Wattmeter Using 2 Meters

Description

A wattmeter design for use with equipment using up to 20 volts DC with a current of up to 10 Amps. I.E. the voltages and currents that you would typically find for Arduino, Raspberry Pi, and other projects in this area and Ham radio equipment with transmitters of up to 50 watts or so.

To use it is simple, read the voltage, read the current, multiply the two together and you have watts. You don't need to connect up 2 separate meters, no messing about cutting supply lines to insert the current meter. The the left hand voltmeter can also be used as a normal meter, but you do have to disconnect BOTH leads first, otherwise you could get a short circuit when using the probes.

These meters used are very cheap, both in price and quality. The ones I have had have always given good service, but I usually calibrate them every so often. I noticed the the model I have used in this Instructable does not have a means of calibration. A good Fluke VOM (Volt Ohm Meter) can cost upwards of $170, even $450. The meters used in this wattmeter are about $5 each, enough said!

To build this device you will need to know the basics of soldering, how to use a hot glue gun, and how to use a sharp knife without harming yourself.

If you have not used a VΩOM meter before, you will need to read the instructions, especially the parts referring to safety. I have had meters blow up on me, because I have forgotten basic safety rules, it is embarrassing to have an meter explode while the customer is standing next to you. Make sure you know what you are doing. If you are working with a 9 volt battery and you accidentally short circuit it, all that going to happen is that you are going to discharge is real fast. But try that on 240 volts or 415 volts and you could die. Having had short circuits on those voltages, trust me I know! Even 12 volts at 10 amps can bite you if you make a mistake. Safety first, always.

Standard disclaimers apply, such don't cut fingers off, burn yourself etc. Don't eat the glue or solder, when do I stop? Please read all the instructions before starting the project. I accept no responsibility, etc, etc!

Step 1: ​Schematic

Schematic Watthour Meter 2 Cen-Tech.sch, in Eagle 7.2.0, PDF

Step 2: ​Parts Required

2 CEN-TECH 7 Function DIGITAL MULTIMETER, $5.99 when on sale. Item 98025

from Harbor Freight http://www.harborfreight.com/7-function-multimete...

3 feet 16 GA, Zip Cord At least 3 feet, in the example shown I used 6 feet, stranded.

I will call these wires the Big Red and the Big Black wires

1 foot 18 or 20 GA Wire insulated wire, stranded, any color, I will call this wire the thin one! (not shown in photos)

4 Power Poles Optional, if used 2 Red, 2 Black

Hot Glue

Solder

Step 3: ​Tools Required

Not all of the tools are shown.

Soldering Iron, with solder.

Hot Glue Gun, with glue sticks.

Utility Knife.

Small Screwdriver.

Dremel Tool with a cut off wheel, or hacksaw blade.

Power Pole Crimper.

Wire Cutters.

Felt Tipped Pen.

Safety Glasses.

WhiteOut.

Long Nose Pliers.

Tape measure.

Step 4: ​Consumables Required

2 painters sticks (or 2 pieces of material the same thickness, I have used chopsticks!).

These need to be thicker than the distance that the front knob sticks above the front of the meter so that the meters do not rock while you are glued them together, you will see what I do in the photo in that section.

Step 5: Unpacking, Getting Read for Assembly

Unpack both meters.

Turn on both meters and make sure they turn on, measure a voltage if you want too, just to make sure! These are cheap meters, so a quick check is a good idea.

Place the meters side by side, upside down.

On the left hand meter, on the left hand side, write the word OUT on it in 3 places as shown with the felt tipped pen. (see photo) [Note: Make sure the the word OUT on the side is a lower than the slot you are going to make later, WHAT slot you say? as you will see I wrote the word a little high, right where the slot is! Opps).

On the right hand meter, on the right hand side, write the word IN on it in 3 places as shown with the felt tipped pen. (see photo) [see note above!]

Remove the backs of the meters, by removing the 2 screws and gently prizing the backs off, from the bottom. Be warned there are 2 slots and 2 little tangs that fit into those slots at the top of the meters.

Remove the batteries, put aside.

