Measuring the Resistance of a Button

Why would you even care what the resistance of a "button" is???????????

I'd ask for help in learning how to use a digital meter first.

Why run a forum if you are too lazy or corrupt to answer anything ?

Stay in your books if that's the case , everyone else has work to do.

Dragon113

Clearly you do not understand how a forum operates. You should keep your misconceptions to yourself.

DVOM

As has already been noted, push buttons in multi-function devices are not always simple momentary contacts, they often include resistors, diodes or other gear.

Assuming that it is a simple switch - you can test this by putting an ohmmeter across the switch and seeing if you have a zero or close to zero reading, reverse the leads to check for a diode, you then have some options.

Normal multimeters don't measure low resistances very well. You can get ones that will do so (normally 4 wire devices) but they're not cheap, and you'll probably use it only a couple of times in your career.

A Wheatstone bridge is one possibility, but failing that, and without too much expense, an easy way to get a close reading is to use a multimeter, a steady voltage (car battery), a known resistor (say 820Ω 1/4w to limit the current to a reasonable level so the resistor doesn't get hot and change its value) and 4 alligator patch leads (say red, black, yellow and white)

1. Set the meter to milliamps scale then connect like so:- Battery +ve - red patch - resistor - yellow patch - unknown resistance - white patch - meter +ve, meter-ve - black patch - battery -ve...Record the milliamps (let's assume 16mA).

2. Disconnect the meter and connect the white patch lead to the black patch lead.

3. Set the meter to DC volts and measure the voltage drop across the unknown resistance, record the readings. (assume 0.4mV)

Now use Ohm's law to get the unknown resistance. R=E/I, = 0.0004/0.016, =0.025Ω

Below is a more accurate method using a percentage calculation but it is more complex.

Again using just a normal multimeter, 3 small alligator patch leads (lets say red, black and yellow), a steady voltage, a known resistance and a calculator.

Using your car battery which will be outputting something like 12.8v fully charged, (not critical as long as it's steady).

Again using the 820Ω 1/4 watt resistor.

1. Connect like this :- +ve lead of meter - red patch lead - resistor - yellow patch lead - black patch lead - -ve lead of meter.

2. Set the meter on ohms scale, wait for it to settle and then record the reading. You are measuring the total resistance of all of the leads and the resistor. We'll call this KR for Known resistance. Assume it comes in at 812.3Ω.

3. Now separate the yellow and black patch lead connection and insert your unknown resistance.

4. Disconnect the meter from the red and black patch leads and connect those patch leads to the battery terminals.

5. Set the meter to DC volts and measure the voltage drop between the +ve battery terminal and the unknown resistance (UR) at the yellow patch lead connection, (connect to the UR terminal rather than to the patch lead to include any voltage drop in that connection). Record the reading (assume 12.79v).

6. Measure the voltage drop across the black patch lead, and then across the unknown resistance (URv) and record the results. (assume 0.0004v and 0.00035v respectively).

7. Add the 12.79v and the 0.0004v to get total voltage drop (KRv) across the known resistances (KR).

8. Now perform a percentage calculation to get the value of UR.

Formula is :- UR = (URv/KRv)*KR, = (0.00035/12.7904)*812.3, = 0.02223Ω,

Get the Haynes or Chilton manual of the vehicle, so you see in the schematics what's the connection of that particualr switch and know what to expect (i.e. open circuit, resistor array of a matrix, or even what type of Sw. it is N.O. or N.C.).

If it is a safety test, use a Megger instead of a digital multimeter but only after you have isolated the button from the rest of the wheel circutry, otherwise you'll fry whatever it is connected to.

Could this be another candidate for the Darwin Award??

What puzzles me here is that the measured electrical resistance of a push button on a steering wheel is a useless datum of information. You have to know what the value should be and most importantly what this information means. If you need to be told how to use a DVM ...

I am trying to give a GA and I already took off 1 'Off Topic". I tell my service techs all the time - we are a service department, not a research department. If you don't know what the voltage is supposed to be, what are you measuring? In service work the measurement is to confirm that proper voltage is or is not present. The same is true for this gentleman. He would need to know what is supposed to be in the steering wheel in order to test it. I also appreciated the comments about the air bag - probing around the wiring could be big mistake. -- JHF

Just to steer the participants that try to make this a forum according to what they perceive it to be. This is the Heading in plain English.

CR4 - The Engineer's Place for News and Discussion, read it yourself in the heading.

We can resolve this continues arguments by either expelling the students with questions or expel the people that answer. Your answer is not necessarily the only and final correct one. The idea is that we do not answer blankly but to steer the person with the question to the right engineering principal, like to use a DVM, first read the manual and try to learn about what you want to know by knowing what to expect as an answer. We do not read the Voltage, we check it, and that suggest we know what it should be. We need drawings if it was designed by somebody else and more important if you design it you need it on paper before you build it.

