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Discussion Starter #1
An Amp Volt Meter is a small, dual read out, digital meter

...that will show the voltage of your battery pack (top reading, in red)

...& it will also show the amperage (Amps) that your motor is drawing from the battery pack (bottom reading, in blue)


This is the one I have/used

https://www.ebay.com/itm/291152585319

...it was only ~$15.00 delivered

...it's 48 x 29 x 21 mm (~2" x ~1 1/8" x ~7/8")


It comes with a (500A/75mV) "Shunt" (basically a giant resistor) that sends information to the meter

...that you install on or near the negative terminal of the battery pack


IMHO It's simple to install & very easy to hook up


It only has (5) wires to connect

The small plug has (2) wires
(they supply the power to the meter)

...the positive (+) red

...& negative (-) black


The bigger plug has (3) wires
(they are the meters sensor "or data" wires)

...the B+ (red) sensor wire goes to the battery packs positive (+) terminal

...the B- (black) sensor wire goes to the (B-) on the shunt

...the P- (yellow) sensor wire goes to the (P-) on the shunt


* The meter can monitor up to 300VDC (the red (3-plug) sensor wire)

...but it can only be powered by ~5V thru 30VDC (the red (2-plug) power wire)


To begin with,I drew a simple wiring diagram

...to show how I connected the meter to a small 24V 12AH battery pack


I added a switch to be able to turn the meter off when not in use (I also show many of the other components involved on a small EV)

...to help illustrate & show how everything is connected & interconnected


** Since this battery pack is under 30V

...the meter can be powered (directly) by the monitored pack voltage

...but, remember this diagram is ONLY for a 24V system


https://www.youtube.com/watch?v=WMmLiqFFuUg
 

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Discussion Starter #2
Before we wire up the meter

...we need to mount it, somewhere


It could be easily mounted

...in/on a small box on the dash board

...or right in the dash board

For demonstration purposes, I'll just mount into a small dash board

...like for a go kart, golf cart, solar panel etc.


I rounded up some plastic (from the back off an old TV)

I figured ~4" x 8" should work

...measured ~8" & marked it

...used a square to "square it up" & as a straight edge, for scoring

...scored it several times in each direction

...lined it up with the edge of the bench top

...& snapped 'er off nice n clean

then,

...measured ~4" & marked it, scored & snapped 'er off too


Evenly, arranged the meter & a couple of switches

...drew out some guide lines with pencil

...marked just inside but, around the perimeter of the meter hole

...& marked the center of each switch hole

...drilled small holes (~1/8") then bigger (~1/4") holes
(to remove as much unwanted material as possible)

...use a razor knife to "connect the dots"

...& then, a lot of whittling
(slowly removing material right up to the line)

...checking several times along the way for "component fitness"

That should do it

https://www.youtube.com/watch?v=icIeGdwQRvY
 

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Discussion Starter #4
To wire it up,

I rounded up some 22-4 wire (22g.- with 4 wires in one outer jacket)

...they look to be about the same size as the sensor wires used for the meter

I have a big role of it left over from when I wired the alarm system in my garage & I also used it for the wiring harnesses on El Dingo & El Moto too


Looking at the diagram

Start off at the meter

...the big plug has (3) "sensor" wires

...yellow (P-)

...red (B+)

...& black (B-)


The 22-4 wire has red, black, white & green

so, we'll use

...white for yellow

...red for red

...black for black

...& the green is just, left over


The small plug has (2) "power up" wires

...red (+) is the power input for the meter

...& the black (-) is the negative for the meter


* For this application (24V) we can draw the power right off of the battery pack (because it's between the 5V & 30V)

...so, the small plug, red (+) "power" wire can be connected together with the big plug, red (B+) "sensor" wire

...& the small plug, black is unused

I put a ring terminal on

...the yellow wire

...the white wire

...& on each of the black wires

Then, I attached the yellow & the white wires together, with a screw, to the wall if the enclosure divider

...& the (2) black wires together with a separate screw


I ran the "power up" wire thru the switch (with spade connectors)

...to be able to turn the meter off when not in use

** I made all connections, with crimp ends, to show that the meter could be easily hooked up without soldering

https://www.youtube.com/watch?v=1J8S26PM-f0
 

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Discussion Starter #5
Last summer, I assembled a mobile meter

This way, it didn't have to be mounted into the dash or hard wired

...it could be easily switched around & used on several different karts

...& it could also, be used on/with higher voltage systems (over 30V)


First, I needed a box to mount everything into

so, I rounded one up & some other stuff too

... a really old smoke detector

...a broken aquarium air pump

...a circuit tester (blown bulb)

...& some wire


Opened up the smoke detector

...& removed the connector & wire leads, for the 9V battery


Opened up the air pump

...needed the top half of the case for the meter box

...& the bottom half to mount the shunt into


Unscrewed the wire lead from the test light/circuit tester


Drilled a 5/8" hole in one end of the aquarium case

...had to "waller" it out a bit (move drill around while drilling)

...needed a little bigger than 5/8"

...while it was still warm (& kinda soft), I threaded the end of the circuit tester lead into the hole


Then, drilled a 3/16" hole in the same end for the other wire leads


The last thing I needed was a front panel


So, I layed out, marked & cut a panel

out of a piece of plastic from the old TV back

A little roundin'-n-shapin' & 4 mountin' holes


Then, I had to cut out a spot for the meter

...& since this is (self-contained) battery operated (not switched on/off with the kart)

