Question about measuring current

[christian]

So all this talk about ammeters lately has me now questioning the unit in my model A. I have a brand new voltage regulator cutout from fun projects I'm about to install and would like to calibrate the third brush using my digital multimeter instead of the ammeter in the car. So my question is, i set the multimeter to current, then measure from where? Does it matter ? I only use one lead from the multimeter right ??

If you can't tell, I'm not really good with electronics

[Roadster62]

You're light years ahead of me, you have test equipment and know how it works. Bob

[Bluebell]

A volt meter has a high internal resistance. It is used across a circuit. (connected in parrellel) Say from a live point in a circuit to earth, or across the two terminals of a battery.
By contrast an amp meter or ammeter has very low internal resistance, in other words if you connect it up in the wrong place you will virtually have a dead short (bang!)
So an amp meter is connected in series. That means your amp meter is connected between two components in an existing circuit. You would usually have to break the circuit to fit the amp meter into it.
Is that what you wanted to know

[SeaSlugs]

what bluebell said but dont forget alot of meters you must put the red lead (or pos lead) in a different socket. and then most are just good for 10A or less.

i would take the wires off the cutout and clip/secure the wires you took off to the black lead and the red lead to the cutout output screw. your effectively inserting the ammeter in series from the generator output to the rest of the car.

[Mike V. Florida]

[B_man_Al]

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Christian ...

All of the above posts are correct, but a word of caution. Digital multimeters are not very good at measuring "noisy" voltages and currents so don't just assume that what you read is correct. They get really "confused" when measuring voltages with a lot of spikes on them like when the car is running.

An analog meter would likely give you better "average" readings.

If the car is not running a digital meter will be fine.

As a sanity check, connect a known load, say from the load side of the cutout switch, (car not running) to ground and watch the ammeter in the car. 1 ohm, 6v = 6A. Just connect it briefly and watch the meter move . Should indicate 6A discharge. If so, your meter is OK. Try other values of resistor if you want to calibrate at different current points . I = E/R = 6V/R ( assuming a 6v system).

[Patrick L.]

Yep, what the B Man said. These monsters don't care much for DVOMs, they like regular ole analog stuff. But, for just setting the third brush, what is wrong with using the old Ford ammeter ?

[Mitch//pa]

i use an external meter all the time when setting generator output and its a good idea not to trust any in dash guage until you can confirm its accurate with an ext meter. as mentioned digital multi meters tend to jump around (not give a steady reading) unless your using a good quality one but i have had success on some cheaper units you wont know till you try. put your meter in the amp mode and touch each lead to one of the junction box wing nuts on the firewall. this is essentially how the stock ammeter is wired into the circuit.

[marc hildebrant]

Christian,

Could you explain your comment ? The Fun Project regulator keeps the voltage constant from the generator. I belive that you set the third brush to max and then let the Fun Project Regulator do the rest of the job.

If you want to calibrate the ammeter with an external multimeter, then the generator third brush/fun projects isn't relevent.

Marc

[Mitch//pa]

Originally Posted by marc hildebrant

Christian,

Could you explain your comment ? The Fun Project regulator keeps the voltage constant from the generator. I belive that you set the third brush to max and then let the Fun Project Regulator do the rest of the job.

If you want to calibrate the ammeter with an external multimeter, then the generator third brush/fun projects isn't relevent.

Marc

marc the reason being is you have to first set the 3rd brush to about 12 amps (not max) then install the regulator...

[marc hildebrant]

Mitch,

Thanks for the information. I didn't know that at all !

Marc

[Willie Krash]

If you go to John's site, Fun Projects you can download the instruction before you buy.
http://www.funprojects.com/pdf/10505R6.PDF

[christian]

Originally Posted by Willie Krash

If you go to John's site, Fun Projects you can download the instruction before you buy.
http://www.funprojects.com/pdf/10505R6.PDF

I have these instructions, they do not address my question.


Thanks Mitch, will try measuring from the terminal box, seems to be the easiest way to go about it.

[stouchton]

christian - if you have access to the back of your original ammeter, simply disconnect 1 wire, and splice your other ammeter from that wire to the original ammeter. Otherwise, you need to disconnect the generator from the battery and put your ammeter between the generator and battery. Mine had a fuse mounted on the starter that made it easy to do. I pulled the fuse out, and put the ammeter on the fuse contacts.

Now I doubt many digital ammeters will work. The ignition system on a model A is a spark gap transmitter without any suppression. I have a plethora of digital ammeters, expensive and cheap. They are all effected by the noise radiated by the ignition system.

Also, cheap digital ammeters usually cannot read 10 amps or so accurately - too much resistance thru the meter - but even though they skew the reading, you can compare them to the car's original ammeter to see if it is calibrated.

An analog meter with a 10A or 20A scale would be a good choice.

There is another way to go about this if you have "normal" headlights. Headlights draw about 10 amps (at least mine do - others can chime in please). Even if your original ammeter has a poor calibration, it will read "0" and swing to charge or discharge. Simply adjust your third brush until the ammeter gets back to "0" or slightly in the charging zone (1 or 2 amps) when you turn the headlights on. This should put you in the 10 to 12 amp category that the regulator is capable of handling.

[jmeckel]

Originally Posted by Mitch//pa

i use an external meter all the time when setting generator output and its a good idea not to trust any in dash guage until you can confirm its accurate with an ext meter. as mentioned digital multi meters tend to jump around (not give a steady reading) unless your using a good quality one but i have had success on some cheaper units you wont know till you try. put your meter in the amp mode and touch each lead to one of the junction box wing nuts on the firewall. this is essentially how the stock ammeter is wired into the circuit.

You need to disconnect the stock meter first for this to work. If you do not you will get a false reading as part of the current will be flowing through the stock meter.

Also I use a FLUKE 337 DVM it works great and the noise (RFI and dirty DC) from an A does not bother it at all. I have used these meters in very high RF areas and on some very nasty looking DC and AC feeds and never have had a problem. Remember you get what you pay for!!

[Tom Wesenberg]

I like the Sperry SP-152A the best.

It's a quality inexpensive analog multimeter with several handy features, such as battery check for 1 1/2 and 9 volt batteries, a buzzer continuity check, and a 20 amp capability.

http://www.toolsnow.com/Analog_Multi...ter_p/5609.htm