Handlobraesing
Banned
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- Feb 14, 2006
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Requirement:
Digital volt meter that can accurately measure down to 0.1mV is a MUST, 0.01mV would be nice. The meter does not need to be able to measure amps directly.
It would be nice if you have two DMMs, so you can measure the voltage and current at the same time. If you do, always use the better one for shunt measurement.
Why one would want to apply this method:
A. You own a meter that does not measure amps
B. Often times the high current range doesn't offer enough resolution and low current range has as high as 0.5 to 1 ohm resistance
Commercially available copper wires have a very well established resistance profile and this design will exploit it, so you can be reasonably accurate by this method.
The ammeter on DMM works fine for higher voltage stuff, because the voltage drop introduced by the internal shunt is insigificant to the total circuit voltage, but with the lower voltages used with flashlights, it often causes the light to run dimmer while the ammeter is in place. You can prove this by connecting an ammeter and placing a short across the meter and watching the light will get brighter.
0.01 ohm shunt: This one is suitable for higher current flash lights. It will produce a read out of 10.0mV per one ampere.
You will obtain about 2.5 m (8.5 ft) of 12AWG wire(DO NOT subsitute size). Solder two small wires across precisely 188cm of the wire. These wires connect to your volt meter. The ends of the 12AWG wire connects between the battery and flashlight. Try to keep the length of wires beyond the 188cm wire minimum.
Once you accurately measure the 188cm section, you can coil it around to minimize space.
0.1 ohm shunt: this one gives you 100mV per one ampere and is more suitable for flash lights using less than one ampere and gives better resolution but introduces more error in higher current flashlight.
Use a 294cm section of 20AWG wire and construct the same way. If you can't find 20AWG, use 467cm of 18AWG, but it will be bulkier.
Usage examples:
0.01 ohm shunt:
6V 55W halogen light. 91.60mV means 9.16A.
0.1 ohm shunt: (Use 0.01 ohm unless it won't give you at least 2 significant digits). Do not use it if it leads to more than 5% drop in voltage (no more than 0.18A in a 3.6v circuit)
4.8V 7.5W xenon light 156.0mV means 1.56A. If you can, try to use 0.01 ohm
2V 0.05A (i.e. 2AA 5mm LED light) = 5mV. This is just fine.
Avoid using cheap 5% tolerance cement resistors. The wire method is much more accurate. You could choose to use a precision 0.1 or 0.01 ohm resistor, but be prepared to pay a lot more.
If you're skilled with electronics, you could mate an op-amp with 10x and 100x voltage gain with 0.01 ohm inside a shielded case for better resolution.
There are two things to remember. The lower the source voltage and higher the load, greater the effect of resistance of measurement instrument.
Anyways, I'm sure most of you know this trick already, but I hope you guys find this useful.
Digital volt meter that can accurately measure down to 0.1mV is a MUST, 0.01mV would be nice. The meter does not need to be able to measure amps directly.
It would be nice if you have two DMMs, so you can measure the voltage and current at the same time. If you do, always use the better one for shunt measurement.
Why one would want to apply this method:
A. You own a meter that does not measure amps
B. Often times the high current range doesn't offer enough resolution and low current range has as high as 0.5 to 1 ohm resistance
Commercially available copper wires have a very well established resistance profile and this design will exploit it, so you can be reasonably accurate by this method.
The ammeter on DMM works fine for higher voltage stuff, because the voltage drop introduced by the internal shunt is insigificant to the total circuit voltage, but with the lower voltages used with flashlights, it often causes the light to run dimmer while the ammeter is in place. You can prove this by connecting an ammeter and placing a short across the meter and watching the light will get brighter.
0.01 ohm shunt: This one is suitable for higher current flash lights. It will produce a read out of 10.0mV per one ampere.
You will obtain about 2.5 m (8.5 ft) of 12AWG wire(DO NOT subsitute size). Solder two small wires across precisely 188cm of the wire. These wires connect to your volt meter. The ends of the 12AWG wire connects between the battery and flashlight. Try to keep the length of wires beyond the 188cm wire minimum.
Once you accurately measure the 188cm section, you can coil it around to minimize space.
0.1 ohm shunt: this one gives you 100mV per one ampere and is more suitable for flash lights using less than one ampere and gives better resolution but introduces more error in higher current flashlight.
Use a 294cm section of 20AWG wire and construct the same way. If you can't find 20AWG, use 467cm of 18AWG, but it will be bulkier.
Usage examples:
0.01 ohm shunt:
6V 55W halogen light. 91.60mV means 9.16A.
0.1 ohm shunt: (Use 0.01 ohm unless it won't give you at least 2 significant digits). Do not use it if it leads to more than 5% drop in voltage (no more than 0.18A in a 3.6v circuit)
4.8V 7.5W xenon light 156.0mV means 1.56A. If you can, try to use 0.01 ohm
2V 0.05A (i.e. 2AA 5mm LED light) = 5mV. This is just fine.
Avoid using cheap 5% tolerance cement resistors. The wire method is much more accurate. You could choose to use a precision 0.1 or 0.01 ohm resistor, but be prepared to pay a lot more.
If you're skilled with electronics, you could mate an op-amp with 10x and 100x voltage gain with 0.01 ohm inside a shielded case for better resolution.
There are two things to remember. The lower the source voltage and higher the load, greater the effect of resistance of measurement instrument.
Anyways, I'm sure most of you know this trick already, but I hope you guys find this useful.
