How do I measure amps?

Showing my ignorance again... I'm developing a test bed to determine the optimum specifications for the track drive and in doing this, am using a 12
volt geared windshield wiper using a car battery for power. What I need is an inline amp meter to record the various phases for different materials. There will be a huge current demand at startup, then the demand should flatten as the device rolls along. With these figures we can begin zeroing in on the most efficient power/track combination. I just don't know how to insert an ammeter without blowing it up... Anybody? And since this is DC I don't think my clamp on meter will work.
Wayne
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Wayne Lundberg wrote:

Get a clamp-around DC ammeter. Yes, they do exist, and they're only about $120 now. Resolution is around 0.1A for most low-cost units, and the upper limit is usually a few hundred amps. DC clamp-arounds use a Hall-effect device. We had a nice one at Team Overbot, made by some Chinese company. Very useful when working with servomotors.
Otherwise you have to get a precision high-current shunt with a value like 0.001 ohm and wire it into the circuit, then measure the voltage across it. Here's how to do that, if you want.
http://www.motorcycleproject.com/motorcycle/text/shunt.html
                John Nagle
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Thank you John... since I'm not looking for a measurement relating to anything already in the system, but a way to measure benchmark (whatever reading when we start the experiments) to changes as we move from one kind of track material to the next and see if it draws less energy or more... and thus tabulate data from the experiments... I'm wondering if the shunt specifications must be as rigid as you suggest. Maybe making up a shunt just using stuff from the junk pile and then measuring the resistance, marking that as starting point, then adding a load and see the difference? Would that work?
Quite frankly, the shunt idea is black magic to me. I can't count the number of times I have wanted to measure current flow in a circuit without going to the expensive measuring devices on the market.
Wayne
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Wayne Lundberg wrote:

Don't know what John had in mind, but in this case a shunt is just a power resistor with a very low resistance. The automotive current meter no doubt works this way, too. As Curt points out in his excellent reply to you, it's not black magic but very basic electronics.
Answering another of your posts here: you do not want to connect your meter inline with your circuit unless A) you are sure the current demand is under the ratings of the meter and B) you are sure the meter uses fuse protection. Some of the cheap meters don't.
-- Gordon
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The meter is fused and I have blown a few over the years, even though I don't know why. I can build you a mechanical device beyond anybody's imagination, but I can't even come up with a simple DPDT for a DC motor to a robotic device and I am getting frustrated.
Thanks for your input and hope you will continue as a mentor.
Wayne
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Wayne Lundberg wrote:

Check out a copy of my book, Robot Builder's Bonanza, which is carried by most public libraries. It includes a detailed diagram of how to hook up a motor to a DPDT switch.
-- Gordon
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On Sat, 12 Aug 2006 19:42:40 GMT, "Wayne Lundberg"

I would think that if you had a good meter and measure the resistance of the wire between the battery and motor, then that wire could actually be used as the shunt. Note the resistance in the wire, then switch the meter to measure voltage. With the motor running note the voltage drop and use it in the I = E/R formula. If you don't want to do calculations, then just note the change in voltage drops between the different track/drive setups. .
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That looks great! I'm not afraid of calculations, after all, Ohms law is not really that compels when your profession depends on proportional formulas and the rule of three. It's a matter of integrating basic electronics ( which I am in diapers) with mechanical stuff, which I am somewhat good at.
Wayne\
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snipped-for-privacy@worldnet.att.net (Wayne Lundberg) wrote in

You could do it the easy way .. and just get an automotive amp gauge (you can get a cheap one at any auto parts store) .. you just put it inline, like a fuse.
mike
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Thank you! I thought there had to be a PC board somewhere behind the ammeter on the car's dashboard. So my next trip will be up to Otay Mesa's junk yard treasury and get the biggest and cheapest and try it out. The shunt idea is also very appealing.
Wayne

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Wayne Lundberg wrote:

