Voltmeter - what am I missing?

[SPYDERLK]

Right you are Rob.

What does this tell us? It tells us the resistance of the battery terminals that permit starting the engine or running the headlights is much much less than 0.1 Ohms. A decent meter has a resistance of several hundreds of thousands of Ohms and more likely millions of Ohms. Just for a simple case lets say the meter has a resistance of only a million Ohms so with a perfect battery connection the total resistance in the circuit (ignoring wire) is 1,000,000 ohms. With a super terrible (you can't start the tractor type battery connection) the total resistance in the meter circuit is 1,000,000.1 so just how much effect do you suppose that will have on the voltage reading? Too small to notice!

That is a reason you want to draw a substantial current through the suspect connections in order to measure a voltage drop.

In the case reported by the OP his corroded fuse to fuse block connections were quite high in resistance, high enough to effect the meter's operation and or reading. The resistance of the fuse connection was NOT just an Ohm or three but several (quite a lot actually), a very bad connection.

Those of you following this thread for credit... note: this material may be on the exam. For you auditors, enjoy.

Pat

Yes, the hi input impedance [resistance] of a V meter ensures that it has little effect on the circuit it is measuring if the circuit is much lower in resistance than the meter. [This is always the situation in a tractor wiring harness unless the circuit is "broken" by a switch - or some other point that has developed "near infinite" resistance.] A voltmeter is used to sample across selected parts of a current carrying circuit to determine the V drop in that part. In so doing, the meter is put in parallel with the circuit element[part] of interest. This gives the electrons another path - some go thru the meter, effectively diverted around the part you are measuring. The meter uses this current to make its measurement. Since the meter ismuch higher in resistance than tractor circuit elements, the miniscule change that it truly makes is as if no change has occurred. Nevertheless, the change has increased the current flowing in the circuit. This is because the part, while being measured is replaced by a new circuit element comprised of itself and the meter in parallel.

Calling them Resistance 1 and Resistance 2, the new R value presented is R1R2/R1+R2. Note that this is always less than either constituent resistance. I think Pat had some alternate avenue of explanation in mind when he presented an addition of resistances - - perhaps V splits in a series circuit being proportional to resistances.​

To move on:
A Digital V meter is extremely sensitive. Even a cheap one will measure down to 0.1mV and as a result it can respond to very small currents flowing thru very small resistances. Since E=IR, for the meter mentioned the product IR has to exceed 0.00005 in order for you to detect it [a round up] on the meter. You are putting the meter across a circuit element having unknown conduction quality [resistance]. Your batt is applying a voltage to the whole circuit of which the element is part. This causes a current to flow thru the whole circuit. You need to have some ballpark clue as to the amount of current. How much current do you need flowing to sure to be able to get a meter reading on any tractor circuit element when using a meter having 0.0001 resolution? Literally, a tail light will do it to test a battery cable. No point in going quite that lo, but the point is you can. With an amp flowing a battery cable wire will give readings on the last digit. Factoring in [adding] the individual measurements taken from the cable to lugs and lugs to post, you may flick the second to last digit. Not good if you do--a volt lost per thousand amps is a bit more than you want to see. At this point tho, you have figures of merit on each junction. With this you can figure out how much heat will come from the junction during cranking:

As a realistic max ballpark in a tractor lets assume max cranking amps is 500A. We have a measured cable wire drop of 0.0001V @ 1A. At 500A The drop will be 500X or 0.05V. Power W= IE = .05x500= 25Watts is dissipated as heat in the cable wire. If this is a short cable this could be a problem [undersized], but if 2 or more ft long it is not because the heat is well spread out and wont amount to more than a few degree change. But what about the crimps? We dont want even a last digit "1" at the cable to adjacent crimp lug because all the heat from the IE is going to be concentrated there, causing greater V loss as it heats which in turn causes more heat. If the meter flicks between 0 and 1 in the ten thousandth place the crimp is suspect. If it reads a solid 1 it will give trouble. In this case a higher test current is a good solution -- turn on the headlights to make the test setup as sensitive as if you had suddenly substituted a meter with 0.00001 sensitivity. Handheld meters like that are readily available but cost agood bit more.​

In the case of the OP, the gauge he is sampling the circuit which is also powered from the circut. This means it is drawing thousands of times more current than would a standalone handheld powered by its own battery. Enuf perhaps to influence the V it receives thru a bad fuse connection to be 0.1V or so lower even if the fuse connection was just 10 ohms or so.
larry

 

[SPYDERLK]

I hear ya. I sometimes wonder about the 0-80 amp scale 2" meters i see for tractors.. and those anemic mounting blts.. and just know that if it was displaying 80 for any length of time, that the plastic case would come apart..

soundguy

Perhaps they just use lo V low impedance meters to measure the shunt. 0.01V full scale would be pretty safe.

0.8W dissipation​

larry

 

[Soundguy]

Perhaps they just use lo V low impedance meters to measure the shunt. 0.01V full scale would be pretty safe.

