Multi meter ?

[kidkaos70]

Hi I am new here and not that familiar with Electronics. I am looking to test a coil on a small engine. I have a basic tester but it seems it dosent go down low enough. The manual for the engine says the resistance should be 0.12-0.18 my meter only goes to tenths not hundreths. Can someone tell me how to test it or what kind of tester(meter) I can use, buy... that wont break the bank. Thanks in advance...

 

[MikeMl]

Do you have a Current-Limited Lab-type power supply? My bench supply has a settable current limit. I would set it to limit at 1A, with an open-circuit voltage of ~1V. Then I connect it to a low resistance coil. I use my DVM in volts mode to measure the voltage across the coil with 1A flowing though it. R = E/I, so since the current is 1A, the resistance of the coil is equal to the voltage.

 

[kidkaos70]

Like I said my knowledge is limited...I think I understand what you mean I do not have a power supply. Is their a DVM that can test a low resistance coil do you know make model. Thanks

 

[KeepItSimpleStupid]

The device is called a milliohmeter and usually requires 4 leads.

Commercial units are quite pricey. **broken link removed** so many hobbyists opt to build one.

The general techniques are: measure the current through it and the voltage across it or some meters had a zero control on them. There is a neat milli-ohmmeter that I found on the net to build: http://cappels.org/dproj/dlmom/dlmom.html

Here is a very simple design, but the parts are not available:https://www.electro-tech-online.com/custompdfs/2013/02/AN106.pdf Now, it may be possible to find suitable components to substitute.

T make this measurement, you could use nothing more than a precision resistor around 600 ohms such as this one, http://www.digikey.com/product-detail/en/ALSR01600R0FE12/ALSR1F-600-ND/257004, and a small battery such as a 3 V CR2032 button cell. Use at least a 1% resistor. Put the battery, resistor and the coil in series. Make a switch that can measure the voltage across the resistor and the voltage across the coil. Use 4 leads to connector to the coil. Or use special Kelvin clips. Oops, these are too expensive too: http://www.etesters.com/see/Kelvin_Clips

The idea would me to make something that would plug into your multimeter and have a set of 4 leads. 2 red and 2 black. One pair of reds would go to one side of the DPDT center off switch and the other pair would go across the resistor. The middle pair to your meter.
Mark the position that goes to the DUT (device under test) as V and the other as I = V/600

You would have the switch set for center off and connect both reds to one side of the coil and both blacks to the other.

We can figure out how to make a "battery test" circuit. In hindsight, you could probably used a DPDT ON-NONE-(ON) toggle switch. In the momentary (ON) position another 600 ohm resiistor would go across the battery in that position.

The ON position would connect the center terminals to the DVM and the previous reference to doing so would be removed.

You would put your multimeter on the volts scale and measure both values. Then compute R= V/I

Extremely cheap and will do what you want, The major parts are a resistor, a CR2032 battery and holder and a switch.
Two test point for battery voltage would also help.
Placing a load on the battery with a push-button would help even more.

As you will find out the case and power supply stuff add up quickly. case, the right kind of "adapter" so you could plug your device into your meter, clips, PC board, leads, grommets etc.

In this design, lead length won't matter. It will get the job done.

No, not as slick as able to read the value directly on a meter, but it's safe enough for what you need it for.

There is another one on the net that looked promising, but I could not find it.

I redid the design because the battery is good for a 6.x ma pulse about 12x/day, so I changed the test current to 5 mA. The battery test probably should draw at least 5 mA, so another 600 ohms resistor to put a load on the battery should work. The discharge cure of the CR2032 i relativley constant.

So, the suggested final design would consist of:
1. Case that could be plugged into the DVM
2. A banana plug that can be made integral to the case (not sure how easy this is to find)
3. Grommets (4)
4. Wires 1- pair labeled Force, 1 pair labeled sense; read and black
5. clip leads (4)
6. PCB breadboard
7. one momentary DPDT center off switch: (ON)-OFF-(ON); Measure V-OFF-Measure I/600
Momentary prevents battery drain from leaving it on.
8. one momentary DPDT ON-NONE-(ON); for Measure/battery Test
9. 1 CR2032 battery
10 1 CR2032 battery holder
11 2 precision resistors
12. 22 Awg solid Wire for internal wiring

A note about wire: Test probe wire is really flexible and high temperature doesn't bother it. Stranded wire comes in different standings, typically specified as x/y; e.g. 18 AWG 7/33 the 7 is the number of strands and the 33 is the gauge of the strands and 18 AWG is the equivalent wire size.

A couple of commercially made test jumpers cut in half could also work.

 

[audioguru]

Why do you want to measure the resistance of the coil?
If it is burnt out then it is either open (infinite resistance) or it is melted into a short circuit (zero ohms).

 

[KeepItSimpleStupid]

An open or intermittent tends to be the norm in my experience. It's usually the secondary or the spark plug end. In my experience, you have to have your meter attached and flex the HV cable along it's length. One place where damage is likely is the exit as it leaves the engine body.

Coil gap and rust on the flywheel are also common ignition problems. If you don't know the gap, an index card makes a perfect pick-up gapping gauge usually.

About 2 years ago, I retrofitted an engine with electronic ignition for about $20. Another engine had the high voltage lead problem as described caused by me. I crimped the HV cable my not being carefull when I put back the flywheel cover. The ignition retrofit was well worth it.

I'm assuming this is the standard electronic magneto or points system without a battery.

 

[KeepItSimpleStupid]

Here is the other design: https://rick.sparber.org/electronics/ramp.pdf