Oh no! The Horror....

A "new fashioned" televisor, using an Arduino to drive the motor and display.

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Oh no! The Horror....

Postby Andrew Davie » Wed Jul 05, 2017 12:12 am

Friend at work today asked me when I was going to bring in my televisor for "show & tell". "Tomorrow!" I gamely say, even though I've not actually tested the new LED matrix in battle. So, first things first, I connect up the LED matrix and it works - sort of. I notice the picture is very glitchy and there are regularly spaced vertical stripes over the whole picture. I've seen this before, though - a clear symptom of not enough voltage to the LED array I thought. Not having a variable power supply at home, my choice was to go from 12V to 17V or to use a power brick which is pretty much untested in battle too. I decided to go with the 12.5VAC-->17.5VDC and keep a really close eye (well, finger) on the Arduino - as it quickly overheated last time I tried 17V. Good news is, this totally fixed the picture with regard to the LEDs - a lovely golden white, almost orange like the original pictures were meant to be. So that's good. I only ran it briefly, worried about the current to the LEDs. I calculated and wired in a limiting resistor, and this dulled the picture a lot.
One issue I was getting, though, is that if I had the speaker plugged in at the higher voltage, the system reset - every time. A big "squawk/pop" from the speaker, then a reset. So I did my further testing with the speaker not connected, and all working OK with it not there. No resets.
Next I thought I'd give the 15V power brick that I'd received a short while back. Here was my mistake. Not having the correct female power connector, I decided to wire up just a red/black wire to the main board, and manually hold the wires - one shoved in the center of the power connector, and the other held to the outside. Just for a test, mind you. So I was doing this, kind of dangling the power lead in the air and *POP* and magic smoke at the power inlet of the board. Looking closely, the wires were not clear and they had shorted :( So I carefully went back to the previous 12V and - no sign of life. I tried the Arduino by itself, connecting to the computer. Dead. Bummer, so that's board #2 dead. I so hate destroying things the day before a demo.
So, dig out board #3 and solder everything on. Everything, that is, except for the IRL540 and the TIP122. Where's my box of FET-things? Search... nothing. Practically tear the whole room to pieces, and remembered I took the box to work on the weekend to do some testing. I distinctly remember bringing it home though. So, search the car - aha! Back to the #3 board, solder in the two components and went for an all-up test. Everything plugged in (except the speaker, I'm now a bit wary of what's happening there). It works! Of course I still have those black stripes in the picture, but time to leave well-enough alone.
Tomorrow I will demo with a bench power supply and feed it 12V at first, and slightly up the voltage until the LED matrix is getting enough juice. Probably will be OK around 12.5V I would guess; it's pretty close now.
So, all's well that ends well - serves me right for being impatient and doing the dodgy "hold it together with your fingers while applying power" thing. Also, on the plus side, I now clearly have some investigation to do into what's happening with the sound at higher voltages. One thing I've been meaning to do is always leave the speaker PWM in a "quiet" state - Im guessing the click/pop I hear on startup/shutdown is where the speaker is just "snapping" to the "far away" value when there's a state change. I might have to put in some sort of smoothing filter for these situations.



Video shows the black banding on picture when LED matrix is under voltage.
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undervolt2.mp4
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Andrew Davie
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Re: Oh no! The Horror....

Postby Andrew Davie » Wed Jul 05, 2017 12:07 pm

Interesting problem when demoing at work. Tried several benchtop power supplies. Base load about 0.2A but when I start up a video and motor spins it goes to about 1.6A which is all OK. The problem is when the disk gets up to speed and the motor cuts out, the power supply appears to be resetting or dropping voltage significantly and the Arduino resets. Only momentary, it seems. I'm thinking there's a spike from the motor acting as a dynamo getting back to the power supply which is then going "uh oh". So, unable to demo using any of the benchtop power units available to me.

This brings me to thinking - I have a generic problem here - perhaps the motor "back current" protection isn't working well enough? I have a Schottky diode across the motor terminals but I figure this can't be enough. Perhaps I could put an oscilloscope on the line and get some idea of what's actually happening. But I'm fairly confident it's a spike due to motor spin.

Mmmh.
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Andrew Davie
"Gomez!", "Oh Morticia."
 
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Joined: Wed Jan 24, 2007 4:42 pm
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Re: Oh no! The Horror....

Postby Robonz » Sat Aug 12, 2017 7:57 pm

Your diode will only stop reverse/back emf voltages, not over voltage spikes. Typically a TVS diode and/or a capacitor is fitted across the back of the motor which reduces over voltage transients. If you have good isolation between the motor circuit and the micro your transients wont matter, I tend to design for that approach.

e.g An important think to watch out for is sharing the ground wire for both the motor and micro. It is best to run two ground wires back to the power supply so motor currents do not appear in the micros ground (star grounding)
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