Blowing Basys2 boards - Page 1
Forum section: electronics / projects

author: EterniaLogic
author_rank: Newbie
timestamp: April 14, 2014, 02:55:30 am
content: Hello,So... my team has blown 3 basys boards on our robot.We currently have an LDO without a pulldown resistor on the output. (Next revision does have one) I have provided the original schematic for what we currently have installed on our robot; along with a very shizzy view of the power vector board.The Project Lab Officials have provided us with an H-bridge board Rev. A, and it seems that the cause of everybody's Basys boards going Kaput is that H-bridge. I have provided their schematic, which has optocouplers and seems legit. The problem is that the pile of broken Basys boards for 15 Groups has gone out of control. it's about 2 feet tall. That has to be around $2880 worth of basys2 boards.What is happening?!?!?

author: Skimask
author_rank: Super Contributor
timestamp: April 14, 2014, 03:44:22 am
content: A) Not an H-bridge- Click the wrong linkB) Where's the optocouplers?- Clicked the wrong one here too!C) Melted insulation on wiresD) Rotten soldering jobE) "Project Lab Officials". What qualifies them to be "Officials" if they can't solve a simple H-bridge (which this schematic is not) problem?

author: EterniaLogic
author_rank: Newbie
timestamp: April 14, 2014, 04:02:59 am
content: Project Lab 1 <-- note the 1, meaning we are all still noobineers in college, melted wires are common I guess. We have a Greenboard which we are still preparing for.... just a bit too late unfortunately.The only thing that is common between all of our groups is that H-bridge board. (Very top link)The very basic schematic on those attachments is a basic power schematic, Rev. A (Currently at Rev. F)I realize that this post should be underBeginners

author: Skimask
author_rank: Super Contributor
timestamp: April 14, 2014, 04:50:38 am
content: meh...maybe...the problem I've got is that the so-called "Officials" aren't able to assist...What rails are common between all of the boards?

author: EterniaLogic
author_rank: Newbie
timestamp: April 14, 2014, 05:29:17 am
content: Ground is common between all of the boards (My group went with this method instead of optocouplers, just using filter caps on main power)H-bridge board is running directly off of the 18 Volt Battery.Electromagnet running directly off of the 18 Volt Battery, 20 Ohm 10 Watt resistor with a coil heatsink to limit 1 Amp instead of 2 Amp at 18 volts.Basys2 board is running off the +3.3 Volt LDO.Sensors are running off of the +5 Volt LDO.In the newest version of the Power board, there are SMD Resettable fuses sprinkled on all of the rails. There is also a 1000 uF filter cap directly on the board to help any spikes... Which could also blow the Basys board? It would've also have been a good idea to sprinkle some optocouplers here and there, but I didn't have time to get to that... (aka: I just thought of it now...) Time to hand-solder a 25-channel optocoupler vector board... We originally avoided using optocouplers because our professor said that the whole point is to avoid a single ground... which is what you get when you use a single battery.Another alternative for port protection(Arduino article)

author: Skimask
author_rank: Super Contributor
timestamp: April 14, 2014, 06:33:16 am
content: 18v H-bridge's, 18v electromagnet, 3v FPGA board, common ground...I smell trouble...

author: EterniaLogic
author_rank: Newbie
timestamp: April 14, 2014, 01:04:58 pm
content: to be fair, it was doing the same thing when we had a 7.2 volt battery. I think I will swear by Optocouplers for the rest of my life for expensive chips... Though computers don't have optocouplers going to the CPU and everything is common ground... How do motherboard manufacturers protect expensive equipment such as CPUs from ESD and EMF from ground?

author: Skimask
author_rank: Super Contributor
timestamp: April 21, 2014, 03:38:34 am
content: They do the same things you've done...ground's everywhere.But you've got something a computer generally DOESN'T have....MASSIVE back EMF (flyback) coming from those solenoids.

author: peter.mitchell
author_rank: Super Contributor
timestamp: April 21, 2014, 07:47:10 am
content: I get the feeling the would would be a better place with more diodes.

author: Psi
author_rank: Super Contributor
timestamp: April 21, 2014, 08:51:57 am
content: Few more ideas..1) 5V tolerant i/o pins on some IC getting a 5V signal when the chips supply rail isnt powered. (This often kills them)2) Analog/Digital ground difference. I've seen a few chips which do not like any voltage difference between their AGND and DGND pins, they fail quite quickly if you have more than say 1V. (The MAX7456 OSD chip for example)3) As others have said, more back emf protection diodes is probably the first thing to try.

author: Niklas
author_rank: Frequent Contributor
timestamp: April 21, 2014, 09:02:28 am
content: Board planningSort out what components that belongs together and group them. Then arrange those subgroups on the PCB. Power groups, like the solenoid activation, should be placed very close to the power connector.Local energy buffering and decouplingI miss a lot of capacitors on that PCB layout. The PCB and the wiring are not an ideal world and will add a lot of series resistance and inductance. Think of the situation when the solenoid is activated. When the MOSFET is turned on, the load will start to draw current, and without a local buffer capacitor, the whole board's voltage levels will be affected.Local buffers and decoupling on the voltage regulators? The decoupling capacitors should almost be hugging the relevant power pins of the IC.In this case a combination of ceramic capacitors and electrolytic capacitors would be a good solution.Skip those PTC resistors, add chokes and capacitors instead.http://cds.linear.com/docs/en/application-note/an101f.pdfAll components and PCBs are non-ideal. Diodes will not open or close instantaneously and during that time strange things can happen. Is the flyback diode on the output fast enough?Force the current to go the way you want it to when routing the board traces. It is really lazy, especially noise and spikes. Place filtering components as checkpoints that must be passed.

author: hlavac
author_rank: Frequent Contributor
timestamp: April 21, 2014, 07:27:34 pm
content: My wild guess would be sensors putting 5V onto basys 3.3V rail thru the input protection diodes on the input pins. Those LDO regulators cannot handle reverse current or bring the output voltage down once it goes over.