Define "huge current." Are you talking about 10 amps, or 100 amps? More?
Whether you can use your clamp-on current meter depends on the features it has. There are AC-only models, DC-only models, and AC-DC models. Hopefully you got an AC-DC model so there's nothing extra to buy.
If you don't need accuracy, old style analog meters with a DC current setting will work. I keep an old Radio Shack meter for just this purpose. It can handle up to 20 amps, which is enough for my particular needs. Beyond that fuses blow.
A 0.1 or 0.01 ohm power resistor will also work. Next time you're in El Cajon try out Murphy's, or Gateway, between I-15 and I-163 at Kearny Villa. Both stock honking resistors you can use for doing inline current tests. You need to be sure to get a power rating that is appropriate for the watts passing through the resistor.
-- Gordon
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Thanks Gordon. My clamp on meter is a Gen-Tech and shows ACA and External Unit reading Ohms in the range of 20M to 2,000M ---- my other VOM is a A.W. Sperry DM-4300A with a plug for up to 10 Amps, and a switch for either AC or DC... but I am hesitant to put the full load in line because I'm not sure if this is the right way to do it. I lost the instructions years ago.
The ideal solution to my problem would be to simply connect the Sperry inline from battery to motor and see what happens under different load conditions. But I need some reassurance here.... Before making the smoky genie come out of the device.
Again, the load is a windshield wiper gearmotor for 12V automotive.
Wayne
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That's a standard clamp-on AC amp meter. It's just a transformer. That only works with AC because only a changing magnetic field will induce a current in a wire. A constant magnetic field will not. That's why it's not as easy to to detect DC currents with a clamp on device.
The DC current probes take advantage of the hall effect. The probe has a current flowing though it, and when that is placed in a constant magnetic field (created by DC current flow), it causes the electrons to curve as they move though the probe sense wire which causes them to pile up on one side of the sense wire. That piling up effect causes a slight voltage to develop from one side to the other in the probe. That can be used to determine the strength of the magnetic field and in turn, the DC current. These devices have in the past tended to be very expensive (I don't know why for sure), but based on the comments I guess they are getting cheaper. I've never owned one. But you can't find them at Home Depot or Radio Shack (I don't think). You have to go to an electronics store.

Yes, that's how you use a normal amp meter. You have to put it in line (in series) with the circuit. But I don't know if 10 Amps is going to be high enough for your application. If your motor draws too much current, it could burn out your amp meter.
The amp meter function has a very low internal resistance (think of it as a wire). Don't connect it direct across a battery or it will fry in an instant! Only connect it in series with a normal load.
I've got a Fluke digital meter with a 10A range option (labeled as unfused). I just checked the manual (which I still have only because it fits in the case that the meter is in). It says the meter can read up to 20A for up to 30 seconds, but that anything over 30 seconds or 20A is likely to damage the meter (aka melt stuff inside that can't be easily replaced).
So your 10A meter might work similar to that. If your load is under 10A most the time and only shoots over it by a little bit for only a short period. I'd suggest you look on the web for the manual however - you can probably find it and then you will know for sure what the limit it.
If your meter has a needle, then a large over-range condition can actually bend the needle as it whacks up against the high range stop - so that's another danger to watch out for.

I have no experience with those so I don't know what they would be likely to draw. But I know my car uses fuses from 10A to 30A or so for various circuits like that.

