it for.

[oldodge41]

15amps is 15amps but as TodFitch says the dc will be harder on the switch. In an application such as yours I would use them with no worries. On an industrial machine with several make/break cycles a minute I would look for a dc rated switch.

[DJ194950]

Many micro swithches I've used in the past had dual amp ratings on them for 120v and 12 v.

Most actually just controlled a function via a relay that carried any heavy amp draw for that function in the equipment that they were used in.

 

Don't aftermarket power window setups use two relays per window? One for up the other for down , therefore w/ relays only a small amp current needed via the switch to activate the relays?.  The ones I've familiar with do.

 

??

 

Doug

[Plymouthy Adams]

voltage rating on switch contacts and fuses are to say that when the switch contacts are open they are safety rated not to allow stated voltage to bridge the contacts and make connection...a fuse will open at 2 amps...but say if it is 250 volt rated when the filament is burnt open...it will be enough to prevent the rated voltage from jumping the gap....that is why it is voltage rating is listed on these devices..

[mrwrstory]

Don't aftermarket power window setups use two relays per window? One for up the other for down , therefore w/ relays only a small amp current needed via the switch to activate the relays?.  The ones I've familiar with do.

 

??

 

Doug

Thanks all, but I'm still confused!

My aftermarket power window setup uses two micro switches, one for up, one for down.  They are wired such that the polarity to the motor is reversed for "up" or "down".  The kit does not supply or show relays in the schematic.  

I have wired the system as shown and the windows work as they're 'sposed to.  However, all the current required by the motor is carried through the switch.  I don't know yet how much the motor pulls under load.  I have fried several switches (really cheap Chinese ones) and blown 10 amp fuses.  I have replaced those switches with quality 15amp 120 vac switches.  One very capable friend locally says "relays",.....another equally capable says "relays not needed".

I'd like to understand what 15a @ 120vac calculates at 12vdc.  I like Tod's and oldodge41's "amps is amps" but before buttoning up the door panels for the gillionth time, I'm hoping for more comments.  Yeah, relays will solve the fuse problem but I will still not understand what's causing the circuit to fail.

[Plymouthy Adams]

sorry..ignore this..already made this statement once..

Edited October 5, 2013 by Plymouthy Adams

[DJ194950]

Question have you just replaced all the window glass felts etc. and they are still very tight which require a heavy amp draw to get them started moving, particularily goig up??

 

Are the motors of your power windows the bigger ones with the field windings for the motor or the newer more expensive permanent magnet type motors? No field winding = less amp draw to operate.

 

Did you buy from a established mfg'er? Product support?

 

Best to ya,

 

Doug

[mrwrstory]

For those who may be interested, I think I'm closing in on understanding this issue.

I have found an online calculator that does the conversion.  I.e. 125vac - 15amps calculates to 12vdc - 172 amps.  So much for the math!  But then I'm thinkin, the switch, and the connectors and the wires ain't big enough for 170 amps.  Ah ha moment    Switch is designed for 125v.  Less volts is not less load   

Now there are some here who will say "dah!"..........but I didn't know that.  Like I said "closing in on understanding"  W/o getting any more of a headache working the math, I'm thinking the switches, because of the scale of their internal components, are probably only good for a fraction of the 15amp rating when run on 12 volts.

Aaaahh, does "relays" sound familiar.  Pretty sure that's where I'm at, at the moment.

 

And yes Doug, I am going over the channel alignment and diggin into the motor specs.

[Plymouthy Adams]

you looking at this wrong....

Edited October 7, 2013 by Plymouthy Adams

[TodFitch]

For those who may be interested, I think I'm closing in on understanding this issue.

I have found an online calculator that does the conversion.  I.e. 125vac - 15amps calculates to 12vdc - 172 amps.  So much for the math!  . . .

Amps is amps. For low frequency stuff this basically comes down to cross sectional area for wire and surface area for contacts. Think hose inside diameter if you are used to working with water or compressed air.

 

Volts is volts. You are basically interested in how much insulation is required to contain it. Think pressure if you are used to working with water or compressed air.

 

Electrical power is volts times amps. 15 amps at 125 volts is 1875 watts. And to get 1875 watts out of 12 volts you'd need 156 amps. By the way, that 1875 watts is the equivalent of about 2.5 horsepower. It seems unlikely that your window lift motors are anywhere near that.

 

So that switch could handle a 2.5 hp motor if it were passing 125 volts. But it can only handle 15*12=180 watts (about 0.24 HP) at 12v.

[mrwrstory]

My original question was, "what does 125vac @ 15a calculate at 12vdc?"  In my ignorance, that turned out to not be the right question.

 

Thank you to everyone who offered advice/opinion/information. 

 

I think Tod's input came closest to what I needed to know although, it took a while for me to get smart enough to comprehend what he offered.  And, as he said, it all boils down to amps is amps,.....that's given the parameters of my project.

I believe the first failures were due to cheap switches and casual  window channel alignment.  The original switches welded and that blew fuses.  That's now fixed.  The channels are tuned and the switches replaced with 15a units.  At the fuse panel, one window draws a max. of 4.1amps, the other 5.1amps,....well within the switch's rated capacity.

However, I have also decided to install relays to protect my now expensive switches.  Again, one knowledgeable friends says, "not necessary", another says, "relays".  My take is, it can't hurt.  

Of course, the debate between my two friends now is whether to switch "power" or to switch "ground".  Don't need any more opinions on that.  It's always somethin,...right     

[mrwrstory]

Wellll, I finally got it done.  Did a buncha head scratching to get my brain around the workings of relays.  Then figured

out how to make them work for my particular application.  I did have help from folks who are smarter than me.  In the

process I learnt how to draw, on the computer, what I had figgered out.  It works and I'm pretty pleased with the results.

They say, to keep dementia at bay, ya gotta keep learning   

 

[DJ194950]

Great that you learned how to do the wiring and use a program to draw it out nicely (which i'm sure was the HARD part of the two)!  Nicely done on both counts!

 

Doug