Contact Resistance Tester

[Killeavy]

Hi

I'm new to this site and I'm looking to design an item of test equipment which will be able to measure contact resistance of relays and then display them on an LCD display. The values of resistance i would be looking at are in the region 100m ohms to approx 20 ohms. Above 20 ohms doesn't relay matter for this instrument. I proposed to use a voltage divider circuit and use a known resistance of approx 10 ohms. I could then calculate the voltage drop across the unknown resistance and hopefully the 10 ohm resistor would be small enough to give a reasonable difference in voltage drops across unknown resistances up to 20 ohms. This is the principal behind a digital multimeter.

I have the circuit for selecting the individual contact selection but the difficulty I'm having is how to take the voltage measurment and then convert it into a resistance value on an LCD display. I have looked on various parts of the net and it looks like I would need a PIC and possibly an ADC. Because the resistances are relatively low then it has to be reasonably accurate.

Can anyone point me in the right direction as to how to convert a measured voltage into a resistance value to be displayed on an LCD display.

Many thanks
Killeavy

[Hero999]

I think you need to be able to measure resistances lower than 100mΩ, perhaps 10mΩ?

You need a constant current source, an amplifier and a volt meter.

For example, if you use a 100mA constant current source, the voltage across the unknown resistance will be equal to 0.1V/Ω, adding a amplifier with a gain of 10 will make this 1V/Ω.

You could use 0-2V a panel meter to measure the voltage across the unknown resistance which will give a reading from 0 to 2Ω.

The voltage reading needs to be taken from as near to the resistance as possible to avoid the effects of voltage drop in the leads.

If you make your own panel meter using a microcontroller you could add a zeroing feature.

I would use AA cells to power the Ohm meter because a little 9V battery won't last for very long with a 100mA load; fully charged AA cells should last for a couple of hours.

[Killeavy]

Alex

Thanks for replying. This project is for professional use. I work in the rail industry which uses 50V d.c. relays for operating safety critical circuits. Because of the safety involved then there is a lot of proving i.e. once a relay is energised/de-energised a 50V d.c. supply is then fed over front or back contacts to prove the position of the relay and then operate other circuits.

Due to the number of contacts which can be placed in series with one another the relay contact resistance is quite important and when performing fault finding techniques a contact resistance tester is sometimes the only way to check, particularly as the relays are in sealed housings and you cannot get access to the contacts when in circuit.

We currently have an analogue relay tester with a moving coil meter but they are almost impossible to obtain now and I was trying to design an alternative perhaps using a digital display. The resistances need to be quite accurate as anything above 10-15 ohms would cause too much of a voltage drop to operate a circuit anyway. When manufactured the relays have a maximum contact resistance of 0.2 ohms. We already have a contact cleaner using RC networks to pass a short burst of current over the contacts. What I'm ideally looking for is an accurate method of calculating the contact resistance and displaying it on a digital display.

Kind Regards
Killeavy

[Killeavy]

Hi Hero

Thanks also for replying to my query. I did think of using a constant current source but wasn't too sure how to go about it. You mentioned measuring resistances higher than 100m ohms and using an amplifier I can see where you are coming from. When you mention the panel meter would this be a voltmeter and due to the ratio of the amp would give an equivalent resistance measurement?

As for powering the tester can I not power it from a mains supply and step the voltage down then rectify it? The problem is I really need 50V d.c. to operate the relay coil also and it would be ideal to have a combined portable tester to perform all the required functions i.e. operate the relay, test the resistance of each contact indivdually and be able to clean the contacts if necessary.

Kind Regards
Killeavy

[Killeavy]

Hi Hero

I did think about the resistance of the wires and thought about using the Kelvin 4 wire method to reduce the effects of the wiring. My preferred method of reading the measurement would be using a microcontroller as it would probably be more accurate and wouldn't require calibration. Also as it would have to be fairly robust due to the working environment.

Making the mircocontroller is the bit I don't really know about.

Kind Regards
Killeavy