It should be in the datasheet for the button unless the button is a custom assembly, even then it may still be in the datasheet.

A digital multimeter is likely not going to help, its accuracy at such a low resistance value is probably going to give you a large false resistance (an order of magnitude or possibly more).

And, why do you need to? Connect your voltage signal up to the button, press it and measure the corresponding voltage in and out and see if it will work with whatever circuit or PLC input you are using to monitor the button status.

Paragraph 1 is not applicable if these are existing buttons on a steering wheel. Paragraph 3 is vital however as resistance is not going to tell you if the button will work with your circuit!

Connect your voltage signal up to the button, press it and measure the corresponding voltage in and out and see if it will work with whatever circuit or PLC input you are using to monitor the button status.

I will tend to agree with you Jack of all trades. These buttons on the steering wheel address a programmable circuit inside the steering wheel that pulse different inputs on the program and then send the output down one of more wires back to a control processor to activate the action/instruction of a specific button. You need detailed information to test this. The buttons are low current devices and to destroy them must be a physical action, like slamming down on them. Electronic damage can be caused by a short circuit and then the assembly may also be destroyed.

Here are your choices.

For the specific "button" (button does not describe the electrical function, that's a switch) you will have one of two results:

1. Button works.

2. Button does not work.

Oh, I have to disagree with you on this Lyn. The problem is that we don't know what he is going to connect with this switch. If it is a circuit that has very low current flow, then it may not be suitable due to the wrong type of contact materials. If it it too high a current flow, he may destroy the "button" or switch fairly quickly.

Clearly more information is required before one can just go out on a limb and specify that it works or it does not.

As for everyone else, a digital volt meter it pathetic at measuring resistance below 2 ohms. The best you can hope for is a rough approximation.

Oh I don't know. I have some very nice four wire digital voltmeters at work that measure some very, very low resistances. I doubt our OP understands the difference between my digital voltmeter and his but not all digital voltmeters are the same.

But given the information so far what are the chances the OP has access to one of those though.

As Lyn asked, I too must ask why do you need to know the resistance of a switch???? Most switches are infinite open and as close to a dead short as possible when activated. Measuring that resistance with a multimeter will be basically a guess with a standard VOM. Most VOM read a set resistance less than 1 on a short. (my current favorite meter will read 0.3Ω on a short.)

If this steering wheel button is a simple switch (as many assume) then I wonder how we can explain how to use a DVOM to somebody in a forum. Therefore I expect that this button is not a simple switch. Therefore to help the OP the OP must not only know the fundamentals of electronics (that they admit to being deficient) but the information of how this steering wheel is configured. We might be able to help the OP understand electronic fundamentals if they would reply to our questions. We certainly cannot identify the configuration of this mysterious steering wheel. We don't even know what continent this steering wheel and OP reside. We certainly do not know the make or model number of the vehicle or even anything as simple as what year it was made.

I am thinking of all the "buttons" on the steering wheel of my current cars and cars past, and can't recall any that were analog in function. They all were digital and most likely a simple switch. Now some did perform an analog function eventually, but they were triggering a circuit that did the analog function for you. (i.e. speed increase on the cruise control - you can almost see the digital steps here)

Digital contact into some accumulating serial communication circuit cannot be fully tested with just a DVM. However, this is once again pure speculation that we can go around and around on. Many of the longer CR4 threads are just this type of speculation iteration festivals.

Then there was this obstreperous bathtub.

Got something better to do?

The shaft beneath the horn switch , the steering shaft , is a ground , normally . It should have a heavy torqued nut or locknut securing it . This retains the actual steering wheel . If you attach the ground lead of the multimeter to this , and find trace voltage either at the live side of your switch or at a steady 12 volt ignition power source , then the meter should give you a closed circuit reading - zero ohms . Now measure the difference in resistance by touching the red ( Live , or Pos ) lead to the 'switched ' side of the steering wheel switch , and depress the switch and release it .

Dragon113

Great instructions on "how to destroy your DVOM"! The OP wil learn the hard way why you do not test a live hot (+) lead while he has meter set on Ω scale

What you must know before venturing into measuring the switch is. 1. A multi function steering wheel gets assembled to fit onto a vehicle that was designed for a multi function steering wheel that can interpret the signals generated by the steering wheel. 2. It is very unlikely that a manufacturer will manufacture a vehicle with all the electronics and slave systems for a multi function steering wheel and then put a normal one without the controls on the car.

I will suggest that you do more home work on the possibility of doing such a conversion before worrying about the resistance, the output after power up of the wheel is probably digital and resistance measurement is a no no.

Lots of posts, lots of good ideas, lots of comments but lots of unanswered questions.

Can you provide more information on WHY you want to measure the resistance and what you are trying to accomplish (what's the button controlling for example) so we can narrow down which of the above options are the best for whatever you are trying to accomplish before we go any further and potentially confuse you with to much information.

Definitely best place to start. Name and make of car also, and if the steering controls are for that make and model car.