Current Measuring - Shunt advice

Oh man - didn't even know that servo board existed - another item on my list.

I'm doing it the hard way via digital output pins .... I'm all in favour of this route for sure ... the Mega and Due have tons of pins spare but it does start to look messy really fast. Both my FET modules (7FET) have only a communication cable (SPI) and power for the loads (LED's).

I needed extra digital pins on the remote and data from the remote (like you will require for the current sensors), so I am planning on using bi-directional serial radios in the 900MHz band.

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.

I think you are misunderstanding the rating of the shunt.
Nobody uses a 100A shunt at 100A. That's the MAXIMUM rating.
The shunt is always chosen for a rating slightly higher than the operational value.
You spec's the operational value as 90A, hence the shunt should be 100A.
No one is suggesting that you would ever draw that much current but it doesn't hurt to plan ahead for worst case.
You can use a 100A shunt at 10A but the accuracy isn't as good as at 90A. Either way,
it is nevertheless the correct rating for your application.

It's good to know that stuff. I went for 100A simply because my peak operational limit was more than the smallest '50A' version I also try to make any sensor mid range = the normal operational value, so 100A seemed logical.

The remote / WiFi / 900MHz thing is the next on my list - as is the storage on SD of the operational data / telemetry.

BenKenobi:
It's good to know that stuff. I went for 100A simply because my peak operational limit was more than the smallest '50A' version I also try to make any sensor mid range = the normal operational value, so 100A seemed logical.

The remote / WiFi / 900MHz thing is the next on my list - as is the storage on SD of the operational data / telemetry.

If you plan on using radios other than an approved transmitter designed for r/c, insure the radio has FHSS (Frequency Hopping Spread Spectrum) capability. In the US, the FCC and FAA frown on single channel radios for r/c control.

Understood, and I agree with their attitude although I think the limits applied to things that fly are pointless in model boats. There's a darn good reason for them on things that fly.

However, that said, control itself I will still do via the Taranis / TX / X8R but stuff like the LED lights / dimming etc I want to remove from the TX's programming.

I also want to build in more control than the standard RX will allow.

I guess a lot of what I'm doing has come about as a direct result of playing with a Pixhawk which has opened up potential and possibilities.

I am not sure if the FCC cares if it flies or not.

The point is you don't want your boat sailing out of sight if the single frequency is used by another more powerful transmitter and jams your frequency.

edit: I always like to read about frequency hopping.

It was co-invented/created by the actress Hedy Lamarr. Good looks and a brain to match.

In the US, the FCC and FAA frown on single channel radios for r/c control.

I'm still using single channel 72 Mhz and I haven't heard anything that says I can't.
(Sunnyvale, CA, USA)

raschemmel:
I'm still using single channel 72 Mhz and I haven't heard anything that says I can't.
(Sunnyvale, CA, USA)

I didn't say you couldn't. Technically you should follow the "old rules" for r/c. Hang a frequency tag on the antenna of your transmitter so other r/c operators will not use your frequency.

We have a "channel board" where I fly but I'm (almost) the only one who has used it in the last 5 years ha ha...

Hence still using the Taranis - which I believe is one of the best available when it comes to 2.4GHz - just my opinion of course. Nobody in any of the places I go to posts anything regarding what they are using any more - most are using 2.4GHz these days - gone are the days when I take my box of ribbons and crystals with me.

Part of my boats programming kills the boat if it loses the SBUS connection - basically if no SBUS message is received for 10 seconds (I may reduce this time) outputs to all motor servo's are 'zeroed'. The X8R RX also has this ability to set servo's to a 'failsafe' on communication to TX loss and this is carried through the SBUS communication. I back it up by zeroing motor outputs in my code to cover for SBUS communications loss. I can also monitor the RSSI on the RX so if that gets too bad I can probably use that as another failsafe input. I can also set a geo fence based on the Neo7M position signal - again aka Arducopter / Pixhawk.

You can take this failsafe stuff to extremes, to be honest I'd never sail it that far away intentionally and most places that I go to you couldn't.

