I ordered some 22 AWG nichrome wire from goldmine-elec.com today. It’s about an ohm per foot, or 3 and a fraction ohms per meter. My plan is to make my own low value resistors for current monitoring.
I often want to monitor the current in a higher power LED or input current to a Joule Thief. This requires a series resistance of a fraction of an ohm, typically 1/10 ohm to make it easy to calculate the current.
The 1/10 ohm resistors are hard to find. I have plenty of 1 ohm resistors which are very handy because 1 volt across it equals 1 amp. But the loss is excessive – 1 watt – and the drop is excessive – 1 volt. By reducing the resistance to 1/10 ohm, the losses at 1 amp are 1/10 volt, and 1/10 watt. At less than 1 amp, the losses are much less and tolerable because they don’t disturb the circuit much (see note at end).
The 1/10 ohm resistor I will make will consist of about 1/10 of a foot (plus a short amount for the leads) of the nichrome wire. That’s a bit over an inch, about 1-3/8 inches or 35 mm.
It’s difficult to solder to nichrome, so I will insert the ends into a terminal strip and clamp the leads with the screws. I’ll use 2 terminals of a Euro terminal block. I saw off two of the terminals and file off any plastic rough edges. One side will clamp the ends of the nichrome wire, the other side, the leads of the current wire and leads to the meter. I can adjust the resistance a bit by sliding the wire in and out of the terminal.
The meter will be a cheap DMM. When I say cheap, I mean free. Harbor Freight had a coupon for a free DMM with any purchase. This meter reads 200 DC millivolts on the lowest scale, so 10 millivolts will equal 100 milliamps current. The resolution will be good enough for making comparative measurements when prototyping a circuit. And the important thing is that the voltage drop will be less than .1 volt for less than 1 amp. Another advantage is that a length of 30 AWG copper wire 0.1 ohm is about a foot long; the same nichrome wire is ten times shorter, and takes up ten times less space.
I’m patiently waiting until the package arrives early next week…..
Update Nov 1 – I received the package and I made up one of the 1/10 ohm resistors as I described. I measured it with my very accurate HP (now Agilent) multimeter and it was 0.1082 ohms. I just have to cut the nichrome wire a bit shorter and it should be exactly 0.1 ohm.
Note: I could put 2 or more 1 ohm resistors in parallel. Two 1 ohm resistors in parallel would be equal to 1/2 ohm, and 0.2 amp would give a reading of 0.1 volt on the DMM. So if the DMM reads 100 millivolts, the actual current would be 200 milliamps. More resistors in parallel would give less voltage drop but a lot of resistors gets bulky and expensive.
