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Alanh · 05-10-2008, 01:59 AM

A couple of things come to mind using a current shunt.

First, if you have 50mV full scale into a 5V ADC for example, you need to look at the number of bits the ADC has to see if you've got the resolution you need. With a 50mV = 150A shunt, you've got 1/3mV per Amp so 1A = .0003333V, 2A = .0006666V, etc. So, used directly, most of the bits of your ADC reading will be zero. You might have to amplify the current shunt voltage to get more volts/amp.

I'm not a current shunt expert, but I've read that you don't really want to use one close to its rated capacity continuously because its basically a resistor and heats up. The number I recall is something like don't use it for more than 60% of the maximum rating on a continuous basis. I don't know enough about car audio amps to say the 50A shunt would have issues there.

Another consideration is where you put the shunt in the circuit. The ADC probably reads voltages referenced to its ground. If you put the shunt in the + wire, you have a voltage reading of 0-50mv except its 12V offset or so from ground. I don't think the Fusion Brain ADC will work with that. You could put the shunt in the ground lead, but now you've introduced a small voltage between the battery ground and whatever you're powering. Now that ground isn't really the same ground the rest of your system uses so you can get ground loops. If the negative power lead into the amps is grounded to the car chassis through any path (like your PC sound outputs), you've now got all kinds of issues including the fact that some of the amp current probably isn't going to flow through the shunt so the readings are off - maybe way off. There are circuits that allow you to use the chunt in the + lead, but it requires some electronic design and fabrication.

As already suggested some of the Hall-effect sensors that aren't directly connected to the wire carrying the current eliminate some of these problems. They have drawbacks too though: like cost, they need a power supply, and also a bit of circuitry to convert their output to a voltage. I've also heard that some inexpensive brands have some type of memory effect that affects their accuracy if you use them near their maximum ratings too.

05-10-2008, 01:59 AM	#5
Alanh Constant Bitrate Join Date: Jul 2002 Posts: 121	A couple of things come to mind using a current shunt. First, if you have 50mV full scale into a 5V ADC for example, you need to look at the number of bits the ADC has to see if you've got the resolution you need. With a 50mV = 150A shunt, you've got 1/3mV per Amp so 1A = .0003333V, 2A = .0006666V, etc. So, used directly, most of the bits of your ADC reading will be zero. You might have to amplify the current shunt voltage to get more volts/amp. I'm not a current shunt expert, but I've read that you don't really want to use one close to its rated capacity continuously because its basically a resistor and heats up. The number I recall is something like don't use it for more than 60% of the maximum rating on a continuous basis. I don't know enough about car audio amps to say the 50A shunt would have issues there. Another consideration is where you put the shunt in the circuit. The ADC probably reads voltages referenced to its ground. If you put the shunt in the + wire, you have a voltage reading of 0-50mv except its 12V offset or so from ground. I don't think the Fusion Brain ADC will work with that. You could put the shunt in the ground lead, but now you've introduced a small voltage between the battery ground and whatever you're powering. Now that ground isn't really the same ground the rest of your system uses so you can get ground loops. If the negative power lead into the amps is grounded to the car chassis through any path (like your PC sound outputs), you've now got all kinds of issues including the fact that some of the amp current probably isn't going to flow through the shunt so the readings are off - maybe way off. There are circuits that allow you to use the chunt in the + lead, but it requires some electronic design and fabrication. As already suggested some of the Hall-effect sensors that aren't directly connected to the wire carrying the current eliminate some of these problems. They have drawbacks too though: like cost, they need a power supply, and also a bit of circuitry to convert their output to a voltage. I've also heard that some inexpensive brands have some type of memory effect that affects their accuracy if you use them near their maximum ratings too.