Put aside removed backs (don't lose the screws!), batteries, instructions and test probes recycle the packaging.

Step 6: Glue the Two Meters Together

Place the 2 meters on top of the painters sticks, one top, one bottom as shown.

The OUT meter to the left, IN meter to the right Make sure that the 9 volt power connectors are out of the way. Slide both meters together, for a test fit.

Now we are at the point of no return! Once glued, well enough said! Also you have to be quick, get it done before the glue solidifies, so I cannot make a movie.

Place a nice bead of hot glue along the dotted line shown, from top to bottom. While the meters are on the painters sticks, slide them together quickly, making sure that they are level on both planes. Press the middle of the meters together if you see a gap. Keep them together until the glue sets (a couple of minutes is good).

I cannot show you a photograph of the glue bead as it would probably set before I could join them.

Step 7: Inspect and Test Fit

Turn the meters over and inspect the glue bead, if you have done it right, it should just be visible in the curved gap.

Now, hot glue again, this time at the 2 groves at the top and bottom, carefully fill them in with another nice bead. (see photo) You could run a bead down the in the slot on top of the meters if you want to, I have done that on other tries, but I cannot seem to get it right. See photos of the other wattmeters I have made at the end.

Remove any excess glue from the back mating surfaces.

Test fit the backs, just to be sure that still fit. Note the 2 slots at the top of the back fit into the little tangs sticking out of the actual meter body (see photo) They should click together nicely, if not check to see if the meters are warped, or if there is glue on the mating surfaces.

Remove the backs and put aside (don't lose the screws!).

Step 8: Cutting the Slots for the Big Wires

The next step is to cut 3 slots in the sides of the meters, 1 by in IN word, one by the OUT word, and one in the center.

The 2 on each side of the meters need to be no wider that the cable that you are using, the one in the middle needs to be larger. (See photo)

I have used a sharp craft knife or a hacksaw blade to do this, BUT you risk damaging the meter displays, so use a Dremel Tool if you can.

The depth of the slots needs to be deeper then the thickness of your wire, as there is a ridge on the backs. Its not too important, as the ridge can be cut off, but the wire MUST be below the level of the meter side, it not the back will not fit on.

Mark the width of the wire at the location of the slots to be cut using the Felt Tipped Pen, by placing the wire across the meters in the position of the slots, note it is on the straight part of the meter side using a sharp blade, score both sides of the wire, then using the pen to highlight the plastic between the scores. (see photo)

Opps, the words IN and OUT on the sides where in the wrong place, ah well!

Now cut the slots out with the Dremel, use Safety Glasses. Now remove the middle bits of plastic sticking up with your pliers, and clean up any edges that may be rough. (see photo)

Lay the wire in the slots and test fit the backs again. Making sure that the 2 slots at the top of the back fit into the little posts. Remove the backs and put aside (don't lose the screws!).

Step 9: Shaping (or Forming) the Big Wires

Cut and shape your wire to the following shape and dimensions, and using the 2 meters as a template. (see photo)

Note you only cut the Big Red wire, not the Big Black one.

When you have finished bending, test fit in the meters, keep bending and shaping intil if fits as shown. (see photo)

Points to note:

1/ Each end is about 0.25 inch longer so as to be able to make the connection, but not too much!

2/ The Big Red wire from the left hand side of the cable goes to the very top (10ADC) terminal of the left hand meter. (see photo)

3/ The Big Red wire from the right hand of the cable goes to the very bottom (COM) terminal of the left hand meter. (see photo)

4/ The Big Black wire is looped, the end of the loop reaching the bottom (COM) terminal of the right hand meter. (see photo)

After screaming and shouting at the stupid thing and doing several reforms of the cable, you will get it correct. This is my 5th wattmeter, so I am getting quite good at it!

Step 10: Stripping the Looped Big Black Wire

You need to strip the insulation off of the loop on the Big Black wire, without losing it's looped position.

