# where to find a digital ammeter for 500amp 50mV shunt?



## Newbiee (Feb 16, 2011)

Hi guys does anyone know where i can find a digital ammeter for 500amp 50mV shunt? i know there are some analog westach and westberg ones but 60$ for a meter?? i would get an analog one too but they seem so expensive. All the digital meters on ebay are 75mV!! grrrr there has got to be some place that sells some decent priced ammeters right?


----------



## rwaudio (May 22, 2008)

Newbiee said:


> Hi guys does anyone know where i can find a digital ammeter for 500amp 50mV shunt? i know there are some analog westach and westberg ones but 60$ for a meter?? i would get an analog one too but they seem so expensive. All the digital meters on ebay are 75mV!! grrrr there has got to be some place that sells some decent priced ammeters right?


You could probably buy a 75mv shunt and meter for less than the cost of a 50mv meter. (from ebay atleast) so unless you are attached to the shunt it might be cheaper and give you many more color/display choices to get a 75mv combo.


----------



## madderscience (Jun 28, 2008)

Ohm's law: V=IR
Power (watts) dissipated in a resistor : P=(I^2)R

shunts are just big, fractional ohm resistors. For example, a 50mv, 500 amp shunt is really just a 0.1 milliohm resistor that can safely dissipate 25 watts. 

Resistors combined in series and parallel to equate to a single resistance as follows:

Series: Rtotal = R1 + R2

Parallel: Rtotal = (R1*R2) / (R1 + R2)

If all else fails and you can't buy a 75mv shunt, you can combine multiple 50mv shunts to make one. For example, you can make a 75mv shunt from three 50mv shunts according to the above equations: two 50mv shunts in parallel equate to a 25mv shunt (half the resistance), Put this pair in series with the third to get 1.5 times the original resistance, or 0.15 milliohms or a 75mv, 500A shunt. Of course, this triples your shunt budget. 

If there is such a thing as a 25mv shunt, get one of those and put it in series with a 50mv shunt to get the same effect.

Another option is to simply make a shunt. You can look up resistance per foot for various wire sizes online. For example, 2/0 cable has a resistance of 0.08 ohms per 1000 feet, or 0.00008 ohms per foot according to the first website I found that claimed to have this info. This means you will need only 1.25 feet of 2/0 cable to equal a 50mv shunt, or 1.875 feet for a 75mv shunt. (check my math to be sure i'm not off by a factor of 10 or something). So you would just find a run of cable that long in your car, and tap the cable at one end and the correct distance away to get your shunt.

Final option: Don't use a shunt at all. Use a hall effect sensor such as one of these:

http://www.lem.com/hq/en/component/option,com_catalog/task,displaymodel/id,64.71.44.000.0/

You will need to supply it with 5v, and scale its output voltage down to the range expected by your meter. The scaling circuit might take a little tinkering but should not be terribly complicated. Without knowing the type of meter I could not recommend anything except to say it probably isn't terribly complicated.

Cheers


----------



## rwaudio (May 22, 2008)

madderscience said:


> Ohm's law: V=IR
> Power (watts) dissipated in a resistor : P=(I^2)R
> 
> shunts are just big, fractional ohm resistors. For example, a 50mv, 500 amp shunt is really just a 0.1 milliohm resistor that can safely dissipate 25 watts.
> ...


This works great on paper, however a shunt is actually "calibrated" most likely at the highest rated current, that's why you see cut marks in the resistance part as they tweak the actual resistance and show the proper reading. As soon as you try to connect a pair of shunts in series/parallel your connections are going to add resistance of their own to the equation and it will throw off your current reading. Not worth the cost and effort to create an incorrect reading. Just use a proper shunt or hall effect sensor. The one work around you could do is use a 750A 75mv gauge with the 500A 50mv shunt since the voltage at 500A would be the same. It might be just as hard to find that gauge as well though. 

There are also gauges out there that are configurable. I bought this one:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=350263616004&ssPageName=STRK:MEWNX:IT#ht_1257wt_1139
You can set the "full scale" voltage yourself, so you can use pretty much any shunt you want and tell it what you want it to read at 75mv. (so to use your 50mv shunt you would set it to 750A) Other than being a bit bright it's a decent looking/feeling meter (considering it's just a cheap made in china meter). It has two "alarm" outputs that you can configure at whatever current ranges you want. I plan to use them for a "driving green" indicator, if I can keep the discharge at or below 1C and a "power hog red" indicator when my current is 500-1000 battery amps. It also has some filter options so that you can somewhat adjust how quickly the display updates, I've found that in bench testing you can smooth out (slow) the display update rate to a point where you can actually read the numbers instead of having a display that changes so fast it's just a blur. However it's still quick enough to know what's going on. The user interface kinda sucks though and you can't do anything without the manual in front of you. I got the matching volt meter and they look good together. I'm going to put some tinted plexi in front of the pair though, as they are just too bright.

