VIN vs USB power, damaged project board, trying to understand what happened

bgp · 2017-04-27 21:53

I have a Propeller project board (this one: https://www.parallax.com/product/32810 ) and I managed to fry it. I'm trying to figure out what happened and if something I'm doing is unsafe to figure out what I should be doing instead.

The state it is in now is that plugging in via USB causes the PWR LED to light up and the port to show up on my Mac, but any attempt to load code onto the propeller fails (so I'm assuming the FTDI chip is working but the prop itself or other important component is toast). Applying 5v to VIN does not make the PWR LED light up any more (it used to). There is also a noticeable temperature increase on the prop chip and another that I don't recognize (or at least I didn't notice it before, it definitely seems hotter to me though).

As for how I've had it hooked up, I have various components I'm tinkering with that need 5v power, for which I'm using a separate L7805. I often have had the project board plugged in so it's: 12v -> L7805 -> VIN pin on project board. At the same time, I'm writing code and loading it onto the propeller via USB - by which the board is also powered.

Is it safe to have power supplied from both VIN and USB? I figured there would be some diodes in there to save me from my own foolishness, but if that isn't a safe configuration then at least I would know what happened. What are the other options for being able to have 5v power while iterating through code?

I guess I could tap into the USB power for the other components (looks doable but tricky - very small contacts; overall load is a concern too). Or maybe there is a way to disable USB power and just always use a separate VIN power - is that possible and would it make it safe?

DaveJenson · 2017-04-27 22:07

Where exactly did you apply the 5V to VIN?
I assume you do know that the Propeller is a 3.3V only device. If you applied 5V to the Prop, it is most likely fried....

bgp · 2017-04-27 22:14

Gotcha - yeah I get that it's a 3.3v device - the 5v went to one of the pins labeled VIN in the lower right of the board (using image from the product shot in the link above). I checked and this pin is directly connected to the "5-16VDC" plate - pretty darned sure I have that part correct.

And I always make sure all the pieces have common ground (including whatever is powering VIN and the USB, and any external components).

Also, I had it running in this configuration on and off for days before it finally crapped out on me. It's always possible that I fried it in some other way that I'm not aware of but I can't for the life of me see what that could have been.

One other possibly related phenomenon is that sometimes when I would plug the project board in via USB the USB port on my mac would stop working (not sure if it's due to overdrawing the power - too much current, or power surge - extra voltage coming from VIN). A reboot would solve it so no harm done, but clearly there is something off. This is also one of the reasons I suspect using both USB power and VIN at the same time may not be "safe".

Rayman · 2017-04-27 22:39

Is the 5V supply you attached to VIN "regulated"? Some unregulated power supplies put out high voltage spikes...

Anyway, a hot prop is never a good sign. I'd guess it's dead...

bgp · 2017-04-27 22:46

Actually now that I look at it in more detail, I was mistaken - power to VIN is coming from four AA batteries - at full charge it's about 6.5v. I'm using the voltage regulator for other components but not the prop board. I doubt my batteries are providing any significant power spike.

And yes I'm pretty sure it's toast. I have another board though, I'm more trying to understand what not to do this next time I hook things up.

Specifically it would be great if someone has the info on whether or not applying power to VIN while it is also plugged in via USB is an intended configuration or not.

xanadu · 2017-04-27 22:57

I fried one and never was able to figure it out. I'm not sure what rev. you have but it's in the PDFs.

Page 5 https://www.parallax.com/sites/default/files/downloads/32810-Propeller-Project-Board-USB-Guide-v1.0.pdf

To supply power to the VIN header, you must apply 5–16 VDC to the external power input connector.
The board can receive up to 500 mA for the 3.3 V rail via USB, but if your project requires additional
current, external power input will be required. It is safe to apply power through the external power input
while the board is connected to USB.

bgp · 2017-04-27 23:04

Thanks! That's what I was looking for. (Board says REV D, btw.)

Sadly it doesn't explain what went wrong. I happen to have a prop plug, possibly I should switch to using that to load code and just power everything externally, removing USB power from the board altogether (even though per that doc I should not have to).

kwinn · 2017-04-27 23:32

Vin and USB power both go through diodes so having both connected at the same time should not be a problem. The output from one of the two diodes (higher of the two voltages) is called REG_IN and goes to the input of the 3.3V regulator.

With the green power led coming on with USB power in and not with just Vin connected diode D1 is open and C1 may be damaged. Diode D2 may be shorted or working. The 3.3V regulator may be working or may be shorted to light the power led. If shorted you will have +5V on the Propeller which would account for it being hot.

The board is probably toast unless you are up to replacing the chips on it. You could check D1 and D2 with a multimeter, and the output of the 3.3V regulator? Posting the results could help determine the cause.

Publison · 2017-04-27 23:37

I also fried one, but I had some suspect voltages coming on board through various diodes. I fried the regulator which is a bugger to replace because it has a thermal pad underneath. Luckily I bought 4 boards for the job.

bgp · 2017-04-28 00:20

@kwinn - thanks. I've been going through the schematic and it's clear what you're asking for on the circuit diagram. On the board itself, some of the components I'm able to identify. Some of the pathways I can see on the top and bottom layers, and some of the capacitors I can identify by the "104", "103", etc. markings, but there's a lot of stuff I just can't easily tell what it is or what it's connected to.

Picture: http://imgur.com/a/TrJY3

Can you point me in the right direction as to where find D1 and D2?
EDIT: D1 appears to be the thing immediately below the big white "+" next to VIN, and that appears to be working correctly: measures 1kOhm in the right direction and full impedance in the other, and that matches the measurement on the fresh board I have.

kwinn · 2017-04-28 03:16

You’re probably correct about D1, and you can measure D1 between the Vin + and pin 2 of the switching regulator.

D2 is a bit tougher. I have a USB to Mini cable that I cut in two and reconnected using a 4 pin .01 spacing female connector to access USB signals. Use the +5V USB pin a pin 2 of the switching regulator to check D2.

The 3.3V can be measured in many places on the board that are well marked.

It is also possible to follow pcb traces and identify components.

Peter Jakacki · 2017-04-28 04:19

Publison wrote: »

I also fried one, but I had some suspect voltages coming on board through various diodes. I fried the regulator which is a bugger to replace because it has a thermal pad underneath. Luckily I bought 4 boards for the job.

Normally I just cut the leads with a blade and flick the chip off, then put a big blob of solder onto the pins and flow them off the board. With thermal pads you can just blob up the leads with lots of solder and while shifting across from side to side with a hot iron the chip will come adrift and you can flick it off the board, especially when you hold it up vertically. Super easy once you have the technique and not afraid to use more solder.

VIN vs USB power, damaged project board, trying to understand what happened

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