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Best Affordable Device To Measure How Much Current A 36v Motor Is Using?

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rs14smith

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Hey everyone,

I need advice or recommendations for a current meter that I can permanently place in a circuit that will have a 36v motor included. The current meter only needs to display how much current the motor is using.

As for how much current the motor may draw, I'd prefer if it's not too expensive to have one that can handle 70A or greater...even though I doubt my motor will ever draw that much.

The meter needs to be something that will perform well outdoors. It will be covered up from direct sunlight, and will be somewhat cooled from a fan, but basically I'm looking for something that's reliable and will last for a long time.
 
We don't have a lot to go on, but the technologies for DC are:
1. Shunt https://www.newark.com/jsp/search/b...onLocale=en_US&catalogId=&prevNValues=1003563 I picked 100 A. Now all you need is a voltmeter capable of measuring a tens to hundereds of mV.

2. Hall effect. It can be used in an isolated condition and their are solid state sensors. Not sure where the limit is.

3. Portable, battery powered, hall effect. just clamp over cable.

There is lots of stuff we don't know such as power source available, read in the dark, how far away do you need to be able to read?, What resolution, bar display etc.
 
Hall effect sensors can handle large currents. ACS755 is a good option.

**broken link removed**
 
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Sorry for double post my browser crashed.
 
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Here is a source for inexpensive current sensors:

**broken link removed**

The cheapest way is to measure the voltage drop across one of the wires that go to the motor. The downside is calibrating it.
 
There is lots of stuff we don't know such as power source available, read in the dark, how far away do you need to be able to read?, What resolution, bar display etc.

Basically something like this: https://cgi.ebay.com/Mini-Blue-LED-...198?pt=LH_DefaultDomain_0&hash=item27b503abae
I'm just not sure how reliable these eBay meters are.

I do not need to see it too far away, just a basic backlit LCD will do.

I'm not too familiar with this Hall Sensor you all are talking about. Does it require less wiring, or what's it's advantage? Something simple is great too. Again this will be mounted "permanently", so a portable multimeter for example, is not what I'm looking for.
 
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The devicecraft website has the hall effect sensors with the conditioning stuff. The advantage to the hall effect is that you don't have to splice into the cable, but you need to pass ONE of the conductors through the hole, not both of them.

You might be looking for a Nema4x enclosure such as these: **broken link removed** with a clear plastic front. Small power supply, sealed power switch etc. the Extra's add up.

I don't see any real issues with the ebay item as long as it's calibrateable.

Many meters cannot use the same ground that they are measuring. Hall effect also creates an isolated output with respect to your motor power.
 
The devicecraft website has the hall effect sensors with the conditioning stuff. The advantage to the hall effect is that you don't have to splice into the cable, but you need to pass ONE of the conductors through the hole, not both of them.

You might be looking for a Nema4x enclosure such as these: **broken link removed** with a clear plastic front. Small power supply, sealed power switch etc. the Extra's add up.

I don't see any real issues with the ebay item as long as it's calibrateable.

Many meters cannot use the same ground that they are measuring. Hall effect also creates an isolated output with respect to your motor power.

Hmm, well is there a "simpler" solution, like your name implies lol "Keep It Simple", as I'd like the device to be fairly small in size, and all I'd need to do is hook it up in series with my motor, and be done with it. Like I'm hoping to find something that no external power will be needed (as I've seen some meters require a 9v battery to power the LCD ehhhh), no worries about the meter having issues using the same ground etc. etc.

I'm not sure if the one on eBay requires an external battery or not, I'm just hoping I can find something small, simple, and reliable for this :)
 
Why not an analogue panel meter across a simple shunt (e.g.a piece of wire). The meter can be adjusted by putting a pot in series with it, or by connecting the pot across the shunt as a potential divider to the meter. Requires no extra power source, is cheap & reliable.
 
Here's an example: **broken link removed**
Under $4 including postage (probably a similar deal to your location).

If you did get one of the above, you'd want to open it up, change the shunt (or adjust its tap point) & change the front panel to reflect your required 0-70A.
 
Why not an analogue panel meter across a simple shunt (e.g.a piece of wire). The meter can be adjusted by putting a pot in series with it, or by connecting the pot across the shunt as a potential divider to the meter. Requires no extra power source, is cheap & reliable.

Because I do not want to have to guess what the current is, I want a easy to read digital output. :)
 
Basically something like this: https://cgi.ebay.com/Mini-Blue-LED-...198?pt=LH_DefaultDomain_0&hash=item27b503abae
I'm just not sure how reliable these eBay meters are.

I do not need to see it too far away, just a basic backlit LCD will do.

I'm not too familiar with this Hall Sensor you all are talking about. Does it require less wiring, or what's it's advantage? Something simple is great too. Again this will be mounted "permanently", so a portable multimeter for example, is not what I'm looking for.

Looking back at the link in this post something just doesn't seem right? This looks to me like a small digital panel meter that can be used with a shunt but does not include a shunt. Figure 50 Amps, OK, that requires by US specifications what? About AWG 8 wire and lugs. I just don't see that little meter handling 50 amps sans a shunt or added hardware. Not at that price anyway.