A little over a 1/4 inch stripped off is fine, not too much. We do this now, to save a little hassle later, be careful not to knick or cut any of the stands, or fingers, squeeze the strands together neatly. (see photo)

I am not going to tell you how to do this, as I have my own way which normally involves the loss of blood!

BE CAREFUL!

Step 11: Glueing the Big Wires Into the Slots

After checking all of the measurements, and a couple more dry runs, it now time to glue the cable into the slots.

Lay the wires into the slots again to see how they fit, do it a couple of times. (see photo ) Remember hot glue always sets quicker when you are not quite sure what you are doing! When you are happy, we will begin.

1/ Place a nice blob of glue into the left hand slot, more inside the meter than outside. Do not get any glue on the white LCD display.

2/ The place the wire into the slot, as shown, press it down lightly so it lies in the bottom of the slot, and hold until the glue sets (at least a minute or so) (see photo)

3/ Place a very small blob of glue into the middle slot, at the bottom (towards you), do not fill the slot with glue yet. (see photo)

4/ Secure the looped Big Black wire as shown, hold until set (see photo)

5/ Place a nice blob of glue into the right hand slot, more inside the meter than outside. Do not get any glue on the white LCD display.

6/ The place the wire into the slot, as shown with the loop of Big Black wire correctly positioned, press it down lightly so it lies in the bottom of the slot, and hold until the glue sets (at least a minute or so)

7/ Place a small blob of glue into the middle slot, at the top (away from you), do not fill the slot with glue yet.

8/ Secure the remaining Big Red wire in the middle slow as shown, hold until set (see photo)

9/ Route the remaining Big Red wire on the left side, rebending if required.

10/ Place a blob of glue as shown on the left, and secure the the both Big Red wires, press it down lightly so it lies in the bottom of the meter, and hold until the glue sets (at least a minute or so) (see photo)

11/ Bend the remaining small thin piece of wire as shown (see photo)

12/ Place a small blob of glue into the wires on the left, place the thin piece of wire as shown (see photo) hold until the glue sets.

13/ Place a small blob of glue into the wires in the middle slot, place the thin piece of wire as shown (see photo) hold until the glue sets

14/ Check every thing, it should look like the photo. (see photo)

15/ Now fill up the slots with hot glue, not all the way to the top, remember the ridge on the backs. Not a big deal as you can alway cut the ridge off of the backs. (see photo)

16/ Test fit the backs again, trimming any hot glue that is in the way with a sharp knife.

17/ Remove any excess glue, especially where the ridges on the backs go.

18/ Test fit the backs again. Making sure that the 2 slots at the top of the back fit into the little posts. Remove the backs and put aside (don't lose the screws!).

Step 12: Preparing for Soldering, and Then Soldering the Wires

Now it is time to prepare the circuit boards for soldering.

1/ Using a sharp knife remove the green solder resist from the 10ADC terminal area on the left hand meter. (see photo)

2/ Using a sharp knife remove the green solder resist from the COM terminal area on the left hand meter. (see photo) .

3/ Using a sharp knife remove the green solder resist from the COM terminal area on the right hand meter. (see photo)

4/ Tin the areas the you have just stripped, 10ADC, COM (left) & COM (right) making sure that you are quick, so as not to melt the meter case (see photo)

Now for the soldering part:

5/ Dress the stripped Big Black looped wire and tin it. (see photo)

6/ Now solder the looped Big Black to the right hand side meter COM terminal. (see photo)

7/ Trim and strip the thin wire, in the right hand side meter, then tin it. (see photo)

8/ Now solder the thin wire to the right hand side meter VΩmA terminal. (see photo)

9/ Trim and strip the Big Red wire from the right hand side connection (the longer one), then tin it.

10/ Now solder the Big Red wire from the right hand side connection to the left hand side meter COM terminal. (see photo)

9/ Trim and strip the Big Red wire from the left hand side connection (the shorter one), then tin it. (see photo)

10/ Now solder the Big Red wire from the left hand side connection to the left hand side meter 10ADC terminal (see photo)

11/ Trim and strip the thin wire, in the left hand side meter, then tin it. (see photo)

12/ Now solder the thin wire to the left hand side meter COM terminal, being careful not to unsolder the Big Red wire. (see photo)

Step 13: Nearly Finished!

