DC Watt Meter Special Modifications and Use
This page gives technical instructions for making modifications to the "Watt's Up" and Doc Watson products. They are intended for an audience with sufficient technical expertise to understand and use them as-is. If you don't understand them, we're sorry, but they're not for you.
The modifications on this page are UNSUPPORTED. They can damage your meter if done incorrectly and we accept no responsibility whatsoever for their use or any consequences of their use. So you proceed at your own risk and accept our terms in doing so.
There may be other ways to approach the addressed topics. The examples given are simple, low cost and work.
Opening the Watt meter case
The case can be opened by cracking its ultrasonically welded seams in a shop vise. Place the meter in the vise with the display facing up and the long sides of the case against the vise jaws. Position one parting line just above the top of the jaw, and the other just below it. Moving quickly (to prevent the stress relaxing), squeeze if gently several mm until the resistance weld "cracks." Then pry the case open noting how things went together inside. Be careful not to squeeze too far or you may damage the PC boards inside. If the weld doesn't pop well enough to pry the case open, reverse the parting line/vise jaw alignment and squeeze again. The black ABS cases usually come apart pretty cleanly. The colored K-resin cases can be more difficult, but usually, it's not much of a problem.
With care you may be able to salvage the case and reglue it together.
Increasing tolerance to transient over voltage conditions
Ensure the SOURCE side of meter always has a low impedance connection to a voltage source like a battery, power supply, etc.
Add an appropriate bilateral transient voltage suppressor (TVS) like a "Transorb" on the LOAD side of the meter between the leads. Make sure the TVS turn on voltage is above that of your operating circuit, but it's clamp voltage is below that of the meter's max rating. A higher Joule/power/energy rating is better, other things being equal.
As an example, the Diodes Inc. partnumber: 1.5KE43CA (Digi-Key carries) is a bidirectional ~ 43 Volt turn on TVS with axial leads that clamps to less than 60V. It would not be appropriate in circuits having operating voltages near or above 43 Volts.
With the case open, carefully remove diode D1. Once done, this will require powering the meter through the Auxiliary Connector, but will also make the meter more robust against overvoltage transients and remove the requirement that, to be the sole power source, the Auxiliary Connector voltage must exceed the measured SOURCE voltage.
Increasing meter's voltage range without opening the meter
To handle higher than the rated voltage you need to both reduce the voltage measured by the meter and the voltage provided to power the meter.
An example solution to allow use in a 72 V environment follows.
Use an external divide by two voltage divider between the 72 Volts and the negative connection for the measurement voltage. Use two 2000 Ohm, 3 Watt (or more is better) , 1% tolerance resistors (e.g. Digi-Key pn ALSR3F-2.0K-ND). Connect the red wire of the meter to the center of this divider. The divider introduces some error depending on resistor tolerances. Probably less than 1%.
Multiply voltage and voltage dependant (e.g. Watts and WH) readings by two.
For the power voltage to the meter use another divider with the same 2000 ohm resistor in the bottom leg to the negative and a 1000 ohm 2 W in the top leg to the 72 V. Output is ~48 volts. It must ALWAYS be a volt or so higher than the output of the measurement divider so the meter is powerd by this and not the measurement leg. These can be lower cost 5% resistors. This voltage goes to pin 2 (center pin) of the auxiliary connector. The resistors will get pretty warm so you'll want to mount them so they have good air flow. The website specs page describes the connector. It's also available from our buy products page.
Increasing meter's voltage range after opening the meter
If you open the meter, you can make some component changes to alter the meter's range.
Do both these mods to increase the "Watt's Up" or Doc Wattson meter's voltage range as noted. Done incorrectly, these mods can easily destroy the meter.
Carefully remove diode D1. Once done, this will require powering the meter through the Auxiliary Connector, but will also make the meter more robust against overvoltage transients and remove the requirement that, to be the sole power source, the Auxiliary connector voltage must exceed the measured SOURCE voltage.
Parallel an 0805 SMT, 0.5%, 19.1K Ohm resistor with the existing 10K SMT resistor at R4. This will increase the input voltage range to ~ 90.2 Volts. You will need to multiply all displayed voltage related values by 1.503 to get the actual voltage. e.g. 47.90 Volts indicated is really 72 Volts after this mod. Be careful! These voltages are dangerous!
Using with an external current shunt resistor instead of the internal shunt
The shunt inside both the Watt's Up and Doc Wattson is a 100 Amp peak, 100mV type. i.e it has 0.001 ohm resistance. It's in the negative i.e. black lead.
Using an external current shunt resistor allows increasing the maximum continuous current of the meter. It also allows using small diameter wires to the meter as only the shunt needs the heavy current carrying wires.
If you have the necessary skills you can modify the meter for use with an external current shunt resistor like we sell on our site (of course, doing this will void the warranty). The CSA100-100 or CSB500-100 would be suitable. With the CSB500-100 (500 Amp) you would need to multiply all current related dsiplay measurements by five and you would use the full dynamic range of the meter. With the CSA100-100 (100 Amp) no corrections to display values are needed since this is the same value shunt (though with higher power dissipation) as is already in the meter.
