Can it be used to measure DC amps? None of my other meters can handle more than 10 amps and I suspect a motor I have draws more than 10 amps. It has stranded 10 gauge wire. Must be a reason for that.

You should be aware that wire size is likely sized to handle inrush current. An inductor will have a massive current spike at the beginning before the field begins to grow, because it is essentially a dead-short at startup.

You can very easily determine your amperage by putting a very low-resistance shunt in series with the ground-lead and then measuring the voltage drop across it with a voltmeter. 1-Ohm would make the calculation easy, but in all cases use Ohm's and Watt's Laws to choose your shunt so it can handle the power-dissipation necessary.

 

@MaxHeadRoom Getting error when trying to quote.

My best meter can handle 10 amps. With the potential inrush current - I don't want to blow my meter out.

@BobaMosfet Yes, I'm aware of inrush. This is why I want to take measurements before completion of my designed motor speed controller. No sense in building one that can handle 20 amps and have it blown the first moment I turn it on because the motor draws 30 amps at startup.

 

I deleted it because I realized you had no DC ammeter.
Another way to test if concerned you will damage any meter with inrush is to run the fan with a blast of compressed air or other source of air volume to the blades and apply the voltage when rotating fast.
And if controlling with PWM then inrush should not be an issue, as the acceleration can be controlled.
Max.

 

@MaxHeadRoom Kind of thought you might have deleted it. That's cool. I've done that too.

This isn't an urgent project. I'm actually busy building cabinet doors for the kitchen cabinets I recently built. My time is pretty much allocated to that task. Only when it's raining - and that has been a lot this spring, second highest recorded rainfall for spring in Salt Lake City Utah do I spend time tinkering at my lab. I'll get there. Forecast is for more rain, so I may be downstairs in Igor's Laboratory with my caldron boiling.

 

If you have an idea as to the model of BMW then you may be able to find a fuse chart which would give the rating of the fuse for that blower motor. The blower motors for Ford cars that I built a tester for years back drew almost 30 amps at max speed. That would tend to explain the #10 wire. But the 100 amp shunt and your 199 millivolt digital meter should give an adequately accurate indication.
I use 100 amp shunts and a ten amp shunt, all quite good quality. "RUE", not quite new, and MUCH cheaper. They came with a circuit board that has a Burr Brown 3656HG isolation amplifier, but I have not bothered to figure out how to power the amplifier, and then I would need to calibrate it, and that gets complicated. AND my DMM does an adequate job displaying the current when I use the series shunt. It is much less convenient than a clamp on, though..

 

DC clamp meters are available for not much. But I caution against the EXtech brand. I bought a model EX730. It works, but when it's not even clamped on a wire it floats around -0.7A, and it gives different magnitude readings if you turn it around (so the current appears to flow in the opposite direction). I keep meaning to crack it open and look for a calibration screw, but haven't gotten around to it yet.

 

DC clamp meters are available for not much. But I caution against the EXtech brand. I bought a model EX730. It works, but when it's not even clamped on a wire it floats around -0.7A, and it gives different magnitude readings if you turn it around (so the current appears to flow in the opposite direction). I keep meaning to crack it open and look for a calibration screw, but haven't gotten around to it yet.

The DC ZERO button has no effect on that?

 

The zero button does zero out the reading, but features like max/min are disabled when using the zero feature, which makes it useless for anything but steady state readings.

 

Was at Lowe's yesterday (big box home improvement store). Glanced at their clamp meters; they had one for $90 (US). I just don't have that much use for a DC amp meter like that to justify spending that kind of money. Otherwise what's starting out as a $20 to $30 project quickly costs over $100. Sure, I have the meter in my arsenal, but I just don't foresee a need for it, unless I want to check current on my car battery when starting, when idle, when off - etc. Most of the time my DMM's can handle what I work with (hobby).

I'll just throw the shunt in and set up the meter to read amps across the shunt. Then since it's 100 amp I can just multiply my reading by 100. Should be WAY close enough for what I want to know.

 

The zero button does zero out the reading, but features like max/min are disabled when using the zero feature, which makes it useless for anything but steady state readings.

Sounds like an odd design! Sorry to harp on your meter here, but I'm really intrigued by this, both because I want a meter with similar capabilities someday and because I've had mostly good experiences with Extech designs. So anyway, one last question for you:

Have you tried the CAL feature within the peak hold mode? From the description in the manual, it sounds like it should do what you need, although I'll admit that it also sounds like a cumbersome and counterintuitive workflow.

 

I tried CAL just now, and it actually made it worse. It's now showing -0.8A sitting by itself on my desk. To be the devils advocate for a moment, its largest range is 800A, so percentage wise 0.8A off isn't that bad.. but then again it's showing 0.8A off when set to the 40A range, which is very significant, bordering on unusable for anything that draws less than a handful of amps. Suffice to say, the current clamp just isn't that accurate. At the time I bought it the goal was to read cranking amps for a starter motor so I needed the big range, now trying to use it for smaller things just leads to frustration. Maybe the solution is (a) spring for a higher tier tool like a Fluke, or (b) get a second meter with much smaller range to use for smaller readings.

 

I tried CAL just now, and it actually made it worse. It's now showing -0.8A sitting by itself on my desk. To be the devils advocate for a moment, its largest range is 800A, so percentage wise 0.8A off isn't that bad.. but then again it's showing 0.8A off when set to the 40A range, which is very significant, bordering on unusable for anything that draws less than a handful of amps. Suffice to say, the current clamp just isn't that accurate. At the time I bought it the goal was to read cranking amps for a starter motor so I needed the big range, now trying to use it for smaller things just leads to frustration. Maybe the solution is (a) spring for a higher tier tool like a Fluke, or (b) get a second meter with much smaller range to use for smaller readings.

