AC Switch in DC Circuit

The voltage is OK but the battery could put out more than 16A in your App you need to measure worst case amperage in your application.

You brought up a good point and something I never thought about. Which is not unusual.

In the describes scenario just for a theoretical what if, would adding a capacitor across the DC load be of any use. I realize it would be a trial and error scenario, it's merely a point of curiosity. Probably more appropriately on the switched side of the load.

It also seems like the freewheeling diode may be of use. As I say I've never thought of that characteristics alternating current vs. direct current in this application.

The Alternating Current (AC) amplitude varies and Direct Current (DC) amplitude is constant with respect to time. AC has zero crossing while DC does not have. So when you break the Current path using a switch in case of AC the current pass thru' zero and the spark gets extinguished, while DC does not pass thru' zero and hence current continues in the air gap for some more time and thus switch gets damaged.

In your case a 16Amps switch can be used for 1.6Amp you can use.

J.N. Karamchetti Hyderabad

If the sensor is purely resistive, you should be OK - I can't think of any speed sensor which would draw more than about an amp. If there's an inductive component to the load, you need to look harder at it. You may get away with fitting a free-wheel diode.

What are the consequences of the switch failing?

2007: go 4 it. perry

Why would you not replace the non-functional switch with factory correct switch?

Existing switch is (at minimum) form C Momentary... maybe DPDT, maybe another form, LEDs may be part of trouble circuit... why replace with an item incorrect all the way around? The nav system may not work, the incorrect switch may actually damage the nav system, whatever mfgr's warranty remains will certainly be voided...

Sometimes, creativity can be a bad thing. Perhaps this is why you are having trouble finding an answer.

Good points, but the OP wishes to void the warranty in the first place.

If it's a bad thing it has nothing to do with creativity. Sorry!

Well, you have a point there. I may have misspoken; how about

Sometimes creativity is ill advised?

Sometimes, I believe, well intentioned creativity goes beyond the practical application of the problem at hand. You are correct, though. Creativity should not be stifled, because you never know what you might come up with...

Well there is no factory switch. My factory nav has a safety feature that does not allow you to input anything while the vehicle is moving...via speed sensor. I understand the reasoning because they do not want driver's attention off the road while driving and I'm not stupid enough to play with it while driving.

However, my wife and i take several long road trips a year and it is annoying that she, as the passenger, cannot access the nav.

Basically, i'm hacking into the speed sensor to disrupt the circuit with a switch to allow input into the nav and then go back online. I know how to do it with a basic SPST switch however, i wanted to use an illuminated switch for night time use. the switch that i liked is AC and it is dual illuminated...Green on and red off.

If the illuminating LEDs are for 120 VAC, how will they work on 12 VDC?

^^^well thats why i'm on this forum. to find out questions about this setup and see if it will work or not.

You can buy very nice illuminated DC rated switches for use in cars. Go that route. Just do a search for "illuminated bi-color automotive toggle switch".

Example:

I doubt the LED will work at 12vdc.

The speed sensor should produce almost no power. Hint will be light guage wires.

I doubt the speed sensor can power the LED.

Just try a plain toggle switch.

If the wires are really light you may even need noble metal contacts.

If the wires are reasonable size then an average switch will work.

As a side note, my travel trailer uses a 110vac standard wall light switch for the 12vdc water pump. That draws lots of amps (over 5amps). No special circuit is used.

I have used 110ac wall switches in a number of 12 volt apps and had no problems. i would much rather pay under $1 for a big switch,than pay 3-$4 for a little 12v switch.You should have seen the look on my friends face when my glowplug relay went out in my truck,and i replaced it with a 50 amp house breaker from an old fuse box in the garage and stuck it under the dash.Diesels start real fast in the cold if you run the glowplugs for about a minuite before hitting the starter. And before i get in trouble here and barraged with dont do thats,they were continuous duty glowplugs, not standard ones.Do not hot wire your glowplugs unless you know what you are doing. Severe engine damage can occour.

It is very unlikely that the amperage in the speed sensor circuit is even close to 1 amp, so the switch is likely to work fine. However, the LEDs will not work. Even in a switch designed for 12VDC it is unlikely that the LED's will work, because it is very unlikely that you are actually switching 12 VDC: the sensor could operate in various ways with various voltages, but it is probable that the sensor voltage to be switched is not more than 5 volts, and it is almost certainly pulsed.

You would also want to be sure that the sensor is used only for the nav system and not for other things. Ordinarily, engineers would not want to duplicate speed sensors for various functions, if the data can be communicated across the CAN bus instead. Speed sensors are used for the car's ECU and ABS, so you'd want to be sure that you are not mucking with those.

It seems odd that the nav unit does not simply rely on its own calculations from GPS data to tell if the car is moving.

A good solution to this would be through software, rather than hardware. (The obvious problem is you don't have access to the software, of course.) To me, such a system seems pretty idiotic -- I ask my wife to reprogram the GPS all the time if we are in the car together -- and not enabling that seems like bad design. Are you certain the function cannot be overridden via a touch screen setup menu, etc?

I have the speed sensor and Switch wired into a relay and the switch is drawing power for the LED from the 12VDC wires in back of the cigarette lighter.

You're fine then. Automotive relays coils typically draw less than 100 milliamps. If you wanted to get exotic, you could put a diode across the coil (opposite to the polarity that would blow fuses) but I think that is overkill.

it is hard to say w/o seeing the actual switch, but if the switch is mechanical then it should work fine on DC. You may not be able to use the LEDs but you should still be able to use it to toggle your speed sensor. chances are it is rated AC because it may have a ground contact. W/ DC you only really need to interupt only one leg of the circuit, altho you could use both legs. + and - . if you could draw a diagram of the switch and/or include a photo, it would help. good luck

Hi,

You can use this switch as far as load is not more than 16Amp. However LED illumination will not work.

Thanks

D M Daga

Hi,

Your switch can carry the DC load but the LEDs will not work , so you have to look for a switch with DC operated LEDs to illuminate.

Thanks,

Joseph

I was going to tap into the cigarette lighter's 12VDC to power the LED's in the switch. What do you think?

You'd have to provide a sketch.

Ordinarily, in an application like yours, the speed sensor would be the switch's input and the nav unit would be the switch's output. The sensor could not provide power for the LED. The third terminal usually found on such switches is for ground , which serves only to complete the circuit for the LED -- the assumption being that the switch input terminal is connected to "power" with a voltage equal to the switch rating.

You would not want to connect both the sensor and 12V from the cigarette lighter to the input of the switch. The sensor could be as simple as a magnetically actuated reed switch with one side connected to ground, to provide a ground level pulse to the nav unit. Connecting this to 12VDC would cause a direct short.

Most "chain" auto parts stores have the switch you need; cheap. Use a dc rated switch. Always use parts that are intended for the purpose and you will avoid many problems.

Quote DC Rule of Thumb For those switches that list an AC voltage rating only, the "DC Rule of Thumb" can be applied for determining the switch's maximum DC current rating. This "rule" states the highest amperage on the switch should perform satisfactorily up to 30 volts DC. For example, a switch which is rated at 10A 250VAC; 15A 125VAC; 3/4HP 125-250VAC, will be likely to perform satisfactorily at 15 amps up to 30 volts DC (VDC). Unquote http://www.carlingswitch.com/products/switches/learn_more.asp?page=switches_amp-rating

That rule-of-thumb may be OK for resistive loads, but in general will not hold for inductive loads.

The rule of thumb is also only for Carling switches that have a certain current rating. I certain Carling makes no guarantee for any other company's product.