Technical electrical question...

[K_W]

I don't know any other electron wizards besides the ones here so...

I have seen switches rated differently for AC (sine) and DC (constant) where DC is rated lower... for example one switch may be rated 10A AC but only 3A DC...

I am using a Molex brand connector on a electrical box in my car that will see very low draw ~3A most of the time then an additional 0.5s pulsed draw of up to 7.5A on occasions. The connector is rated "250V 5A" per pin and the manufacturer says that is when used with AC. They do not know the continuous DC maximum as " they never tested to failure "... weird way to put it.

My question is... is 13.5V 7.5A DC more or less actual load than 250V 5A AC ?

If it is I will have to make a small change to my design.

 

[hammerd13]

K_W,

Here are some suggestions/thoughts that come to mind...

1) As it relates to switches having a higher AC current rating than DC, it's due to the alternating nature of AC and the arcing you get when you open a circuit. The DC arc you get when opening a switch, always breaks the current when it's flowing in one particular direction. With AC, sometimes current is flowing one way...at other times the current will flow the other way. Essentially, you get a more violent arc that tends to wear away one side of your contacts with DC. With AC, the arc is less severe and the contact deposition works in both ways and isn't as detrimental to the contacts.

2) I recommend you find a connector with a higher ampacity. Without knowing all the details of your electrical load, I'd pick something capable of at least 7.5A @ 14.7VDC (typical max vehicle voltage). It all has to do with I^2R losses in the contacts of the connector and resultant heat that's generated. You MIGHT be able to get away with your existing connector, but if your assumptions are incorrect the result could be a melted connector and potentially a fire!

3) You really can't equate a switch to a connector, with respect to current and to some extent voltage. A switch is designed to break the circuit, whereas a connector just has to carry the load without melting from the current or arcing from the voltage potential.

 

[ChristianPatriot]

Not an expert, but I have worked with electricity for a while now. In some lab somewhere there might care about “sine wave” ac amps vs “steady-state” dc amps, but for normal work like automotive, I would go with amps are amps. In my opinion that switch would be fine at a constant 3A load with an occasional small spike, but again, just my opinion. Will it alter your plans dramatically to get a bigger switch?

 

[Mr Evilwrench]

The reason that switch contacts are rated lower for DC than AC is that the DC load will cause a larger arc when you open the connection than the AC will. The arc will damage the contacts, requiring the switch be replaced. I've seen them weld themselves together. For a connector, you're not going to be connecting and disconnecting under load, so AC/DC doesn't matter.

 

[K_W]

I wish I could remember more of my electronics classes... the next size up in connector is rated 13.5 A per pin... that will work just fine if I run the final ground wire separate... I will just have to redo all the work I did the last two days, or run the offending wires outside the current connector.

I changed the design today and forgot about the per pin maximum... which will be exceeded on 3 of the current sized pins with the new design.

 

[jkaetz]

Another thing you may or may not be taking into account, in DC applications, if source voltage goes up, amp draw goes down. The inverse is true also. Other things that will impact the switch, how often will it be turned on and off? What will the load be when it is turned on or off? One more alternative, use the switch to trigger a 12v automotive relay. The are usually rated at 20-30 amps.

 

[K_W]

Another thing you may or may not be taking into account, in DC applications, if source voltage goes up, amp draw goes down. The inverse is true also. Other things that will impact the switch, how often will it be turned on and off? What will the load be when it is turned on or off? One more alternative, use the switch to trigger a 12v automotive relay. The are usually rated at 20-30 amps.

It's a small box that has two relays that have a capacitor and resistor on their coil circuits so that when the button for each is pressed they pulse 12V for 0.5s to the trunk release and a fuel door release I am adding. I added additional wires to allow me to connect other items now and later, which is where I was going to exceed the connector's rating.

Here's a picture of it in progress, before I added the additional wires.

