mike48
11th December 2014, 12:46 PM
I had to roughly measure the current drawn by my 12 V vehicle winch, and made up a R.A.G. model current shunt as per photos.
It is made from a 304 stainless steel M8 x 100 ISO coarse set screw (aka bolt), and some brass bar, nuts and washers, all ex bits and scrap bins.
The washers are SS316, the nuts zinc plated steel.
Nuts torqued up very, very tight, and the "punched" SS washers are smooth edge side to the brass bar for best connections.
The critical "electrical dimension" is the distance along the bolt between the centrelines of the brass bars, and for my bolt was 48mm.
This was set so as to give a shunt transfer characteristic of 1.0mV per 1.0 amp.
This means that your millivoltmeter directly reads in amps.
So, 400 amps reads as 400mV.
The shunt resistance is 0.001 ohm, and at 500 amps dissipates 250 watts.
Use this shunt at high currents intermittently only; a few minutes on ten minutes off to stabilise the system.
Use at higher currents will be limited mostly by external factors like connecting lead resistance, source internal resistances, and even the resistance of the brass connector bars.
It will work, but special techniques and awareness of limitations are required.
Place your meter leads on the brass near to the nuts.
Meter lead length is mostly of no consequence.
Connect the shunt as shown, not to the end of the bolt using more nuts.
I calibrated my shunt at 10 Amps, a bit low, but I have an accurate millivoltmeter.
If you use an M8 SS304 bolt and brass "lugs" for yours, it will be accurate enough (10% or so), and good enough for the task without calibration.
Must use an M8 SS304 bolt, not SS316, not steel or brass or other.
Test 304 with a strong magnet as slightly magnetic; 316 is (almost) non-magnetic.
Be aware that DVMs have a measurement time lag to display varying currents correctly.
An analog meter will work also, and with less time lag.
Caution connecting up to the battery; this shunt can carry very high current.
Not my original idea; saw it on the web somewhere.
Works well.
cheerio, mike
333877333878
It is made from a 304 stainless steel M8 x 100 ISO coarse set screw (aka bolt), and some brass bar, nuts and washers, all ex bits and scrap bins.
The washers are SS316, the nuts zinc plated steel.
Nuts torqued up very, very tight, and the "punched" SS washers are smooth edge side to the brass bar for best connections.
The critical "electrical dimension" is the distance along the bolt between the centrelines of the brass bars, and for my bolt was 48mm.
This was set so as to give a shunt transfer characteristic of 1.0mV per 1.0 amp.
This means that your millivoltmeter directly reads in amps.
So, 400 amps reads as 400mV.
The shunt resistance is 0.001 ohm, and at 500 amps dissipates 250 watts.
Use this shunt at high currents intermittently only; a few minutes on ten minutes off to stabilise the system.
Use at higher currents will be limited mostly by external factors like connecting lead resistance, source internal resistances, and even the resistance of the brass connector bars.
It will work, but special techniques and awareness of limitations are required.
Place your meter leads on the brass near to the nuts.
Meter lead length is mostly of no consequence.
Connect the shunt as shown, not to the end of the bolt using more nuts.
I calibrated my shunt at 10 Amps, a bit low, but I have an accurate millivoltmeter.
If you use an M8 SS304 bolt and brass "lugs" for yours, it will be accurate enough (10% or so), and good enough for the task without calibration.
Must use an M8 SS304 bolt, not SS316, not steel or brass or other.
Test 304 with a strong magnet as slightly magnetic; 316 is (almost) non-magnetic.
Be aware that DVMs have a measurement time lag to display varying currents correctly.
An analog meter will work also, and with less time lag.
Caution connecting up to the battery; this shunt can carry very high current.
Not my original idea; saw it on the web somewhere.
Works well.
cheerio, mike
333877333878