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Free high current shunt.

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SirSparks

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I thought people using high current low voltage circuits might like to know this;

a "Big ANL" 300 amp fuse has the same resistance as a 75 mV ammeter shunt. I have wired a 0-200 amp (75mV) ammeter across my 300 amp fuse and it is reading perfectly. Of course should the fuse ever blow so will the ammeter but the fuse costs more than $8 and the ammeter only $2.50 from China (incl. shipping) so no big deal (buy a spare meter).
 
I imagine any fuse type could be used in this manner for both high and low currents once calibrated. Nice tip.
 
Uebergeek; I'm confucuious. θλψ
Does it cost less than $2 and will it cook my breakfast?
 
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sorry missed the free part... they are mostly lower currents...

if you are not worried about accuracy you can create a shunt from PCB FR4 stock. at those currents you might just use thin copper pipe
 
Good ,idea. Thanks for your input. Actually NEC tables (section 8) gives all resistances for copper wire, for smaller shunts that might be the way to go.
 
I doubt that the resistance across the fuse will remain constant with load. Current shunts are designed to have linear response and temperature stability.
 
This may be an older thread but another thing to consider is that the fuse may be rated to blow and isolate a certain voltage while the meter probably isn't.

The fuse could blow then the meter could melt but create an arc which could cause a fire. This is the reason some fuses are only rated to say 32 volts while others are rated to 100-10,000 volts ......
 
Well the fuse shunt has been working fine for 18 months now, I can tell the accuracy at FSD is at least 95% and that is all I need. I agree with the overcurrent issue but all meter wiring is exposed and very small so burning would be minimal but in all likelihood the meter coil would burn out first with no further damage. It's the same scenario if a regular shunt connection became loose.
 
It's not the burnout that I'd be worried about - it is the possibility of arcing - the meter may only have a few mm effective isolation once fully burned out so you could end up with some rather nice light shows. If you get the chance to, yank the fuse under controlled conditions and see what melts / fries / arcs / explodes (obviously wear protective clothing and goggles !)

Having worked on three phase installations I've seen some very pretty fireworks when things have gone wrong.
 
As long as its voltage rating is enough for whatever the main fuse is rated at then that should be fine.

Unless you're saying replace the meter with the fuse and see what happens when the main fuse blows ?
 
Hello,

A little later reply, but sometimes you can get away with connecting a small value resistor in series with the meter and it wont hurt the accuracy too much. It depends on the meter used.
For example, a 100 ohm resistor in series with the meter might only change the reading slightly, yet in the even of a blown main fuse, the current would be limited to about 120ma. But my guess is that the meter movement is of higher resistance than that anyway, but you'd have to check it.
Of course you have to test this too to make sure it still works ok.

If we knew more about the meter movement spec's we could figure this out more definitively.

Another idea is to use an op amp between the "shunt" and the meter as a voltage follower. That would protect the meter.
 
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Whenever I have needed a high current shunt I have just used a copper wire resistance chart and the corresponding size and length of wire to get the resistance I need.
 
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