 

[hammerd13]

Neat project K_W! One other point I thought I'd mention is the current capacity of the wire. It looks like you've already made some deliberate choices in your picture, so everything is probably already good to go. I believe 18AWG would work up to 8A, depending on ambient temperature, etc.

Here are some links for reference on wire gauge:
How Do I Determine My Wires Ampacity Rating? photo - Compass Marine How To photos at pbase.com

Also, check the power rating of your resistors. Those looks like 1k Ohm 1/8W resistors to me. I don't know your exact circuit, but 1k Ohm at 14.7VDC is 0.22 watts...almost a 1/4 watt.

Have fun with the project!!

Model Railroad Wire Ampacity

 

[Mr. Habib]

An AC arc will tend to self extinguish because the voltage and current go through zero each half cycle. A DC arc will maintain itself until the distance between
the contacts is too great, hence the difference in ratings for things like switches, contactors and relays. For a connector the voltage rating has more to do with
the dielectric properties of the housing material and the pin spacing. The current rating is based on heat from the current flowing through the resistance of
the connections, I squared R losses. Those will be based on continuous current. Since your application will have very short current pulses with a very low duty
cycle, you're not going to be opening your trunk 1000's of times a minute, you will likely be fine with the current rating on the connectors you've chosen.

 

[K_W]

Neat project K_W! One other point I thought I'd mention is the current capacity of the wire. It looks like you've already made some deliberate choices in your picture, so everything is probably already good to go. I believe 18AWG would work up to 8A, depending on ambient temperature, etc.

Here are some links for reference on wire gauge:
How Do I Determine My Wires Ampacity Rating? photo - Compass Marine How To photos at pbase.com

Also, check the power rating of your resistors. Those looks like 1k Ohm 1/8W resistors to me. I don't know your exact circuit, but 1k Ohm at 14.7VDC is 0.22 watts...almost a 1/4 watt.

Have fun with the project!!

Model Railroad Wire Ampacity

Resistors are 1/4w I think... they only see power when the switches are pressed. The testing that I've done they never got above room temperature and I triggered the circuit far more often than it would ever be on the vehicle, but I will look into changing them to 1/4's.

I'm using 16 gauge for the main wires, 18 gauge for the two motors and extra circuts, and 22 gauge for the relay trigger switches.

Both switched power amd constant will be on 10 amp fuses tapped to 15 or 20A circuits that have near 100% certainty that they will not be used at the same time as the new circuits. (Washer pump and sunroof)

 

[K_W]

Here is the completed box...

I switched to the bigger 13A connector, ran new wiring, added a new feature, and came up with a new cleaner (serviceable) layout. I used fully insulated connectors and added replaceable fuses inside the box just for the latch motors (10A fuses on the taps). I was able to run both latches on the bench and car with 3 and 2A fuses, so I will keep those in them to start with. That frees up 8 amps for each the constant and switched circuits and there will be 2 taps for each. I did have to run the ground wire separate to avoid overloading the connector in the unlikely event both circuits are drawing 10 amps at the same time.



I'm real happy with how it FINALLY turned out...

 

[jkaetz]

Here is the completed box...

I switched to the bigger 13A connector, ran new wiring, added a new feature, and came up with a new cleaner (serviceable) layout. I used fully insulated connectors and added replaceable fuses inside the box just for the latch motors (10A fuses on the taps). I was able to run both latches on the bench and car with 3 and 2A fuses, so I will keep those in them to start with. That frees up 8 amps for each the constant and switched circuits and there will be 2 taps for each. I did have to run the ground wire separate to avoid overloading the connector in the unlikely event both circuits are drawing 10 amps at the same time.



I'm real happy with how it FINALLY turned out...

That looks pretty good. Regarding your fuses, keep in mind that a fuse's primary purpose is to protect the wiring from overheating/burning. So long as all the wiring can handle 10 Amps, you can stick with a single 10 amp fuse at the source rather than in line. For that matter even with the inline fuses you still want a proper sized one at the source in case the wiring would somehow get shorted to ground. It does sound like you have that already.

