High current on PCB? - Page 1
Forum section: electronics / beginners

author: michael.hill
author_rank: Unknown
timestamp: September 22, 2013, 12:55:17 am
content: I'm wanting to design a current monitoring board, but I'm having a bit of a problem figuring out just how to manage high current on the PCB. This will be used to measure the current through a set of motors (up to 6) in a robotics application. Typical running current is on the order of 40-60A, but at stall, the current gets up to 133A. Is there a way to safely handle these currents on a PCB? I'm sure there has to be since the motor controllers seem to handle it just fine. What amperage should I design for? 150A worst case? I've seen design rules for trace widths, but they're only valid to 30A. I've also seen something about a PCB bus bar, but I'm not entirely sure how to use them.Thanks

author: george graves
author_rank: Super Contributor
timestamp: September 22, 2013, 01:15:43 am
content: Depends a lot on what kind of shunt you're using(or current sensing device). Sounds like at the currents you're working with you might want the shunt to be off the board.

author: michael.hill
author_rank: Unknown
timestamp: September 22, 2013, 01:42:06 am
content: Thanks for the quick reply!So possibly use something like a Chassis Mount shunt? I'm not entirely sure how current sense resistors are rated. I think I want to aim for 150 Amps to be safe, so am I on the right track by playing the game of P = I^2 * R and finding which resistors are rated for that power? Or are resistor powers over rated a significant mount?

author: c4757p
author_rank: Super Contributor
timestamp: September 22, 2013, 01:47:28 am
content: What about a Hall effect sensor like the ACS758? Comes in up to a 200A version and you don't have to worry about a shunt resistor.

author: michael.hill
author_rank: Unknown
timestamp: September 22, 2013, 02:01:30 am
content: Quote from: c4757p on September 22, 2013, 01:47:28 amWow, that's a great idea. Didn't even think about it. That does solve quite a few problems (and is a heck of a lot cheaper too!)

author: michael.hill
author_rank: Unknown
timestamp: September 22, 2013, 02:41:56 am
content: Hmm, on second thought, it looks like I still need to get the wires to connect some how. Looks like these guys sell something similar to what I want, but I want like...6 of them on 1 board.http://www.ebay.com/itm/50A-100A-150A-200A-Bi-Uni-AC-DC-Current-Sensor-Module-arduino-compatible-/111040360152It looks like they're using 4 oz copper with gold plating.

author: PedroDaGr8
author_rank: Super Contributor
timestamp: September 22, 2013, 07:31:04 am
content: I have some 0.2 mOhm 1% 10W resistors. Those would allow measurements up to ~150-200A based on my calculations.Sent from my EVO 4G LTE using Tapatalk 4

author: AndyC_772
author_rank: Super Contributor
timestamp: September 22, 2013, 08:22:02 am
content: Is this an ac or dc application? If it's ac, you could just pass the conductor through a current sense transformer, and then you don't need a series resistor at all.

author: Psi
author_rank: Super Contributor
timestamp: September 22, 2013, 08:52:39 am
content: keep in mind HALL current sensors aren't quite as accurate as other methods of current monitoring.Which may not be an issue depending on your application.

author: BravoV
author_rank: Super Contributor
timestamp: September 22, 2013, 09:36:43 am
content: Using copper trace as resistor, just curious when the temperature coefficient of the copper material will become a significant factor in affecting the resistance accuracy ?

author: Psi
author_rank: Super Contributor
timestamp: September 22, 2013, 09:41:42 am
content: Depends on how thin and how long you make the track since that will govern how hot it gets.If you make the track thick and short the heat buildup will be smaller but you need more amplification on the voltage across it.

author: AndyC_772
author_rank: Super Contributor
timestamp: September 22, 2013, 10:01:20 am
content: This is quite an interesting investigation to carry out if you have a 6.5 digit multimeter. Pass a constant current through a copper wire or PCB trace and see what happens to the voltage over time as the copper warms up - I've been surprised at just how much it varies.

author: michael.hill
author_rank: Unknown
timestamp: September 22, 2013, 11:13:29 am
content: Just had another thought. I could use about a foot long piece of copper wire, measure it's resistance (4-wire method), and put that in series with my motor on the high side after the motor controller. I would effectively be using just the wire as my sense resistor, then I could bring in the voltage taps at the ends of the wire to measure my high side voltage drop across the wire using standard current sensing ICs.What do I have to do to limit any induced current if I run the voltage sensing wires parallel to my high current motor wires?Thoughts?

author: SeanB
author_rank: Super Contributor
timestamp: September 22, 2013, 11:18:33 am
content: Nothing, the induced voltage will be very small. If you want use a RC network on the input to reduce RF noise induced into the amplifier if you want, but generally the induced noise will be attenuated by the low resistance of the shunt. Just run the sense wire very close to the shunt to reduce the loop area for noise pick up, and use a twisted pair for the sense wires back to the control.

author: Bertho
author_rank: Regular Contributor
timestamp: September 22, 2013, 11:21:26 am
content: At 150..200A you can easily use your mounting wires as sense-resistor.A 10mm^2 wire (about gauge 7) has ~1.72mOhm/m, or 0.258V/m at 150A.A 25mm^2 wire (about gauge 3) has ~688uOhm or 0.138V/m at 200A.That should be plenty to sense the current.What your should worry about are the inter-connections of the wires to devices. A screw-terminal has easily 1..5mOhm resistance. At 150A and 1mOhm resistance, you are dissipating 2.25W just in that connection.

author: michael.hill
author_rank: Unknown
timestamp: September 22, 2013, 11:22:38 am
content: Would coiling around the high current wire be of any benefit? It's been a while since my last E&M physics class, but IIRC, the most current was induced when the wires were parallel. If I coil, will that reduce it or will the increased length of wire nullify the fact it's no longer parallel? I'm leaning toward the latter now that I think about this.

author: Simon
author_rank: Unknown
timestamp: September 22, 2013, 03:37:19 pm
content: the thing to consider with a resistor is that it will be on the positive or negative rail so if you are say using an op amp you need to make sure it is a rail to rail one, the hall effect sensor is the best idea. Alegro make them and they are not too expensive.