To modify the meter you need to crack the case open and remove the internal shunt. See instructions for opening the case elsewhere in the FAQ.
Done incorrectly, these mods can easily destroy the meter.
Remove the internal shunt. This is most easily done using two high thermal mass soldering irons - one at each end of shunt. After removing the internal shunt, you will see pads underneath it for connection to the kelvin sense terminals of the external shunt. Use a short length ( a few feet) of 22 Ga. twisted pair wire to make the two Kelvin connections to the shunt. The positive side (nearest the load) of the shunt must be connected to the meter's load side Kelvin contact. Run the measured current through heavy wiring to the shunt and connect the meter's load side black wire to the same place the shunt's negative side goes. You could instead use both of the meter's black wires for connection to the Kelvin terminals, but accuracy will suffer a bit, as IR drop from meter 's power return current (typically 7 mA) will offset the measurement slightly across the wire's IR drop from the shunt.
Note that in still air, external shunts are rated for use at 66% of their full scale value. So you must always ensure that you're continuous current is not exceeding 0.66 x Shunt max Current through the current shunt resistor. Or 0.66 x 500 = 330 Amps.
Decreasing the current range to get more resolution at lower currents
If you open the meter, you can replace the 100 Amp, 100 mV, 0.001 Ohm shunt with a higher resistance valued current shunt. As long as the power dissipation is kept the same (2.5 W) or less, this should work.
Of course, you will have to divide down the displayed values that are current related.
Divide Value = Shunt Resistor You Installed / 0.001 ohms.
Example to get a 1 Amp max range meter:
Replace the existing shunt with a precision 0.1 Ohm resistor to decrease the current range to 1 Amps max. Since the power dissipation has dropped, the maximum continuous current would now be 100 / (0.1/.001) the full 1 Amps. The current resolution will drop to 0.1 mA. although the display can only show 0.xx. Current related display measurements would be divided by 100. So 15.5 A on the display means 0.155 Amps are being measured. 8.4 Ah actually means 84 mAh. Accuracy at lowest currents may suffer, but repeatability should be very good.
Paralleling an external shunt with the internal shunt
This doesn't require opening the meter case, but is a bit more complex to interpret and you still run large current through the meter.
The shunt inside both the Watt's Up and Doc Wattson is 0.001 ohm. It's in the negative black lead. At a peak reading of 100 A, 100 mV is developed across the shunt. For continuous use you will want to stay under the 50 A maximum continuous reading (and also use the 3 wire connection method). This means there will be 50 mV across the internal shunt. With a 100 mV external shunt, only half that shunt's range will be used, but that's not so bad because only 66% is available for continuous duty (see below).
With an external shunt in parallel with the negative (shunt) lead of the meter, we need to calculate how much current is going through each of the shunts and figure out what your correction factor for the meter's displayed values would be for the combination.
The correction factor = 1 + (0.001 ohm / external shunt ohms).
For example, using the CSA100-100, 100 A, 100 mV shunt available on our site:
shunt ohms = 100 mV / 100 A = 0.001 ohm
Current readings correction factor = 1 + (0.001 ohm / 0.001 ohm) = 2 (because each shunt equally shares the current).
So you would need to multiply all of the current based readings (amps, watts, watt hours, and amp hours) by 2 to get the correct reading. This setup would handle 50 + 50 = 100 A continuous current and 200 A for several seconds or more.
Note that in still air, external shunts are rated for use at 66% of their full scale value. So you must always ensure that you're continuous current is not exceeding 0.66 x Shunt Max Current through the current shunt resistor. Or 0.66 x 500 = 330 Amps.
Using a CSB500-100, 500 A, 100 mV shunt:
shunt ohms = 100 mV / 500 A = 0.0002 ohm
Current readings correction factor = 1 + (0.001 ohm / 0.0002 ohm) = 6
So multiply all of the current based readings by 6 to get the correct reading.
500 x (50mv/100mV) = 250 Amps which is less than the continuous external shunt rating of 500 x 0.66 = 330 Amps. So this setup would handle 250 + 50 = 300 A continuous current and 600 A for several seconds or more.
Using a 500 A, 50 mV shunt:
shunt ohms = 50 mV / 500 A = 0.0001 ohm
Current readings correction factor = 1 + (0.001 ohm / 0.0001 ohm) = 11
So multiply all of the current based readings by 11 to get the correct reading.
500 x (50mV/50mV) = 500 Amps which is greater than the continuous external shunt rating of 500 x 0.66 = 330 Amps. So this setup would be limited to handle 330 + 50 = 380 A continuous current and 600 A for several seconds or more. Because the max shunt voltage is 330/500 x 50mv = 33mV, this configuration uses less of the 50 mV internal shunt's continuous range and is a lower resolution choice due to the inevitable noise.