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Sorry kind of wrong forum for robotics but i need some quick help and i love the community here :)

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LAN_403

Legit_bacon

Tue Jul 22 2014, 08:43AM

Registered Member #4034 Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138

I used to be into High voltage/ power electronics a while back and posted here a bit and now im into robotics,
anyway to my problem, Ive designed and made up some PCB's for 5 arduinos networked over I2C
It takes in 12V from a liPo and has 2 Vregs, one 8V on for a external motor and a 5V one for all the arduinos.
The SN754410 dual H-bridge chip i use for driving motors as far as i know is hooked up correctly however when i plug it in the 5V line goes up to 6V?!
Only happened after i installed the 8V Vreg and i have till Saturday (nz time so friday for you guys most likely) till i have a competition i need it to work for. 6V is right on the limit of damaging the arduinos which if they blew i cant replace in time at all so it needs to be fixed....
I've attached the schematics, board trace file (in a picture) and a picture of the finished board itself.

Thanks for the help and time in advance :)
//EDIT: notice the Vreg isnt installed in the picture, as it is a older picture from when it was in construction, however it is there now and is just like the other one

Carbon_Rod

Tue Jul 22 2014, 09:58AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

The SN754410 has two VCC1 (Pin 8 for motor drive outputs +12v) and VCC2 (Pin 16 logic level +5v). In the schematic/PCB these pins are shorted, and may be leaking around +6 into the logic side.

Pull the chip to check if the problem subsides, and replace with a new one after correcting the problem.
We tend to cut the legs off damaged chips to prevent problems re-cycling into our parts stock.

Cheers,
Rod

Legit_bacon

Tue Jul 22 2014, 10:34AM

Registered Member #4034 Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138

Problem solved! pays to take a good look at the datasheet... Id allways used these on 5V motors and logic and thus having them shorted never gave me any problems, only have the 1 chip so its probably dead ill need to test it and hopefully i can get one ordered within 2-3days...
Thanks for the help, you're a life saver :)

Sigurthr

Tue Jul 22 2014, 11:34AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Do those mini arduinos not have an onboard Vreg? Full size arduinos can handle 9v with no problem (I forget the maximum input voltage).

Glad you sorted it out though.

Legit_bacon

Tue Jul 22 2014, 09:45PM

Registered Member #4034 Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138

They do have Vregs yes, to 12V i believe. But there are alot of external 5V devices i needed to power from the same board and their onboard Vregs dont have anywhere near the capacity needed to drive them so it just seemed cleaner to keep all the arduinos and sensors on the 1 5V line. Also I2C benefits from all devices being as similar as possible in voltages etc, probably wont have much effect as these arduinos use a crystal oscillator but i figured id do it anyway. Ie i use the straight post-regulator 5V logic input.

The limits of the atmega328b is from 3.8-6.1V I believe from the top of my head so it was right on the limit.

Carbon_Rod

Tue Jul 22 2014, 10:38PM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Most people use the similar L293D chip:

These are under $11.29 for 4 h-bridges on a filled shield (also fits the Mega2560):

Not sure why you used multiple Arduino boards, but it would be an interesting design challenge coordinating a multi-master I2C bus.

Best of luck,
Rod

Legit_bacon

Wed Jul 23 2014, 12:01AM

Registered Member #4034 Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138

The multiple arduinos are for sheer compute power :D
the robot uses stepper motors for extreme precision in its movements and those are a hog on processing power, so they get a arduino of their own, then theres a large array of IR reflectance sensors which take some time to read, halting other things in the program, so they get a arduino of their own too.

and while i was at it i figured i may aswell give everything its own arduino (get em from sparkfun for $3 so what the hell, why not) to make the master code nice and streamlined.

As for the Multi-Master I2C yes it was very ...Interesting to set up indeed. But works brilliantly now. The only problem is about a 25hz update rate with all the slaves reporting in, however it hasn't caused any problems as of yet.

thanks for the links to those chips, however i noticed they dont have reverse diodes inbuilt? i believe that the sn754410 does when i checked out its data sheet a while back so ill go check again, also it is available locally so I think i'll stick to that for now :)

I'll post the finished product here soon if you're interested, Its been my fist custom PCB build so their have been a few errors so far but its still looking good :)

Thanks again for the help!
-Matt

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