Launch controller switch question

I was thinking about using this to replace the cheap pushbutton switch in my controller (it went kapizzle this past weekend): http://www.radioshack.com/product/index.jsp?productId 62545&cp
It's rated 3A at 125V, which ought to be good for 30 amps at 12.5V, correct? (~21.2 amps DC equivalent, if I remember the conversion right)

I've used these: http://www.radioshack.com/product/index.jsp?productId 62496&cp
Double the voltage value of the one you picked at the same amperage.
I have not had one melt on me using car battery power and any known igniter (including the thick wire used on Estes igniter leads).
-Fred Shecter NAR 20117 http://search.ebay.com/_W0QQfgtpZ1QQfrppZ25QQfsooZ1QQfsopZ1QQsassZshreadvector

It doesn't quite work that way. The rating of 3 amps is the maximum rating for the switch, as is the 125VAC rating. In other words, don't put more than 3 amps through it (at any voltage), or 125VAC across it (at any current).
That having been said, it will probably work for most controllers -- but you may want to consider the 20-30 amp automotive switches that they sell, instead...
David Erbas-White
Eekamouse wrote:
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Incorrect. See what they are rated for at 12V (or greater) DC. The contacts may not take more than 3A regardless of the voltage. The contacts may also depend on the current being AC instead of DC to cleanly break the circuit. DC arcing may damage or even weld the contacts.
I second the other poster: get a good automotive switch instead. And don't buy stuff at radio shaft.

Fred / Dave / Bob -
Thanks for all your advice.
A couple of questions -
1., Does an automotive momentary pushbutton switch even exist? I checked the local auto parts place for one, but all I could find were push-on/push-off switches and rocker switches. (Keep in mind too, this has to fit in a small project box enclosure, so I don't have room for a remote starter switch.)
2. I can understand why you shouldn't use a switch at a higher-than-rated voltage, as the contacts might begin to arc. But I can't see why you couldn't use a switch at a lower-than-rated voltage. Also, if the total watts through the switch is unchanged, how is running 3A at 125V any different than 30A at 12.5V?
(FWIW, I need enough current to fire a cluster of up to 8 estes-type igniters, so I really am going to need a switch that can handle that many amps).
Thanks again for the any enlightenment...

Eekamouse wrote:
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I'm sure they do, but it's been so long since I've looked for them that I can't tell you where to look (other than your local auto-supply store).
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A switch is not "intended" as a resistor, yet you're trying to treat it as one in your calculations. The voltage is generally picked based on the contact area/arcing that is expected. The amperage rating is based on how much current can safely pass through that contact area without providing any appreciable resistance and/or heating. That's why each of the 'ratings' is a maximum value.
David Erbas-White
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The switch never sees the max voltage and current at the same time. Hence, it never dissipates that much power. When the switch is open, it sees the voltage. If the max is exceeded, the switch may then arc. But when the voltage is dropping across the switch, there's no current flow, so there's no power being dissipated (unless it's arcing :)
When the switch is closed, it sees current flow, but very little voltage drop across the contacts. Again, very little power dissipation. But excess current can burn up the switch or weld the contacts.
I'm sure you understand it now. So when you see the voltage and current ratings, you know they don't imply a power rating, and thus you cannot pick a different current-voltage pair with the same V*I product.
Dig around the auto parts stores. Somewhere, somebody will have a high current momentary contact switch.
HTH.
Doug

I'm not an electrical engineer, but I will give it a shot.
Voltage is pressure or "potential". Imagine a bucket of water on top of a ladder and another on top of a 50 story building. Pour them down a pipe and one will produce more pressure than the other. If you fill the entire 50 story pipe with water, if the pipe is too weak, it will burst. With electricity if you exceed a rated voltage, the electricity will "leak" out under the pressure in the form of arcing. It can arc across open contacts, or through the insulation material if you exceed the voltage it is rated for.
Amperage is the total flow of electrons. Like the total flow of water. You can flow a stream of water from a hose at a certain pressure against a window. If you add 100 hoses - each with the same flow rate and pressure, you might blow through the window. in the case of electricity, the metal contacts can only handle so many electrons. if you exceed the rated flow rate (amps) they will heat up or melt or weld to each other remaining on. The old FSI controllers used the smaller Radio Shack pushbutton switches and they would burn out in no time.
-Fred Shecter

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I wouldn't limit myself to automotive searches. Here's a 20A momentary toggle that wouldn't take up any more space than a pushbutton:
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber 0-373
You may want to search various electronics supply sites such as :
http://mcm.newark.com /
www.mouser.com
www.digikey.com
Some of them do have a minimum order of $25 or so.
Or look in your local yellow pages for electronics equipment & supplies.
Tim

Eekamouse wrote:
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Automotive relay. Available in 30A or 40A versions. There are some rated higher but those are harder to locate.
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The Ohmic heating of the switch parts is I^2 * R. I have seen the results of using a 3A rated toggle switch as a safe/arm switch in a relayer. It makes a great fuse. :-)
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You should seriously consider building a relayer.
The total current will be, assuming a good 12V battery, 12V/(Rigniter/8 + Reverythingelse). Divide that by 8 to get per igniter current.
My measurements of Estes igniters show them as being consistently around 0.6 to 0.7 Ohms. So use 0.7 Ohms.
Reverthingelse includes everything besides the igniter in the circuit: battery internal resistance, wire, and all unsoldered connections which includes switches.
You should be designing for at least 5 amps per igniter. 30' of 12 AWG wire (this assumes you don't cross the line into high power, which would require 200') has ~0.1 Ohms of resistance and I like to use a figure of 10 milliohms for unsoldered connections. The resistance adds up quickly and it will be hard make it work. All it takes is one marginal connection and all is lost.
Reverythingelse Current per igniter 0.1 8.00 0.15 6.32 0.20 5.22 0.25 4.44
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Eekamouse wrote:
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Automotive parts houses carry momentary push-button switches that are uses as ignition switches. A key switch is usually used to supply power to the ignition and the push-button switch engages the starter.
http://www.action-electronics.com/switches.htm#Push

Thanks everyone. I think I'll go with a 3A switch for the controller fix, but limit that to single-motor use, and build a relay box with a high-amp automotive relay for doing the large clusters. That way I can put the battery right next to the pad and keep the wiring length to a minimum to reduce power loss.

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BTSOOM. Try a switch designed for a lawn tractor or snowblower. They all use the same key switch, which can handle plenty of DC current.
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Nothing wrong with using it at lower voltages. But lower voltages don't translate to increased amps. Unless otherwise marked your 3A 125V switch should only be used at 3A 12V, not 30A.
|(FWIW, I need enough current to fire a cluster of up to 8 estes-type |igniters, so I really am going to need a switch that can handle that many |amps).
Then you want a relay launcher anyway.
And don't forget that if those 8 motors are bigger than C6s, you have an HPR rocket that requires a 200' safe distance, L1 certification, and an FAA waiver to fly.