0.8W dissipation​

larry

My issue is the 80a going thru the anemic mounting studs.. if you get extended load times.. you can get thermal problems.

As for the vom issue.. Easiest way to look at it is to take the input impeadance of the vom ( in the manual ).. and apply it as a parallel resistor to the circuit. For instance.. my scope meter is 10meg input Z.. When measuring a circuit with a comparitively low resistance.. like a circuit that was 12v/20a dc, to yeild 1.67ohms..., for instance.. your paralleled circuit change is less than a drop in the bucket.... IE.. the parallel circuit resistance drops to 1.669999.. mind you this is actually enough to make a voltage change measurement with a good sensitive volt meter.. but it's all still academic for the average automotive application.. unless you are dealing with extremly low voltage values like for sensors.. etc.. like 02 sensors.. etc.... which.. the Z would be much higher.. and thus the change much much more..

( At this point.. I'm guessig we have lost 85% of the readers after this electronics 101 discussion! )

soundguy

 

[RobJ]

uhhh, I just buy my amp gauges at the autoparts store!

 

[SPYDERLK]

My issue is the 80a going thru the anemic mounting studs.. if you get extended load times.. you can get thermal problems.

As for the vom issue.. Easiest way to look at it is to take the input impeadance of the vom ( in the manual ).. and apply it as a parallel resistor to the circuit. For instance.. my scope meter is 10meg input Z.. When measuring a circuit with a comparitively low resistance.. like a circuit that was 12v/20a dc, to yeild 1.67ohms..., for instance.. your paralleled circuit change is less than a drop in the bucket.... IE.. the parallel circuit resistance drops to 1.669999.. mind you this is actually enough to make a voltage change measurement with a good sensitive volt meter.. but it's all still academic for the average automotive application.. unless you are dealing with extremly low voltage values like for sensors.. etc.. like 02 sensors.. etc.... which.. the Z would be much higher.. and thus the change much much more..

( At this point.. I'm guessig we have lost 85% of the readers after this electronics 101 discussion! )

soundguy

Good point. Yould need to ensure good connection to the meter. Depending on whether the posts themselves do the conducting they may not need to be large. The studs may just clamp the leads against a robust conductor that is soldered to the shunt.

Scopes typically have good hi impedance but I didnt think they had measurement sensitivity down to the tens of microVolt range typically offered by even the cheap digital meters. Without that level of sensitivity you are blind when using lo current to assess circuit elements having lo resistance. Not sure I understand the parameters of your example correctly. Are they inverted?

I imagine anybody that is interested can ask questions. Its the quickest way thru the jumble to the eureka! point.
larry

 

[patrick_g]

Yes, the hi input impedance [resistance] of a V meter ensures that it has little effect on the circuit etc etc.
larry

Larry, thanks the the review lesson from remedial electricity 1-A.

I have several digital meters but still have analog meters too that I like fine and still use including one of my favorites a Simpson 260. I have some pocket sized meters as well. Even with young eyes it was tough to read beyond a digit or two to the right of the decimal. I challenge anyone even with a hand magnifier to read the forth digit.

I mostly have lost, broken, given away, or who knows what most all the little shirt pocket sized analog meters, the simple ones without any switch so you had to choose different jacks to change from AC to DC to Ohms to mA. Yeah, some of these were not setting world records in ohms/volt. If was fair to say they were numb and may have required a little more current through a circuit to get a reliable reading across a bad connection. I don't even have (or don't know what box in storage to look into) a VTVM anymore.

I have at least 3 of the Radio Shack "clam shell" shirt pocket sized DMM (with continuity tone so high it is in my hi freq dropout and I have to cradle it at my ear to hear it) and a couple of the yellow HF shirt pocket sized they sell on sale for $1.99 which work fine. Some of these don't display so many significant digits but otherwise work fine and if you find a way to smash a $2 HF it is no great loss.

Not the quality of the John Fluke you can throw into a block wall and not hurt but just fine for many practical purposes (excluding perhaps detecting high resistance in a connection by putting a couple pico amps through it (likek that is somehow better.)

Now, on that other topic... A dirty 20 amp fuse/holder connection. If the voltage drop was a volt with just the current it takes to run the little digital panel meter, what would it have been if you tried to get 15 amps through it and how many Watts would that have generated. I have soldering irons for smaller electronic connections that are only 12.5 Watts and they get pretty hot. I suspect any plastic involved in the fuse or holder would have melted if the 20 amp circuit was used at any reasonable percent of its capacity.

Pat

 

[SPYDERLK]

Larry, thanks the the review lesson from remedial electricity 1-A.

I have several digital meters but still have analog meters too that I like fine and still use including one of my favorites a Simpson 260. I have some pocket sized meters as well. Even with young eyes it was tough to read beyond a digit or two to the right of the decimal. I challenge anyone even with a hand magnifier to read the forth digit....snip...