...should add a switch (so, it can be turned off, when not in use)

so, I cut out a hole for a switch too
 

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Now, to wire it up

Remember, the meter has (5) wires

...(2) go to the "power plug" which supplies the power (5V to 30V) to operate the meter

...& (3) go to the "sensor plug" which supplies the data to the meter


I figured a little 9V battery, with it's clip on power leads, would be the simplest/easiest to mount & hook up

So, inside the box, I mounted a 9V battery with a small piece of velcro

...then, I soldered the 9V battery's red (+) wire & the "power plug" red (+) wires together

...& soldered the 9V battery's black (-) lead wire to one terminal of the switch

...& also, soldered the "power plugs" black (-) wire, to the other terminal of the switch, to complete the "power up" circuit


Keep in mind

...the meter needs to be seen "up front" by the driver

...but, the sensor data comes from "in the back" at the battery pack

I figured, ~3' should be plenty

...so, I used ~36" of 22-4/cable to connect the "sensor plug" from the driver compartment to the battery pack

...the red (B+) "sensor" wire is soldered to the red wire in the 22-4/cable

...the yellow (P-) "sensor" wire is soldered to the white 22-4/cable

...the black (B-) "sensor" wire is soldered to the black 22-4/cable


Back at the battery pack

The shunt is to be connected "inline" with the battery pack's negative (-) cable.
(I mounted the shunt into the other half of the old air pump box with a couple of zip ties)

...the (B-) (battery -) side of the shunt, is connected to the negative terminal of the battery pack (or as close to it as possible)

...the (P-) (power -) side of the shunt is to be connected to the negative (-) battery cable (the negative cable going away from the battery)


The black wire (B-) from the 22-4/cable goes to the battery side of the shunt (B-)

...the white wire (P-) from the 22-4/cable goes to the motor side of the shunt (P-)

I attached an alligator clip to the red (B+) wire coming from the "sensor plug" (so It can be easily attach it to the positive (+) terminal of the battery pack)
 

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Discussion Starter #9
Other than a purpose built buck converter?

No expensive for such low current.
I know there are 48V to 12V 3A buck convertors on eBay for ~$10.00
https://www.ebay.com/itm/DC-DC-Converter-Module-Buck-Voltage-Step-down-Regulator-Power-Waterproof-MA1045-/123112071726?_trksid=p2349526.m4383.l4275.c10#viTabs_0

or

48V to 12V 10A voltage convertors (for stereos on golf carts) ~$12.00
https://www.ebay.com/itm/GOLF-CART-DC-BUCK-CONVERTER-10-AMP-48V-36V-VOLTAGE-REDUCER-REGULATOR-TO-12V-10A/332636769522?hash=item4d72b24cf2:g:PVoAAOSwzINa52f-

but, they seemed like "overkill" (these little meters only draws ~50mA)

I know, from testing, a voltage divider will "do the job"

While doing research, the info says they are not for "powering a load"

So, I guess I'm asking
...would using a voltage divider to reduce 48V to 12 & supplying under 50mA to power one of these little meters, be considered "powering a load"?

...or would it be a "safe, simple & acceptable" way to "accomplish this task"
 

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Discussion Starter #10
I know there are 48V to 12V 3A buck convertors on eBay for ~$10.00
https://www.ebay.com/itm/DC-DC-Converter-Module-Buck-Voltage-Step-down-Regulator-Power-Waterproof-MA1045-/123112071726?_trksid=p2349526.m4383.l4275.c10#viTabs_0

or

48V to 12V 10A voltage convertors (for stereos on golf carts) ~$12.00
https://www.ebay.com/itm/GOLF-CART-DC-BUCK-CONVERTER-10-AMP-48V-36V-VOLTAGE-REDUCER-REGULATOR-TO-12V-10A/332636769522?hash=item4d72b24cf2:g:PVoAAOSwzINa52f-

but, they seemed like "overkill" (these little meters only draws ~50mA)

I know, from testing, a voltage divider will "do the job"

While doing research, the info says they are not for "powering a load"

So, I guess I'm asking
...would using a voltage divider to reduce 48V to 12 & supplying under 50mA to power one of these little meters, be considered "powering a load"?

...or would it be a "safe, simple & acceptable" way to "accomplish this task"
OK, I'll take that as a "give it a try, report back with the results" :D

I have another question for the EE's (well a few more)

It seems like many different combinations of resistors will accomplish the same task.

In the previous test, I used a 3K Ohm resistor for R1 & a 1K Ohm resistor for R2, to reduce 48V to 12V

But, it looks like (mathematically) as long as R2 is 1/3 of R1, you will get the same results.

So, in this situation (48V to 12V ~50mA) would it be better to use lower Ohm resistors like (300 Ohm = R1 & 100 Ohm = R2)
...more of a mid-range like (R1 = 600 Ohm & R2=200 Ohm)
...or stick with the higher Ohm resistors (3K Ohm = R1 & 1K Ohm = R2)

& what about wattages?

In the test, I used 1W resistors & noticed that R1 got a little warm (not hot) just warm but, I did not notice a temp difference in R2.

So, would higher watt resistors handle the heat (warm) better?
 
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