What is wrong with this circuit?
4 Attachment(s)
Powered by 5V battery, all works as expected.
When powering AVR with 5V regulator, result is second video. What is the fault? Code:
// the setup function runs once when you press reset or power the board https://www.youtube.com/watch?v=UkOGMdlGNM0 |
Re: What is wrong with this circuit?
David,
In your first circuit, you have the same voltage (5V) on the emitter and base. You are able to pull the base up to the emitter when you drive the output high and thus turn off the transistor. In your second circuit, your emitter is at 6.6V but you can only drive the base to 5V (the voltage fed to the AVR through the regulator). You are not able to drive the transistor off in this configuration. You would be better using an NPN Darlington in your second circuit as the AVR and your LED's share a common ground reference. Drive the output high to turn on the LED's (you only need a few volts above ground to fully turn on the transistor and the 5V AVR output is more than sufficient). Cheers, Adrian |
Re: What is wrong with this circuit?
So could I solve this circuit by using two additional transistors that are driven by the AVR and their amplified output drives the Darlington high enough to turn off?
Reason I ask is looking at components to build this I looked at NPN circuits which had the load between Vcc and the transistor. This would require a common positive and separate negative for each LED switched by their respective transistors. That would normally be no problem but I already built the wing and wired it with common negative. Sort of put the cart before the horse on this one.:BangHead: The only thing I am really trying to accomplish with the 5V regulator is the ability to run the whole thing from 2S LiFe and keep AVR Vcc @ or below 5.5V. |
Re: What is wrong with this circuit?
What if he connected both transistor emitters to the same 7805 output rather than straight off 6.6V?
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Re: What is wrong with this circuit?
A generic 7805 is rated at 1A. You can push them to a little over 2A peak if you have 5V "headroom" but this will not be available with a 2SLiFe. So we will be limited to 1A.
If the average current draw of the LEDs is below 1A and the peak current draw does not trigger the internal short circuit protection (a big if), then you could run the AVR and the LEDs off the 7805. It is easy enough to test. If it does work, the 7805 may require a heatsink. Cheers, Adrian |
Re: What is wrong with this circuit?
David,
If the LEDs cannot run off the 7805, here is a link for circuit design considerations - microcontroller using NPN to switch PNP with higher voltage load. http://www.w9xt.com/page_microdesign...switching.html Cheers, Adrian |
Re: What is wrong with this circuit?
Quote:
I thought it was all good running very cool with heatsink but then something goofy was happening with the flash pattern and I didn't know the reason why so I thought about just running the AVR and that got me here. I will certainly look into that page on High Side switching. I actually started laying out that circuit on experimenter board. The page you linked will make it an easier job of understanding and selecting components. Thanks for the great help here. The way you explain is easy to understand. Got about what seems to be the same advice in other forum but described from a "Geniuses" point of view and hard to piece together. I understand basics and enjoy electronics but need a bit of "hand holding" if you will. Thanks again. |
Re: What is wrong with this circuit?
Quote:
I went through the calculations, figured out the R values and it works like a charm. I even stumbled across an LM78L05/TO-92. I have not actually measured current in the circuit but with R2 @ 4.7K, R1 @ 740K and R3 @ 4700K there must be very little through the Output pins of the AVR and it should be pulling way less than the max ability of the voltage regulator. That page is getting bookmarked with a HUGE asterisk. Your assistance is much appreciated. |
Re: What is wrong with this circuit?
Just a comment: It may be better to use a standard common collector circuit to drive the LEDs, by having the load between emitter and positive rail.
Also, It's been a while since I worked with one, but I believe the 7805 requires minimum 3V between input and output. You need to use an LDO regulator. |
Re: What is wrong with this circuit?
Quote:
Dropout voltage is around 2V for 7805 or 78L05 that I ended up using but only at or near max rated current. With small current draw actually being used to power the AVR output is reading about 4.9V |
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