 

[K_W]

That looks pretty good. Regarding your fuses, keep in mind that a fuse's primary purpose is to protect the wiring from overheating/burning. So long as all the wiring can handle 10 Amps, you can stick with a single 10 amp fuse at the source rather than in line. For that matter even with the inline fuses you still want a proper sized one at the source in case the wiring would somehow get shorted to ground. It does sound like you have that already.

It's the motors that I'm worried about... if my system somehow fails and puts constant power to the motors, the latch mechanism will only go so far and then stop the motors and they will basically be shorts... hope is the fuse will blow in that case.

 

[K_W]

This was me all this week during this project... (credit: Aquachigger)

 

[JettaKnight]

Why is DC and AC switch rating different? Observe:
[video=youtube;Zez2r1RPpWY]http://www.youtube.com/watch?v=Zez2r1RPpWY[/video]

 

[russc2542]

Why is DC and AC switch rating different? Observe:
[video=youtube;Zez2r1RPpWY]http://www.youtube.com/watch?v=Zez2r1RPpWY[/video]

Sweet!

 

[Mark 1911]

Don't exceed the AC current rating, even if you are using a lower DC voltage. If it's rated for 5A , don't exceed that for DC, even at a lower voltage. If anything, the amperage rating might be less for DC because switching transients are higher with DC, so DC contact ratings are always lower than AC. Current ratings for conductors are essentially the same for AC or DC.

I would suggest finding something with a current rating that is AT LEAST 125% of your continuous DC current, that way you allow a little margin and don't have to worry about frying something. The other way to look at it is this, whatever the rating, be sure not to exceed 80% of that.

NEC guidelines generally/typically are not to exceed 80% of the continuous current rating (or be sure your components are rated for at least 125% of your continuous current) that can vary depending on circuit types and voltage levels, but that's a good starting point. There are other de-rating factors that come into play on some circuits depending on other sources of heat, such as sunlight, heating from other circuits, etc.. You probably don't need to worry about that for your application. Just use the 80%/125% rule, you should be ok.

Since I'm a day late and a dollar short as usual, I took a look at your project. You should be fine.

 

[K_W]

Don't exceed the AC current rating, even if you are using a lower DC voltage. If it's rated for 5A , don't exceed that for DC, even at a lower voltage. If anything, the amperage rating might be less for DC because switching transients are higher with DC, so DC contact ratings are always lower than AC. Current ratings for conductors are essentially the same for AC or DC.

I would suggest finding something with a current rating that is AT LEAST 125% of your continuous DC current, that way you allow a little margin and don't have to worry about frying something. The other way to look at it is this, whatever the rating, be sure not to exceed 80% of that.

NEC guidelines generally/typically are not to exceed 80% of the continuous current rating (or be sure your components are rated for at least 125% of your continuous current) that can vary depending on circuit types and voltage levels, but that's a good starting point. There are other de-rating factors that come into play on some circuits depending on other sources of heat, such as sunlight, heating from other circuits, etc.. You probably don't need to worry about that for your application. Just use the 80%/125% rule, you should be ok.

Since I'm a day late and a dollar short as usual, I took a look at your project. You should be fine.

Thanks, I did. My max draw is 10A, the connector I have now is rated for 13A.

The switch I ordered arrived today and I'll start putting it in the car, tomorrow if the rain holds out.

 

[JettaKnight]

Then you'll be fine, it looks good.

Remember, if it starts ticking, cut the red wire.

 

[K_W]

It's done, and works. Only took me 20 minutes longer than I had planned (about 6 hours). The box is in, everything is wired up, secured and loomed neatly, the trunk release works, the fuel door release works, and my Homelink rearview mirror is tied into The Box for power.

I'm going to order a hardware kit for my dash cam and put that in next week using The Box, a switch, and a couple small diodes for selectable key or constant power as a parking lot mode.