Now, on that other topic... A dirty 20 amp fuse/holder connection. If the voltage drop was a volt with just the current it takes to run the little digital panel meter, what would it have been if you tried to get 15 amps through it and how many Watts would that have generated. I have soldering irons for smaller electronic connections that are only 12.5 Watts and they get pretty hot. I suspect any plastic involved in the fuse or holder would have melted if the 20 amp circuit was used at any reasonable percent of its capacity.

Pat

Analog meters certainly have their place. I like them for the needle swing that you dont have to look at to see. Also interpretation of swing characteristic in R mode can give a clue as to what reactive components are connected to various leads in a "black box". Digital is my tool of choice for field trouble shooting because of its discrete step reading and hi sensitivity and its automatic +/- ability.

On the issue of gauge current, I was speaking to the small discrepancy in the no load voltage run thru the fuse when the only "load" was the gauge. The effect of major loads elsewhere in the feed circuit was introduced and dealt with well elsewhere.

With a lot of help from Shaun (Blackmagicwoman) I bought and installed a 40 amp alternator on my BX 1800, fired up the rig and tested the voltage readings across the battery terminals with my multimeter. I got a reading of 12.6 volts with the tractor off and 14.46 volts with the engine running just above idle and 14.32 volts at idle. I also got a reading of 14.32 volts with the engine just above idle with the lights on. Very exciting, nothing caught fire, no explosions, I finally installed something without a semi-major injury, so I was happy!

I bought a digital voltmeter gauge and tested it across the battery terminals and got the same readings as I had with the multimeter at the various rpm's and with the lights on and off. I then mounted it to the tractor and hooked it up to a keyed power source, so it would only display when the engine was running. For a keyed power source, I used the wire for a rear light that came installed on the tractor, but hadn't been used. I pulled the wire back through to the battery compartment and used only the length I needed for the positive lead and I just hooked the negative terminal to the battery negative auxiliary post. I then turned the key on and got a reading of 12.0 volts with the engine off and 14.1 volts with the engine running above idle. At one point, and at a little higher rpm's, it displayed 14.2 volts. When I turn the headlights on, the reading dropped to 13.2 volts on the newly mounted meter. I ran my multimeter across the battery terminals again while this reading was displayed and got a reading of 14.32 volts on the multimeter.

 

[Soundguy]

Flipped or or inverted.. etc.. should read .6 ohms.. got my integers swapped. same difference aside from the mathematics error.. the parallel 10meg ohm resistor to the .6 ohm resistor drops it about a hundreth of a volt or less fromt he original reading.. ie.. an analog may not even notic ethe change.. and depending onthe autorrange digital.. it may or may not.. depends on t is has 2 or 3 digit sensitivity.

soundguy

Good point. Yould need to ensure good connection to the meter. Depending on whether the posts themselves do the conducting they may not need to be large. The studs may just clamp the leads against a robust conductor that is soldered to the shunt.

Scopes typically have good hi impedance but I didnt think they had measurement sensitivity down to the tens of microVolt range typically offered by even the cheap digital meters. Without that level of sensitivity you are blind when using lo current to assess circuit elements having lo resistance. Not sure I understand the parameters of your example correctly. Are they inverted?

I imagine anybody that is interested can ask questions. Its the quickest way thru the jumble to the eureka! point.
larry

 

[patrick_g]

Flipped or or inverted.. etc.. should read .6 ohms.. got my integers swapped. same difference aside from the mathematics error.. the parallel 10meg ohm resistor to the .6 ohm resistor drops it about a hundreth of a volt or less fromt he original reading.. ie.. an analog may not even notic ethe change.. and depending onthe autorrange digital.. it may or may not.. depends on t is has 2 or 3 digit sensitivity.

soundguy

Soundguy (et al), I do have sensitive and accurate DMM's too, not so old fashioned as to avoid them. In fact I was an early user of digital voltmeters (1964, primitive, bulky, cumbersome to use but damned accurate) provided by the USAF, built by John Fluke. I bought my first personal digital meter from Valhalla Scientific (San Diego) which was about 10x12x2 inches had a wall wart and 4 (I think) internal D cells. It measured AC-DC volts and amps (up to 10) capacitance, resistance, and frequency too up to 10MHz. It was really leading edge stuff way back then, ca 1970-73. This was back in my medical electronic instrument designing days (dual beam ultraviolet spectrophotometers for blood analysis and other fun instruments and stuff.)

Pat

 

[Skyco]

aaah- the Simpson 260, have one out in the shop somewhere. Its probably hiding with my old HeathKit VTVM.
Digital stuff has it's place and other times analog is the best. I remember tuning radio front ends and duplexers with IFR service monitors. Then along came our first digitally based HP service monitor, what a pain that was to use in very intricate tuning work, you'd go right past a peak or dip because the response time of the equipment was too slow. No analog meter, but rather a digitally created version of a meter on a screen.