Measuring DC AMPs W/Clamp-on meter

[radioman]

Has anyone used a clamp-on meter to measure DC amps troubleshooting golf carts?

[Volt_Ampere]

Many clamp on meters only measure AC but there are some that measure DC. It is much more difficult to measure DC inductively. I don't have any direct experience with clamp on DC ammeters and I don't know how they react to the chopped DC that a cart generated. I would opt for a current shunt and a good DC meter made to work with the shunt.

[scottyb]

I looked into this - a DC meter with this capacity was expensive.

[radioman]

Originally Posted by Volt_Ampere

Many clamp on meters only measure AC but there are some that measure DC. It is much more difficult to measure DC inductively. I don't have any direct experience with clamp on DC ammeters and I don't know how they react to the chopped DC that a cart generated. I would opt for a current shunt and a good DC meter made to work with the shunt.

I thought about the distorted DC wave forms in golf carts interfering with the reading after I posted the topic. Sometimes it would be interesting to be able to measure DC current inside a cable without having to break the connection. I used a DC clamp-on current meter once and it was susceptible to ghost readings but it was a cheap one. I never used one to work on golf carts. It might be a good troubleshooting aid in the hands of a experienced technician.

[rlw]

I'd be willing to bet someone else's paycheck that JohnnieB could come up with the math it takes to use one of the cables between the controller (or wiper) as a shunt for a milliammeter. The resistance of the "shunt" would depend on the type and gauge of cable of course, and there'd be all kinds of multiplying and dividing that needed to be done.

I'd also guess that there's a linear relationship between the duty cycle of the chopped DC voltage and the current flow.

Just a thought (and maybe not a very good one!)

RLW

[cgtech]

Just to be able to get a accurate enough ohm reading on a battery cable to be able to do this would require a ohm meter of a level of accuracy that we cant afford (lab stuff). You could apply a known amp load across a cable and measure voltage at both ends of the cable to determine ohms, but then you need a volt meter of a level of accuracy we cant afford. The closest thing i have (clamp-on meter) only goes to 200a, and it cost $160 when i bought it, but it will do AC or DC. It seems to do ok on the output side of a PWM controller, but, it only goes to 200a.

[rlw]

The other way to do it is actually build a shunt - my late dad used to make 'em from copper bus bars. I don't remember the math (I'm sure the Google would tell me if I looked hard enough), but he could make a fairly precise shunt for a DC milliameter to get it to read DC amps.

With a given length/width/thickness of copper - one end bolted to a terminal, the other bolted to a cable, and taps where the milliameter goes, one could probably get a good idea of the current flowing through the circuit.

That might be a fun winter mad-scientist experiment for me. I think I still have one of dad's calibrated ammeters laying around here somewhere to compare it to.

If I ever get a "round tuit", I'll post the results here.

EDIT: The first interesting hit I got on The Google was this link:

http://www.rcgroups.com/forums/showthread.php?t=393591

Of course, that's for measuring current on an R/C motor - our shunt would need to be MUCH beefier, but the concept is the same.


RLW

[jjance]

I picked up one at Home Depot. It measures up to 400 Amp AC/DC. I think it was about $139.00. I use it mainly with my solar panels and rarely with my cart. The one time I did use it was to verify I had a 3.3 hp motor.

[cgtech]

Originally Posted by rlw

The other way to do it is actually build a shunt - my late dad used to make 'em from copper bus bars. I don't remember the math (I'm sure the Google would tell me if I looked hard enough), but he could make a fairly precise shunt for a DC milliameter to get it to read DC amps.

With a given length/width/thickness of copper - one end bolted to a terminal, the other bolted to a cable, and taps where the milliameter goes, one could probably get a good idea of the current flowing through the circuit.

That might be a fun winter mad-scientist experiment for me. I think I still have one of dad's calibrated ammeters laying around here somewhere to compare it to.

If I ever get a "round tuit", I'll post the results here.

EDIT: The first interesting hit I got on The Google was this link:

http://www.rcgroups.com/forums/showthread.php?t=393591

Of course, that's for measuring current on an R/C motor - our shunt would need to be MUCH beefier, but the concept is the same.


RLW

That will totally work! Now i want to find a thin piece of copper and build one of my own (for golf cart sized applications). Although a real shunt is available on the interwebs for not-too-much money, its way more fun to do it your self.
Android app store has a free app called "ohms law" to do most of the math for you, just do a "before and after" copper shunt voltage reading. We are spoiled with the easy stuff phones can do for us, when i learned it myself by doing the math back in the day. There is no smart phone replacement for understanding the theory though. I use Ohms law every day in my own projects (a resistor and a capacitor to power a AA powered analog clock from USB to use as a timer most recently, easy & fun stuff). Heres the stuff without Google.

[rlw]

CGTech,

I used to do the math, too - I'm just old and lazy these days... ;^)

There's a good visual example of a shunt in my charger, an ancient ferroresonant EZGO model (perfect match for my resistor cart). There's a bar across the legs of the ammeter inside - the bar is the shunt.

What I'd need to ask Google about is how to calculate resistance based on the cross-section and length of a piece of copper. For very high currents like in our carts, you'd need more than just a copper strap, you'd be looking at a copper bar a couple of inches long, maybe 3/4" wide, and 3/16 or a quarter inch thick. The trick to calibration is to already have a calibrated meter. Place one probe from your not-yet-calibrated meter at one end of the shunt, read the correct value across the whole shunt on your calibrated meter, then move the other probe from the not-yet-calibrated further away or closer to the other probe 'til both meters read the same. That distance is the correct location for your taps on the shunt.

This won't be a $150 precision shunt from Fluke, but probably within +/- a few percent of reality, good enough for the girls I go out with!

Somewhere in the far recesses of my tiny brain is a memory of dad cutting 3" sections of 1/2" copper pipe, flattening them in a vise, then stacking and drilling the pieces to make a shunt for a meter.

Dad worked for the power company his whole life - had 43 years with 'em at retirement (they gave him credit for his 3 years in the Army during WWII). He started in the old trolley shop (Monongahela Power started out as an electric trolley company) working on motors, then moved to the meter shop. After the war, he used the GI Bill to get his engineering degree, and worked in substations and powerplants on what we'd call "remote sensing" equipment today (the infrastructure that let the power company monitor voltage, current, etc. from the substations at a central location). His last few years there, he was the liaison between the phone company and the power company.

He'd have loved to play around with an electric cart.

RLW