Mesuring resistance using a multimeter?

[Lumencraft (Matt)]

I would like to know how to mesure resistance using my multimeter?
I have been attemting it with this meter

However using what I think is the ohms settings on this one I have had no luck. Using the settings from the top each click to the right, reguardless of what I touch it to, wether its the leads to each other or to a pair of pliers etc. it always reads 0.0 The only setting that does anything is the 200 ohms it will register .2 or so when you touch the leads together and maybe .3 if you touch it to some pliars or somthing. with the leads together. on pliars or any thing metal.

What am I doing wrong?

 

[Mr Happy]

What you are doing wrong is measuring things that have essentially no resistance, like wires and metal objects. Therefore the meter will read close to zero.

It's hard to find everyday objects with enough resistance to measure with a meter like that.

You could try a mains 60 watt light bulb (not while connected to the mains!), that should have some resistance. Try a graphite pencil: sharpen both ends and measure from one end to the other.

Unfortunately, for the kind of resistance in flashlight springs and switches, it is too small to measure accurately with a meter like that. To get a good measure you have to use a different technique (for example pass a known current like one amp through the thing you are trying to measure and then measure the voltage drop across it on a millivolt range).

 

[TigerhawkT3]

If you want to give the ohm setting on your multimeter a good workout, go down to RadioShack or something and buy a pot or rheostat. Then you can turn the knob and watch the numbers change!

 

[PhotonWrangler]

There are several household items that will show resistance. Try putting the leads across the plug on a toaster and then activate the toaster's plunger. You'll be reading across the heating element.

Find a cadmium-sulfide photocell and connect it across the leads, then shine a flashlight on the cell and watch the reading change from high to low.

Put the meter on the 2M range and grab the leads with your fingers. Vary the strength at which you grab the leads and watch the reading change; you're measuring skin resistance.

 

[Bullzeyebill]

There are several household items that will show resistance. Try putting the leads across the plug on a toaster and then activate the toaster's plunger. You'll be reading across the heating element.

Find a cadmium-sulfide photocell and connect it across the leads, then shine a flashlight on the cell and watch the reading change from high to low.

Put the meter on the 2M range and grab the leads with your fingers. Vary the strength at which you grab the leads and watch the reading change; you're measuring skin resistance.

For the OP, and maybe others, make sure that the toaster is not plugged in.

Bill

 

[Bullzeyebill]

There are several household items that will show resistance. Try putting the leads across the plug on a toaster and then activate the toaster's plunger. You'll be reading across the heating element.

Find a cadmium-sulfide photocell and connect it across the leads, then shine a flashlight on the cell and watch the reading change from high to low.

Put the meter on the 2M range and grab the leads with your fingers. Vary the strength at which you grab the leads and watch the reading change; you're measuring skin resistance.

For the OP, and maybe others, make sure that the toaster is not plugged in using the ohm scale.

Bill

 

[PhotonWrangler]

Thanks for pointing that out, Buillzeyebill. I should have clarified that the toaster should be unplugged from the wall, and the multimeter probes touched to the plug's contacts.

 

[Lumencraft (Matt)]

Thanks everyone for your replies. I think what Mr. Happy said is most in the direction I was wanting to go. I am actualy trying to mesuer the changes in resistance before and after the fixes, ie adding braided wire to the switch, pro gold etc. I just didnt understand how to mesuer it. I figured it was a matter of using the meter correctly. I have seen post where mesurement changed in millaohms. If any others have suggestions on mesuring small changes like this I welcome the advice.

 

[Mr Happy]

If any others have suggestions on mesuring small changes like this I welcome the advice.

How I do it is like this: I arrange somehow to measure the voltage across the bulb terminals at the bulb holder. This is not easy and you will have to be creative.

First I measure the voltage with the bulb in the holder and with the bulb not in the holder. Call these two voltages V(cc) and V(oc). Then I measure the bulb current using the 10A/20A range on the meter via the tail cap (remove the tail cap and use the meter to complete the circuit). Call the bulb current I(cc).

With these three measurements, the total circuit resistance including the battery is:

R = [ V(oc) - V(cc) ] / I(cc)

In absolute terms one measurement of R like this does not tell you everything you need to know, but you can use it to find out how R changes after making modifications. You should normally find R is small, from 1 ohm down to fractions of an ohm.