The Taranis appears to be a nice unit. It has a STM32 ARM Cortex M3 60MHz processor onboard and almost guaranteed it uses FHSS. That is basically the Due and my choice of transceiver. I guess I chose well for my system.

Well mine has ACCST which is perhaps the same thing with a different name but FrSKY do list an FHSS and an ACCST module as separate items. I've never yet lost a signal on it, since it carries telemetry such as the RSSI so this provides you some idea how good your signal is, so you can tailor your behaviour to suit.

As you say it is an ARM CPU so it's quick as far as TX's go, from power up to control is in a blink (unlike some TX's I've had). It is also open source so I can do pretty much anything that I can think of with it - unlike my old F12 Futaba that cost 4 times as much - I can also throw other vendor compatible modules into it such as Spektrum and Futaba - or I can add another FrSKY ACCST module to give me 32 channels - problem is you run out of switches etc, and then you can do so many things with the inbuilt logic / lua scripting via the OpenTX Companion software that I'll never go back to an 'off the shelf' closed shop unit.

Oh and I could return my AMPs readings to the telemetry screen of the Taranis - I have a Teensy board programmed with Mavlink for that very purpose (telemetry transmisson)....

100A Current Shunt

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.

This one is a little smaller.

Let Vfullscale = 0.075V (75mV)

FYI, by Ohm's Law, if the full scale voltage is 75mV @100A, then:

R = 0.075/100 =0.00075 ohms

P = I * V = 100A * 0.075V = 7.5W (less than 10W resistor)

Let Vfullscale = 0.100V (100mV)

P = I * V = 100A * 0.100V = 10W

I do appreciate that, really should take a class I suppose, the application of the math and then component selection feels like a dark art.

For instance there seems no consistency in how resistors are identified, as in one of the formula above a resistor of 0.00075 ohms I had a quick look for - they weren't exactly jumping off the page.

And how would you describe 0.00075 - milli / micro ohms ??

What you did was usefel for me though because it showed how to work out the W across the load - in this case the shunt in a way that made far more sense than some pages I've been reading.

First things first.
Ohms XXX
milliohms 0.XXX
microohms 0.000XXX

Hence, 75mohms = 0.075 ohms

Now, when you talk about current shunts, you are talking about a component specifically designed to have a very low resistance to avoid heating up when conducting large amounts of current.
The 100A 75mV full scale shunt for example : R = V/I = 0.075 ohms/100A (obviously you move the decimal point 2 places to the left , arriving at 0.00075 ohm , or 750 uohms.

What you did was usefel for me though because it showed how to work out the W across the load - in this case the shunt in a way that made far more sense than some pages I've been reading.

I've learned from experience on the forum (Date Registered: Nov 04, 2013, 09:49 pm) (you can see any members stats by clicking on their Avatar) that half of the people are not specifically mentioning things because they already understand them and the other half are not specifically mentioning things because they don't have a clue and don't want to look stupid by asking about it. I look at a persons stats when I read their replies and I ask myself "Does this person have a clue ?" If the answer leans toward "No", then I will work at the examples so they can learn how to do themselves. I mean, if you think about it, it is very convenient to be laying in bed , unable to sleep because you are wondering about something and realize that all you have to do is turn on your computer and read what someone on the other side of the world wrote yesterday about the topic .

I would stay with the hall effect unit if it was my project. You can use it easily on either the ground or supply side.
Just my opinion...

edit: Also less series resistance in your circuit. Your resistor is 750 microOhms. The hall effect unit is 100 microOhms.

I've used a Hall Effect Current Shunt before but it was a different brand. It was part of my OSD board for my FPV quadcopter. It displayed the motor current onscreen. I think they are more compact and probably more accurate and certainly better suited for RC since that's what they were designed for (unlike the shunts I linked which are Industrial Power Electronics Grade).

raschemmel:
I've used a Hall Effect Current Shunt before but it was a different brand. It was part of my OSD board for my FPV quadcopter. It displayed the motor current onscreen.

That is what I am using it for also, except not a quadcopter.