Compared to the its total ranges, the error levels aren't terrible, but they're not great either. Pretty disappointing, really.

Thanks for sharing your experiences. If I were in your shoes, I'd probably end up with a second meter for lower ranges - since I'm not doing any serious electronics work professionally, it's hard to justify top tier equipment prices. Either way, good luck!

 

Just finished the wood working for the kitchen cabinets. Now back to this project:

IF I connect the blower to the battery through the shunt and put an amp meter parallel to the shunt, the shunt being rated 100 amps, and suppose I get 0.05 amps reading, would that mean 5 amps is going through the circuit? I ask because this is where the grey gets a bit fuzzy.

 

Just finished the wood working for the kitchen cabinets. Now back to this project:

IF I connect the blower to the battery through the shunt and put an amp meter parallel to the shunt, the shunt being rated 100 amps, and suppose I get 0.05 amps reading, would that mean 5 amps is going through the circuit? I ask because this is where the grey gets a bit fuzzy.

Maybe I'm misunderstanding you - if you mean a typical DMM set to current mode, you don't want to put its probes across your shunt resistor! That would blow the fuse in your meter and possibly damage it as well. You want to have your meter set to read volts, not amps (with meter leads plugged in accordingly.) You should take a voltage reading across the resistor, most likely in the low-millivolt range.

Unfortunately, I'm not sure what the calculation to convert mV to A will be, because there seems to be more than one standard. You mentioned that your shunt is a 100A shunt, but nothing else. Does it have any other numbers on it? Quick product searches come up with 100A/100mV shunts and 100A/75mV shunts. You may have one of those, or perhaps some other value that I haven't come across. The conversion from mV to A will depend on the resistance of the shunt, which will be quite low. You'd be looking at 1 milli-ohm or 0.75 milli-ohm for the two specs I mentioned above. Anyway, if it has more numbers on it, hopefully they'll reveal the exact specs.

 

To be honest, an advertisement via e-mail for Amazon featured a cheap volt meter with amp meter combined. It came with a 100 amp shunt so that the meter could read up to 100 amps. My DVM has two amp settings, mA and 10A, depending on how you wire it. Since the motor in question can draw more than 10 amps I don't want to put my meter in series and then wish I didn't. Beyond the fact that this volt/amp meter with the shunt came from China I know nothing more about it. When measured for ohms it's so low I don't have confidence the meter will be accurate enough to determine the drop across it. But like I said, I've been busy building cabinets and drawers and doors. That project finally wrapped up today and I'm in the process of cleaning up the garage (my woodshop space). I'm tired and want to watch some TV. So maybe tomorrow I'll do a little more digging into the matter. Or maybe I'll just figure out how to wire up that Chinese meter and see what it says. After all, I'm not looking for accuracy, just a decent idea of the draw at startup and at run. However, some people in the reviews complained it takes too long to read volts and amps. So I may never get that information. Perhaps once I have an idea of what it's reading I can hook up a scope and see the trace to get a good idea of how much it draws at startup.

Just occurred to me - put an automotive bulb in series with it. What'cha all think?

 

To be honest, an advertisement via e-mail for Amazon featured a cheap volt meter with amp meter combined. It came with a 100 amp shunt so that the meter could read up to 100 amps. My DVM has two amp settings, mA and 10A, depending on how you wire it. Since the motor in question can draw more than 10 amps I don't want to put my meter in series and then wish I didn't. Beyond the fact that this volt/amp meter with the shunt came from China I know nothing more about it. When measured for ohms it's so low I don't have confidence the meter will be accurate enough to determine the drop across it. But like I said, I've been busy building cabinets and drawers and doors. That project finally wrapped up today and I'm in the process of cleaning up the garage (my woodshop space). I'm tired and want to watch some TV. So maybe tomorrow I'll do a little more digging into the matter. Or maybe I'll just figure out how to wire up that Chinese meter and see what it says. After all, I'm not looking for accuracy, just a decent idea of the draw at startup and at run. However, some people in the reviews complained it takes too long to read volts and amps. So I may never get that information. Perhaps once I have an idea of what it's reading I can hook up a scope and see the trace to get a good idea of how much it draws at startup.

Just occurred to me - put an automotive bulb in series with it. What'cha all think?

My 100 amp shunts say 100 mV, meaning that 1 amp -1 millivolt. Using the meter, it is set to display millivolts DC. Consider that the shunt is doing the conversion of amps to millivolts, and that is why the meter must be set to read voltage. To be accurate you do need to use the "meter" terminals on the shunt for the meter, and the "big terminals" for the power and load. You already knew that, I was just repeating it because it is very important.

 

OK, rain outside. Can't do garden work like I was supposed to.

My shunt says 100A 75mV. So I'm assuming for 75mV I have 100 amps. 7.5 mV = 10 amps. 0.75mA = 1 amp.

So am I going about this right: 7.5 volts = 100 amps on my shunt? You see - this is why I call math a four letter word.

[edit] I'M SO CONFUSED! One amp = 0.75 millivolt?

 

Voltage across shunt when:

Starting = 27 mV
Running = 12.7 mV

Don't give me the answer, give me the formula I'm so badly missing. It's probably stupidly obvious, which is why I can't see it.

EDIT: I'm coming up with 20.25 Starting Amps and 9.5 running amps. Am I even close? It sounds reasonable. Just want to be sure.

 

Amperage = (reading in millivolts/75) * 100

so:

A = mV *4/3

or approximately:

A = mV * 1.333

 

So you're saying at 27 mV my starting current is 36 amps?