 

[2xTrinity]

First I measure the voltage with the bulb in the holder and with the bulb not in the holder. Call these two voltages V(cc) and V(oc). Then I measure the bulb current using the 10A/20A range on the meter via the tail cap (remove the tail cap and use the meter to complete the circuit). Call the bulb current I(cc).

With these three measurements, the total circuit resistance including the battery is:

R = [ V(oc) - V(cc) ] / I(cc)

In absolute terms one measurement of R like this does not tell you everything you need to know, but you can use it to find out how R changes after making modifications. You should normally find R is small, from 1 ohm down to fractions of an ohm.

You can also solve for your battery's internal resistance separately using a similar method, then subtract it from the total.

What you will need to do is discharge the battery at a known current, then measure the voltage across the battery both open circuit, and under load. You can do this if you have two meters by simply hooking the battery up to a load (such as an incan lamp) through the current-meter, while simulatenously measureing voltage across the battery.

R(batt) = ( V(oc) - V(load) ) / I(load)

If you subtract this figure from the figure you found for yourt flashlight resistance, you will then be able to determine the resistance of your light by itself. The meter's resistance will also contribute to that slightly, so inreality it will be even lower still.

 

[Alan_P]

Resistances that you are trying to measure cannot be accurately measured with a multimeter. In order to measure low resistances you need a milli-ohm meter. Unfortunately these cost severl hundred dollars.

 

[Lumencraft (Matt)]

Thanks again to all. The posts in the last day or two have been what I am looking for. Even knowing about the mila-ohm meter is a plus although several hundred dollars is a bit of afor me.

 

[Mr Happy]

Resistances that you are trying to measure cannot be accurately measured with a multimeter. In order to measure low resistances you need a milli-ohm meter. Unfortunately these cost severl hundred dollars.

They can however be measured with two meters costing less than $10 each (in my case $2.99 each). The several hundred dollars is $20 for the electronics and several hundred dollars less twenty for the convenience.

 

[Lumencraft (Matt)]

They can however be measured with two meters costing less than $10 each (in my case $2.99 each). The several hundred dollars is $20 for the electronics and several hundred dollars less twenty for the convenience.

Nice . I think for what I am doing this would be fine.

 

[LuxLuthor]

Resistances that you are trying to measure cannot be accurately measured with a multimeter. In order to measure low resistances you need a milli-ohm meter. Unfortunately these cost severl hundred dollars.

My technique requires an adjustable power supply that I used for my destructive bulb testing (sig link). AWR walked me through this, and now it is as simple as child's play. Granted, you need the adjustable power supply, but they are relatively inexpensive, and may have another intended purpose as in my case.

• 1) Set your DMM to measure current (use higher 10A setting).

• 2) Clip PS output aligator leads to DMM leads while in Amp mode.

• 3) With PS turned off, set PS voltage dial to medium setting. Turn PS Amp dials to off.

• 4) Turn on PS. Slowly increase fine tune PS Amp dial until your DMM reports as close to 1.000 Amp as possible. Leave dial at that now calibrated 1 Amp output setting. You can double check Amp readings turning PS on/off several times, but mine stays where I left the dial.

• 5) Turn off PS & disconnect DMM leads from PS, and now change DMM back to detecting voltage.

• 6) Use PS aligator clip leads to attach to item you wish to get milliohm reading from, and turn PS back on. This will now be putting a calibrated 1.000 Amp through the item you are testing.

• 7) Take DMM volt reading on tested item, just inside of where two aligator leads from PS are attached to the item....otherwise if you touch your DMM to aligator clips, your reading will also include the substantial milliohm resistance from aligator clips.

Using the relationship of Ohm's Law, this DMM volt reading of say 0.012V (12 mV) will now equal 12 milliohms.
​

 

[Lumencraft (Matt)]

My technique requires an adjustable power supply that I used for my destructive bulb testing (sig link). AWR walked me through this, and now it is as simple as child's play. Granted, you need the adjustable power supply, but they are relatively inexpensive, and may have another intended purpose as in my case.

• 1) Set your DMM to measure current (use higher 10A setting).

• 2) Clip PS output aligator leads to DMM leads while in Amp mode.

• 3) With PS turned off, set PS voltage dial to medium setting. Turn PS Amp dials to off.

• 4) Turn on PS. Slowly increase fine tune PS Amp dial until your DMM reports as close to 1.000 Amp as possible. Leave dial at that now calibrated 1 Amp output setting. You can double check Amp readings turning PS on/off several times, but mine stays where I left the dial.

