# Current Measuring - Shunt advice

I need to measure the current being drawn by an ESC - a motor controller that is operating on 12 VDC from a LiPo battery using a demand value from a remote transmitter (have this bit working). The current could reach 90A even under normal operation but more than 120A will destroy the ESC and probably the motor so I want to build in a soft trip to guard against motor stalls (motor is driving a water jet that could get debris in it).

I've done a little reading re using shunts etc and measuring volt drop across them using an op amp such as the AD623 but I can't decide which components I need, I'd be happy if I could get a resolution of tenths of an amp.

Any recommendations re components ?

Hi,

What is the model/type of the ESC, and what size battery.

12V x 90A = 1080W or about 1kW. Most ESC, especially one that are designed for that level of current, should have current control facilities, ie a soft start and overload protection.

Tom.... :)

Yup that's what I figured wattage wise on the motor, I don't think that the ESC has any overload protection on the motor size, there is some degree of control on the start speed but this is limited. The ESC itself is a 90A hobbyking item see here >>

90A Marine ESC

Battery is a 3S so 11.7 Vnominal but > 12V fully charged, 8000mah capacity.

I'm driving the ESC via a pulsed binary based on this although it is tweaked a little due to the ESC not being a servo but I'm also driving 6 servos >>

MegaServo

My concern is the shunt design - maybe my maths is flawed since this is a new topic for me but I used P=I2R meaning an optimum shunt of 0.01 ohm (to use the AD623 with decent resolution) would need to handle circa 200W based on a current of 140A (bit of safety margin) - I could be barking up completely the wrong tree though since I have yet to find such a resistor.

Current Measuring

Hi, How long do you expect the battery to run the motor and the other equipment, how are you going to keep it charged

The 8000mAh or 8Ah battery I looked at had a discharge rate of 30C. 30 x 8 = 240A, so it will be okay with 90A.

But 90A for how long.

8/90= 0.088Hours or 5.3Minutes, if you assume 100% efficiency.

Not sure how long you want your project to run. Also at those current levels, you will need decent gauge wire and neat efficient wiring and connections.

Tom.... :)

Please go and read the discussion on the ESC page you link to, some reliability probs by the looks.

I actually don't anticipate long periods anywhere near the full load of 90A, the motors themselves are only rated to 66A anyhow, this is basically flat out. Re the ESC issues I agree this is pretty new territory, got to learn some place, somebody needs to be first I guess. The ESC's are easily changed out anyhow.

To make the picture a little clearer this 'plan' is going into a 1.6 metre long model of a 'super yacht' (Moonraker) that has three brushless motors (XK3674-1650KV), these are each driving a water jet (KMB Jet28). So three batteries, three motors, three jets - theoretically that's 3kW on full load but the ESC's and motors are water cooled. I'm estimating a run time in excess of 30 minutes.

The jets however have pretty tight tolerances so they aren't going to swallow cigarette ends / feathers etc happily, I want to not only protect the jets but also the motors / ESC's although these are easily replaced - the jets not so easy as they are in integral part of the hull.

This is a little off topic I guess but I'm using an ATMega2560 in a similar manner to the 'Pixhawk' used for drones although I'm not planning any autonomous operation.

The ATMega2560 will operate all motors, servos and also pitch controls (ultimately via a 9 axis gyro) all via an FrSKY X8R coded for 16 channels communicating using SBUS and a Taranis X9D transmitter, I also have a Ublox Neo7M GPS and compass installed. I have an additional 7 channels needed for LED / lighting control but I plan to build a touch screen WiFi controller to handle this. The model control is pretty complex but that's a whole other thread.

Thanks - I’ll put those on a list - pity they aren’t in the UK but I reckon 100A should be more than enough - if it gets that high I would be killing the load anyhow.

Wonder how hot this would get in service ? - looks a lot bigger than I’d expected - so locating three of them in a suitable place could be a challenge.

Hall effect units are normally better. http://www.ebay.com/itm/50A-100A-150A-200A-Bi-Uni-AC-DC-Current-Sensor-Module-arduino-compatible-/111689533182

Didn't think a Hall effect would work on DC circuits - something else to dig into.

BenKenobi: Didn't think a Hall effect would work on DC circuits - something else to dig into.

Sure it will. Get a 100A uni-directional unit.

Be a lot easier to locate three of those than three 4" long shunts ...

Why 3? Do you have 3 ESCs?

I do indeed - 3 motors, three batteries …

I’ve asked the seller if they will ship to the UK - since these things don’t come up on E-Bay UK search but they seem ideal - and are a lot less messing than the AD623 route I had in mind - and they will work with a Due so double plus

The web page says that EBay store ships wordwide.

Item location: Changhua, Taiwan Shipping to: Worldwide Excludes: Hong Kong, Taiwan, China

Yup - but he only lists 2 as the available quantity for anything other than 200A - which is more than a little over range for my needs. 100A would be ideal since the motors rated max current is 66 amps - mid range almost for a 100A - I could trigger a 'kill' to that ESC channel on anything over that.