If 1/2 W or even 2 W is too little for 2.2 ohm and these wattages cause the resistor to fail, then why are they even produced and sold?

To "measure with the circuit energized" would I put test leads on the original wiring while hooked up to the resistor with the car on? How do I compute the current based on a resistance that I have not measured?

 

Review what I said in post #358. If you use a new resistor, the light stays off, right? So measure the resistor's value before installing. Measure the voltage across resistor once installed and powered. Compute current and power based on previous formulas.

Resistor is in series with the seat circuit right? In a series circuit, current remains the same across all loads. Voltage drops differ. Knowing the supply current you can even compute the seat's apparent resistance (subtract voltage drop across your resistor from supply then calculate for R using the computed current).

Edit. Components are designed to handle particular power requirements. It's all based on applications. Most electronic circuits are very low power so 1/4 watt rating is plenty with margin to spare.

 

The light stays off with a new resistor for a while (4-6 months) I have measured the resistors before and they are typically right on for the ohm rating they are claiming to be. The value you are talking about measuring is in ohms, correct?

The resistor is wired up between the two wires for the airbag under the seat where they would normally be plugged into the seat. So should I measure the original wiring with a resistor hooked up to see if the voltage remains constant?

So I should measure the volts going across the original wires powered up and multiply that by the ohm rating of the resistor and double it to make sure I have enough ohms in the resistor (using your previous example of 2.5V)? How do I know I have the correct ohm rating? Any theories as to why these resistors seem to work for 2gornot2g, but not for me?

Thanks for all your help- I am learning a lot. Now make sure to explain everything to me like you are talking to a 5 year old when it comes to electricity.

 

5 yr old explanation; Don't stick your finger into a powered empty light bulb socket!!!! Been there, done that.

Correct, resistance is measured in ohms Ω.

Can you post a pic as I'm having a hard time visualizing this. By your description, it sounds like the resistor(s) placement is in parallel with the seat.

So lets clarify that first before moving forwards.

 

Well, I am beyond the light socket so maybe a 6yr. old explanation.

Here is a picture- the resistor is under the electrical tape at the top of the pic

 

I need to see how it hooks up relative to the seat's wiring.

I kid you not.. as a youngin (4-5 yrs old?), I had this bright (pun intended) idea to stick my finger in the socket and turn the switch on.

 

I'm not sure I understand what you need. Normally there is a plug hooked up to both ends of the yellow wires, but now there is just a resistor with a wire off each end wired up in its place.

Does this picture help? It is an older resistor, but this is essentially how it looks without the electrical tape.


My dad did something very similar when he was much older. To see if a light socket worked he stuck a screwdriver into it and got quite a shock. To be fair he was extremely intelligent, but definitely not nearly as handy as me. He used to joke and refer to himself as "handyman Jim." One year he got in the mail a membership for the Handyman Club of America. Everyone got these membership offers in the mail, but he played it off like they knew he was an excellent handyman and it was an exclusive offer. Nothing could have been further from the truth.

 

Ok. I think I understand now. In this case it's even simpler then.

You know the value of the resistor (X ohms). Using a dmm, take a voltage reading across the resistor leads with the circuit energized.

Compute for I (current) = V/R. Now you can figure out what power rating you need, P=(I^2)R, or just I*V. Use a resistor at least 2-3x for safety margin.

As an example, lets assume 12V and 2Ω, so that's 6A and 72 watts. Highly doubt there's a 12V across the resistor as you'd be frying them let and right.

 

Awesome! Thanks man- I will try that as soon as I get time.

 

Jsolo- When I took the reading on the airbag wires with power I got a range of 2.0-2.8 ohm. It usually settled around 2.2-2.4 so I think I have the right ohm rating. Does this mean I have the right wattage, but should consider doubling the wattage of my resistors to 4.4 watts to be safe? I tried getting a reading here in volts on this setting, but nothing really came up in AC or DC unless I did a "relative" reading. When I did that the numbers fluctuated wildly.

 

I'm not sure what relative means. Relative to what?

What range were the number fluctuating between? This is one of those times an analog meter would be more useful to see the 'swing'.

 

That's what I was thinking- relative to what? It was always a negative number and it fluctuated a lot, probably almost 100 points. Does these readings even mean anything in "mV?" You mentioned earlier that I would check the volts in ohms. If that is the case then, my ohms are reading right around 2.2-2.4 most of the time. So should I interpret that to mean I should look for 2.2 ohm resistors that are in the 4-5 watt range (double the ohm reading)?

 

Assuming you were measuring DC, negative just means you had the tester leads polarity reversed.

mV means millivolts - thousandths of a volt - really small value. I can't answer the watt question without an accurate volt reading.

 

I tried the leads on both wires in AC and DC and it always resulted in a fluctuating negative number. Earlier you made it sound like the voltage I was looking for was measured in ohms. Or am I just reading your quote below incorrectly?

 

Huh? Ohms is a measure of resistance. Voltage is voltage, resistance is not voltage.