• 5) Turn off PS & disconnect DMM leads from PS, and now change DMM back to detecting voltage.

• 6) Use PS aligator clip leads to attach to item you wish to get milliohm reading from, and turn PS back on. This will now be putting a calibrated 1.000 Amp through the item you are testing.

• 7) Take DMM volt reading on tested item, just inside of where two aligator leads from PS are attached to the item....otherwise if you touch your DMM to aligator clips, your reading will also include the substantial milliohm resistance from aligator clips.

Using the relationship of Ohm's Law, this DMM volt reading of say 0.012V (12 mV) will now equal 12 milliohms.
​

Thanks Lux,

Lets make sure I am unerstanding the pricipal, and not just tracing a pattern. Doing as you said with the power suply set at one amp etc. If I connect to each end of a steel spring with PS and put DMM leads close to the PS clips at each end an I get a reading of 1.0 volts that= 1 ohm? Also to be clear. Does the actual voltage output from the power supply change the conversion. Like say I put the vo at 10v up high rather than say 4v in the middle? And if I were to use a setting of two amp rather than one and got the reading of .012v would that simply= 24 millaohms?

Also where is a good place to get a bench power supply?

 

[Mr Happy]

You can actually use the same principle without a bench power supply, especially if you have two meters. The bench power supply is a luxury for this purpose (such an item is of course very nice to have, but they do tend to come up a bit pricey).

What you can do is make up a circuit with a large stable battery, a bulb, the item to be measured, an ammeter (use the 10A range), and a switch. When you close the switch the bulb current will go through the thing to be measured and the ammeter will tell you the current. It need not be exactly 1 amp in this case, but something about that value would be good.

Now use the second meter and attach the probes across the item whose resistance you want to measure. When you close the switch, simultaneously read the current on the one meter and the voltage on the other.

With those two readings you can use the formula

R = V / I

to get the resistance.

 

[LuxLuthor]

Power supplies are not necessarily pricey unless you get higher Volt/Amp output, linear (vs. switching) output, multiple channel outputs, and other higher specification standards. Here is a page that has many that would serve your purpose discussed in my last post for very cheap.

It is up to you, but I would rather use this than dealing with "make up a circuit with a large stable battery, a bulb, the item to be measured, an ammeter (use the 10A range), and a switch, and a second DMM."

To answer your other questions, you mostly have the correct understanding in your examples. It uses Ohm's Law (V=IR), and solving like MrHappy did, you actually use: R=V/I

So the nice thing about using a calibrated 1.000 Amp output is I=1, so R=V, and in your example yes 1.000 V measured would mean you had 1.000 Ohms (or 1,000 milliohms).

If you calibrate the PS to output 2.000 Amps, then you would need to divide by 2 as in this setup:

R=V/2.000 so a voltage reading in this setting (again touching DMM probes just inside of PS aligator clips on spring to be measured) of 0.012V would yield:

R=0.012V/2.000A or R= 0.006 Ohms

The PS voltage only needs to be high enough so you can drive 1 Amp through the item being measured, and becomes otherwise irrelevant, and does not enter into your DMM measurement which is specifically measuring the voltage drop accross the item with 1.000 Amp running through. If you are putting 2 Amps through the item, you will need a higher PS voltage to drive the current through the item, so you don't gain anything with higher Amp.

 

[Mr Happy]

Hey, I've nothing against bench power supplies, but sometimes it is actually more fun to do a MacGyver and solve a problem making use of materials to hand

In my case my "stable battery" is just a bunch of rechargeable AA cells in a battery holder, I have a handful of multimeters picked up for $2.99 apiece at Harbor Freight, and my "switch" is just touching the wire to the battery terminal.

In actual fact, the most important thing is to maximize the voltage at the bulb terminals -- so if you can measure that you don't need to care about measuring the resistance. Just keep making resistance mods until you can't increase the bulb voltage any more.

Most of all is to keep up the encouragement to learn! :twothumbs

 

[Meterman]

I did have a lot of pleasure reading the different approaches to measuring small resistances by LuxLuthor and Mr Happy, both keenly suggesting their individual ways. :twothumbs

Both of them are right of course and both of the methods seem quite different philosophies to me.

I (being very fond of measuring) prefer the third way: using a Milli/Micro-Ohmmeter, but - alas - that's the expensive way.

Wulf