I think you said the idea of a shunt was magic to you in another post? Here's a quick course in basic electricity that might help you (if you don't already know this)....
All you do is hook a very low value resistor in series with your load. This extra resistance will reduce the voltage to your load slightly (depending on how low the resistor value is and the load your motor is creating). But because of Ohm's law, the voltage across this resistor, will be directly proportional to the current flowing through it. And the current flowing though it, will be identical to the current flowing though the motor, because they two are hooked in series.
So, if you connect a resistor like that, and then hook a normal volt meter across the resistor, and measure the voltage across the resistor, you can calculate the current flowing though it.
If you use a .1 ohm resistor, and you measure a voltage across it of 1 volt, then the current is:
I = E / R = 1 V / .1 Ohm = 10 Amps.
The amount of power turned to heat in the resistor is E * I, or E^2 / R watts. In the example above, for 10 Amps of current, that would 1^2 / .1 or 10 Watts. So you would need a 10 Watt power resistor in the above case to be sure you didn't burn up your resistor.
So, if you dealing with currents higher than your amp meter can deal with, and you don't own a DC current probe, you can just put a low value but high wattage resistor in series with your circuit, and use your voltmeter to measure the voltage across the resistor.
If you are dealing with a starter motor that draws 150 amps, then you better use something with a lot less resistance to keep it from melting - which is what the "high current shunt" was about - just a short and heavy duty wire with a known but very small resistance.
All wire has resistance and acts like a resistor so the above formulas work to calculate voltage across any length of wire with current running though it. You can just connect a volt meter at two points on a wire and measure the resistance when it's out of the circuit, then measure the voltage when it's in the circuit for the same connection, and calculate the current flowing though it. But unless you have a very sensitive ohm meter and volt meter, that won't work very well for most wires because the resistance, and the voltage, will be too small to measure accurately.
--
Curt Welch http://CurtWelch.Com /
snipped-for-privacy@kcwc.com http://NewsReader.Com /
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Thanks Curt, now off to the shop to run the experiments.
Wayne
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Wayne Lundberg wrote:

Not in answer to your question, but... Be careful about using windshield wiper motors. I recently had a disastrous robot project using windshield wiper motors as drive motors. Your motors may be different than mine, but two things bit us: 1. One of the DC motor leads was tied to the motor case (and therefore the gear box case). This causes a direct short when the motors are mounted to a conductive chassis and driven with opposite polarities. 2. The motors' gear boxes were designed to be driven in only one direction. The current draw in the "wrong" direction was over 5 times the current draw in the "correct" direction. This dropped the battery voltage and the motor was obviously "bogged down" even under no load.
BRW
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12
Thank you! The business about working best in one direction is good to know. I did not know this. As the the ground, yes, and for this experiment it is on a breadboard and for limited time usage, so the motor is held in place with wood. The 'real' anti-IED-Robot will have made to order motors, drives and the like once we determine optimum drive systems.
Thanks again!
Wayne www.terrasegura.org

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Wayne Lundberg wrote:

Not in answer to your question, but... Be careful about using windshield wiper motors. I recently had a disastrous robot project using windshield wiper motors as drive motors. Your motors may be different than mine, but two things bit us: 1. One of the DC motor leads was tied to the motor case (and therefore the gear box case). This causes a direct short when the motors are mounted to a conductive chassis and driven with opposite polarities. 2. The motors' gear boxes were designed to be driven in only one direction. The current draw in the "wrong" direction was over 5 times the current draw in the "correct" direction. This dropped the battery voltage and the motor was obviously "bogged down" even under no load.
BRW
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Thanks! I shifted from the windshield wiper motor and went to a standard 1/2" drill connected to a speed control I made for another application years ago. I can turn the dial up until the motor stalls, measure the Amps through the clamp-on meter, and get a pretty good idea of what the power requirements will be under different loads as I brake the shaft with strong hand... then mechanically... then with weights on wound string around the shaft.
Wayne

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Those who forget history are doomed to repeat it. (or reinvent it!)
Find an old auto mechanic who still has an old $2 starter motor current draw tester which is held against the start motor wire and is not connected to the wiring circuit in any way. It is for DC only.
This is just a circular meter about the size of a silver dollar (I just looked in my old mechanics box) and is accurate somewhere within about 10 amps or so. But it tells you if you can hook your more accurate meter in line without burning it out!
It is held against the starter wire (the big wire carrying all the amperage needed to start the car) and as you know this can be several hundred amps. The current in the wire pulls the needle off center and gives a reading that acts as a first step in diagnosing starting problems.
Next step, if needed, was to use a shunted amperage meter, but not if the starter was pulling 1000 amps. I have one of those meters too.
Pure simplicity. Ah, the old days.
HobbyBot
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