Good luck with your meter hunt.


----------



## dtbaker (Jan 5, 2008)

Newbiee said:


> Hi guys does anyone know where i can find a digital ammeter for 500amp 50mV shunt? i know there are some analog westach and westberg ones but 60$ for a meter?? i would get an analog one too but they seem so expensive. All the digital meters on ebay are 75mV!! grrrr there has got to be some place that sells some decent priced ammeters right?


why don't you look at some of the multi-function meters intended to display amps, ah, volts, etc? The Cycle Analyst is inexpensive, and shunt-based. http://www.ebikes.ca There are other solutions that are Hall Effect based as well.


----------



## madderscience (Jun 28, 2008)

In regards to "works great on paper" resistors (and shunts) are linear devices. Putting two in series or two in parallel will not affect either one's calibration, and the only additional resistance will be that of whatever conductor is used to connect them. Done right, the extra resistance of that should be negligible, or certainly within the noise when being used with a simple analog or digital gauge. 

Making a shunt from a calculated length of a piece of 2/0 wire would certainly have more error than a factory manufactured shunt, but on the other hand, it would be easy to calibrate (just move the tap) presuming you have a way to accurately measure how far it is off.

A configurable gauge could certainly solve the problem though.


----------



## rwaudio (May 22, 2008)

madderscience said:


> In regards to "works great on paper" resistors (and shunts) are linear devices. Putting two in series or two in parallel will not affect either one's calibration, and the only additional resistance will be that of whatever conductor is used to connect them. Done right, the extra resistance of that should be negligible, or certainly within the noise when being used with a simple analog or digital gauge.
> 
> Making a shunt from a calculated length of a piece of 2/0 wire would certainly have more error than a factory manufactured shunt, but on the other hand, it would be easy to calibrate (just move the tap) presuming you have a way to accurately measure how far it is off.
> 
> A configurable gauge could certainly solve the problem though.


I only mentioned it because I actually tried it and it didn't work so well. I was using a single 0.0001ohm shunt and I needed a higher voltage signal, so I put two of them in series with a nice clean secure bolted connection between the two, no wire or anything. The end result was ~3x the voltage of a single 0.0001ohm shunt instead of 2x. I found this because my results were off at first (showing higher than expected current levels) so in my case the good connection between two shunts was somewhere in the range of 0.0001ohms giving me a total of 0.0003ohms. In short, if you can adapt to the resulting signal then go for it, however if you are expecting EXACTLY the mathematical equivalent that you get on paper it's not going to work. 

A pair of taps on a 2/0 wire would probably be the best bet as long as you have a reference to calibrate it to, I probably could have done that instead of buying a 1000A 75mv shunt.... doh
Thanks for the tip.


----------



## Newbiee (Feb 16, 2011)

thanks guys for your help! stupid me i bought a 500amp 50mV deltec shunt on a bid on ebay because i noticed it was cheaper than everywhere else. and now that i have it the analog meters are the only ones i can find and they cost 60$


----------



## EVfun (Mar 14, 2010)

Yea, but they are cool 

Plenty of people have had issues getting digital meter working because they can be noise sensitive. Analog meters are usually trouble free. Some reasonably priced analog volt and amp meters are available from EVparts. They conveniently fit in a standard automotive 2 inch gauge holder.


----------



## GizmoEV (Nov 28, 2009)

Newbiee said:


> thanks guys for your help! stupid me i bought a 500amp 50mV deltec shunt on a bid on ebay because i noticed it was cheaper than everywhere else. and now that i have it the analog meters are the only ones i can find and they cost 60$


All you really need is a volt meter. Your shunt will read 0.1mV/A. I have a cheap meter which came with a 50A 50mV shunt and it has a pin to short to indicate where to put the decimal. Taking the decimal out would give you exactly what you want. It is powered by a 9V battery which lasts quite a long time. It is an LCD display but doesn't have a back light.

As someone else mentioned, you should really consider the CycleAnalyst at ebikes.ca. It is very reasonably priced and gives a plethora of useful information. You just program in the resistance of your shunt and it works.


----------



## Newbiee (Feb 16, 2011)

thanks a bunch all of you!! i decided the large screen high current cycle analyst will be a good choice for this situation. Thanks again guys!


----------