Double check all of the connections, glue bonds, etc.

1/ Attach the Power Poles on each end of the Big wires, if required. (see photo)

2/ Reinstall the batteries. (see photo)

3/ Install the backs, with screws (that you did not lose, did you?)

Step 14: Marking the Fronts of the Meters

1/ Turn the meters over so you can see the switches and displays. (see photo)

2/ Set the left hand meter rotary switch to 20 DCV (So that the arrow on the rotary switch points to the 20). (see photo)

3/ Mark the center of the rotary switch with a V (for Volts) with the Felt Tip Pen. (see photo)

4/ Using WhiteOut put a little white box around the 20 on the scale. (see photo)

5/ Then put a little WhiteOut on the arrow end of the rotary switch. (see photo)

6/ Set the right hand meter rotary switch to 10A (So that the arrow points to the 10A). (see photo)

7/ Mark the center of the rotary switch with a A (for Amps) with the Felt Tip Pen. (see photo)

8/ Using WhiteOut put a little white box around the 10A on the scale. (see photo)

9/ Then put a little WhiteOut on the arrow end of the rotary switch. (see photo)

Yes the left hand side could have been a little neater, I know!

Step 15: Testing the Wattmeter.

Now for testing

You will need a power supply of about 12 volts, about 1 amp and a 12 watt resistor of 12 Ohms

1/ With everything turned off

2/ Connect the power supply unit to the IN wires. It is advisable to get the polarity correct but not necessary
(if connected the wrong way around, the meter(s) will simply show a - (minus) sign).

3/ Turn the PSU on.

4/ Turn the left hand (Volt) meter on and read the voltage, it should read the PSU voltage!

5/ Turn off the PSU.

6/ Connect the 12 ohm resistor to the OUT wires.

7/ Turn the PSU on

8/ Turn the right hand (Amp) meter on and read the amperage BUT the current will be the actual Voltage measured divided by the actual value of the resistor, plus or minus the accuracy of the meters. With the values given the current should be about 1 amp (see explanation below).

The 12 Ohm resistor may only be 11.5 Ohms (say) and the voltage will drop to something like 11 volts (say), which would give you 0.96 Amps. You will have to calibrate each set of meters yourself, there will be some differences. As the say in the trade, your mileage may vary!!!

Also remember the following, if you use the resistor and the voltage recommended, the resistor WILL GET HOT, VERY HOT, be careful you don't burn yourself.

Step 16: Some Notes

If you have not had or used this type of meter before please read the instructions before using them, I have designed this system for use on very low voltages (up to say 20 volts) and a medium amount of current (up to say 10 amps). The the power supply must be fused or otherwise protected from short circuits. Remember 12 volts and 10 amps is 110 watts, you won't get shocked, but you could get burnt.

I used zip cord, but you could use individual wires, I find zip cord is neater.

IN and OUT are relative terms, the meter will work both ways, ie with IN and OUT reversed, and the polarity reversed, but they will show minus signs in their displays depending on the polarity and the direction of the current flowing.

The reason for this marking (IN & OUT) is that the voltage is measured on OUT line, removing the very small volt drop that occurs across the meter reading current.

It is possible to use the Voltmeter (the left hand side meter) as a regular V?OM one, seer the next section.

Step 17: Using the Test Lead Probes.

Yes, you can use the test leads and probes that came with the meter, but a few rules apply.

----------------------------------------------------
From the schematic:

PROBES

If used the following rules must be obeyed:

.............. There MUST BE NO connections to the INPUT and/or OUTPUT wires

THEN

...............connect both VOM+PROBE+ and VOM-PROBE- to the Volt meter (the left one) ONLY

The VOLTAGE meter can be used as a normal VΩOM meter, using the probes.

If there are ANY connections to either of the INPUT and OUTPUT wires

THEN

............... YOU MUST NOT, REPEAT NOT USE THE PROBES

You risk shorting the power supply lines. Big bangs, smoke, destruction, you get the picture!