Changing/removing limiting resistors on a MiniPOV3

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pneumatic
 
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Changing/removing limiting resistors on a MiniPOV3

Post by pneumatic »

I built my MiniPOV3 kit, and have it working, but the messages are very hard to read in anything other than a very dark room. The kit I got had 100 ohm limiting resistors, and that's a large value even if they were being driven by 5V, rather than 3 (or 2.4 if I use NiMH.) In fact, based on the source pin characteristics outlined here: http://www.atmel.com/dyn/resources/prod ... oc2543.pdf (Figure 106 on page 196)
it looks like when Vcc is 2.7 volts (a good approx) it might not be necessary to have a limiting resistor at all, since it would seem to hit 17 mA at 2V, which is pretty close to what standard LED should be driven at.

As a first step I was thinking about replacing them with 50 or 33 Ohm resistors, but then I had a wacky idea. To avoid de-soldering all of them, I could piggy-back an additional 100 ohm resistor on top of the existing one to give the equivalent of 50 ohms (or piggy back a 50 ohm for 33 ohms.) I don't know if this would end up being a mess or not. Does anybody have experience trying to solder on piggy-backed resistors?

-- Mitch

adafruit
 
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Re: Changing/removing limiting resistors on a MiniPOV3

Post by adafruit »

one leg at a time :D

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pneumatic
 
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Re: Changing/removing limiting resistors on a MiniPOV3

Post by pneumatic »

adafruit wrote:one leg at a time :D
With that (very small) encouragement, I decided to give it a try using a 75 ohm resistor in parallel.) I did R1 as a test, and that seemed to work fine, making the bottom-most led significantly brighter. I decided to do the rest in order of increasing accessibility, so I started next with R5. To avoid damaging the uC, I pulled it out, and when I put it back in afterward, I got no lights, and the ATtiny chip just got really hot. I checked for solder bridges and other obvious faults, but I suspect I just got bitten by static discharge taking out the chip. I suppose I could put it in a breadboard and just give it Vcc/gnd and see if it still heats up.

Is there anything else I could have damaged that would cause this? If I order a replacement ATTiny2313 from mouser, do I have to just start with a "make burn-fuse" after powering it up for the first time?

mtbf0
 
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Re: Changing/removing limiting resistors on a MiniPOV3

Post by mtbf0 »

don't supposeyou put the chip in backwards?
If I order a replacement ATTiny2313 from mouser, do I have to just start with a "make burn-fuse" after powering it up for the first time?
yes.

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pneumatic
 
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Re: Changing/removing limiting resistors on a MiniPOV3

Post by pneumatic »

mtbf0 wrote:don't supposeyou put the chip in backwards?
Wow... that was embarrassing. I missed the notch and mistook the circle on the package for the registration mark.

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Re: Changing/removing limiting resistors on a MiniPOV3

Post by mtbf0 »

Pneumatic wrote:Wow... that was embarrassing. I missed the notch and mistook the circle on the package for the registration mark.
don't let it go to your head. you're hardly the first.

and look at the bright side. it wasn't your mods that did it. and there's a good chance the tiny2313 survived the ordeal. did it?

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pneumatic
 
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Re: Changing/removing limiting resistors on a MiniPOV3

Post by pneumatic »

mtbf0 wrote:....

and look at the bright side. it wasn't your mods that did it. and there's a good chance the tiny2313 survived the ordeal. did it?
It did, and now I have a spare, which I also put in backwards, and also survived.

On the downside, I cut out the resistors I had added as part of the diagnostic, and now no longer have enough 75 ohm resistors to complete the project unless I can reuse the ones with extremely short leads. It may be easier just to desolder and put in new resistors after all. I have a strip of 33 ohm resistors that should do the trick.

-- Mitch

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Re: Changing/removing limiting resistors on a MiniPOV3

Post by sparr »

I am a fan of driving LEDs directly from AVR pins, but only if you are sourcing current at the AVR. If you are sinking current, I have heard it's a Bad Idea(TM) to send unrestricted current into the chip.

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