How accurate do you want to measure the current? Will the current always be unidirectional? How inexpensive is inexpensive?

I tend to agree with Dougy as to a simple shunt with analog meter as a simple and viable solution.

Ron
 
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Dougy:

Running current <50mA

Power Supply DC6-15V (5V optional)

36 is bigger than 15 V, hence some type of power supply/regulator is necessary.
Unfortunately 36 V is non-standard voltage.
 
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Ron:

1st line of the specs of the ebay item:

DC 50A Mini Blue LED Digital Amp Current Panel Meter & Shunt

Unforunately 36 V is non-standard and they say < 50 mA but don't tell you at what voltage which is probably 5 ish. That's 0.25 Watts.

Again, just looking at two suppliers, I found a 12 V 25 W (10 x more power than needed) power supply that's EXPENSIVE and overrated for the application: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=RP30-2412SEW/N-ND, but it has the right input voltage range.

This http://www.newark.com/tracopower/thp-3-2412/dc-dc-converter-3w-12v-0-3a/dp/51R5278 is a bit cheaper.

The 36 VDC and low cost digital are competing with each other.

The LM317THVT is a linear regulator that works to 57 V input: http://www.futurlec.com/Linear/LM317HVTpr.shtml it's also available from other suppliers. This: http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_117621_-1 could become a basis (PC board) for the regulator. The voltage ratings of the caps need to be changed and the variable resistor changed to a fixed one and the LM317 changed to an LM317HVT. Still inexpensive. Probably won't work, because the ebay item say an "independent supply" is required. This means isolated.

If 120 V is available, then the power options get better and could be an inexpensive "wall wart".

The ebay item says a 75 mV shunt and 0.1 V resolution. If this were the case , the meter would always read zero. They may mean 0.1 A resolution. Asked seller question.
 
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Ron:

1st line of the specs of the ebay item:

DC 50A Mini Blue LED Digital Amp Current Panel Meter & Shunt

Unforunately 36 V is non-standard and they say < 50 mA but don't tell you at what voltage which is probably 5 ish. That's 0.25 Watts.

Again, just looking at two suppliers, I found a 12 V 25 W (10 x more power than needed) power supply that's EXPENSIVE and overrated for the application: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=RP30-2412SEW/N-ND, but it has the right input voltage range.

This http://www.newark.com/tracopower/thp-3-2412/dc-dc-converter-3w-12v-0-3a/dp/51R5278 is a bit cheaper.

The 36 VDC and low cost digital are competing with each other.

The LM317THVT is a linear regulator that works to 57 V input: http://www.futurlec.com/Linear/LM317HVTpr.shtml it's also available from other suppliers. This: http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_117621_-1 could become a basis (PC board) for the regulator. The voltage ratings of the caps need to be changed and the variable resistor changed to a fixed one and the LM317 changed to an LM317HVT. Still inexpensive. Probably won't work, because the ebay item say an "independent supply" is required. This means isolated.

If 120 V is available, then the power options get better and could be an inexpensive "wall wart".

The ebay item says a 75 mV shunt and 0.1 V resolution. If this were the case , the meter would always read zero. They may mean 0.1 A resolution. Asked seller question.

Wow didn't know this was going to turn out to be so complicated lol. If using an analog meter is the "simplest" solution, I may have to go down the road. I wish there was a tutorial stating how to hookup a shunt to a meter as from the pictures I've seen, I do not really see their overall purpose...or I understand the purpose (for allowing high current) but do not understand how it works.

From the pictures, it just looks like two metal blocks that you tie your connections to, then you make another connection from the shunt to the meter...but from that, it appears you could just easily hookup the connections straight to the meter and not bother with the shunt...even though I know that's wrong, that's how it appears.
 
Thanks KISS, now I see all the details. Damn, now that is as inexpensive as it gets! Granted the shunt isn't likely much but at that price not a bad deal at all. So you need an external DC6-15V (5V optional) power supply but depending on the setup use as you suggest a regulator or depending a two bit wall wart. I would be tempted to buy a few just to see how good they actually are.

Ron
 
Wow didn't know this was going to turn out to be so complicated lol. If using an analog meter is the "simplest" solution, I may have to go down the road. I wish there was a tutorial stating how to hookup a shunt to a meter as from the pictures I've seen, I do not really see their overall purpose...or I understand the purpose (for allowing high current) but do not understand how it works.

From the pictures, it just looks like two metal blocks that you tie your connections to, then you make another connection from the shunt to the meter...but from that, it appears you could just easily hookup the connections straight to the meter and not bother with the shunt...even though I know that's wrong, that's how it appears.

Actually it is pretty simple. :)

The shunt is no more that a very high power low resistance resistor. When current runs through the low resistance there is a small subsequent voltage drop. That voltage drop is proportional to the current passing through to the load. The tie points on the block for the meter are precision points across the resistance. I can post a few images of shunts I have laying around if it would help but what you see is what you get.

Ron
 
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