0

Schematic Resistors
Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.

Schematic Resistors

by Jon128 on Thu Dec 13, 2007 2:24 pm

I just built a tvbgone recently and am very impressed.

However, there is a problem with the schematic. The 120 ohm resistors are wayyy too small! (It still works fine because the current is limited by the output pin)

To get 100 mA in the LED you only need a 3.3k resistor. The base current would be (3 - 0.7) / 3.3k = 0.7 mA, times the current gain (150) the collecter current would be 105 mA. I tested this with pspice and on a breadboard.

You could get away with more current by using a 1k or 2k resistor.
Jon128
 
Posts: 3
Joined: Thu Dec 13, 2007 2:03 pm
Location: Toronto

by maltman23 on Fri Dec 14, 2007 5:53 am

Yeah, true, but if you use 47 ohms you get lots of current through the IR emitters, and the range is even further. The tradeoff is that there is no way to program using an ISP if the resistors are small.

I tried a bunch of different values empirically on my lab bench, with an IR detector at the other end of the bench measuring the intensity of IR it receives, and using 47 ohm resistors significantly increased the brightness over 120 ohms.

I've been using 47 ohms in mine for the last month, with the same batteries, and nothing has blown, and the batteries are still good (and I do lots (LOTS!) of field testing). :)

maltman23
 
Posts: 134
Joined: Fri Jul 13, 2007 4:54 pm
Location: San Francisco

by Jon128 on Sat Dec 15, 2007 3:15 pm

Interesting, I guess the current is limited by the output pin and the resistor causes a voltage drop. So the resistor is only there to split the current evenly

It would be a good experiment to add an emiter follower to buffer the voltage from the pin, then use the proper sized resistor to limit the current. I don't have any extra room on my board though.
Jon128
 
Posts: 3
Joined: Thu Dec 13, 2007 2:03 pm
Location: Toronto

by adafruit on Mon Dec 17, 2007 1:27 am

the resistor values go against pretty much everything i learned about properly biasing transistors. still, mitch did the testing and he said it works better so thats what we went with :)

adafruit
 
Posts: 12151
Joined: Thu Apr 06, 2006 4:21 pm
Location: nyc

by maltman23 on Mon Dec 17, 2007 3:27 am

I've never really been very orthodox in much of anything I do. :)

I am pumping as much current through the IR emitters as I can. I'm relying on the internal resistance of the batteries to limit the current enough to not fry anything. I haven't actually done the math, but I've been field-testing with these parts in this configuration for a few months, and nothing's blown yet. And quite a few TVs have turned off from quite a far distance. :)

Mitch.

maltman23
 
Posts: 134
Joined: Fri Jul 13, 2007 4:54 pm
Location: San Francisco

by caladan on Mon Dec 17, 2007 12:49 pm

It's strange, that there was change between 47R and 120R... It shouldn't be so.
Even if collector current is 0.5A, the B=100, base current should be like 5mA, so R should be 470R or something like this...
Well, I will test this on my own. Now I used transistors with small current, but will try to use "bigger" ones.
caladan
 
Posts: 19
Joined: Mon Dec 17, 2007 12:16 pm

by adafruit on Mon Dec 17, 2007 1:31 pm

maltman23 wrote:I've never really been very orthodox in much of anything I do. :)

I am pumping as much current through the IR emitters as I can. I'm relying on the internal resistance of the batteries to limit the current enough to not fry anything. I haven't actually done the math, but I've been field-testing with these parts in this configuration for a few months, and nothing's blown yet. And quite a few TVs have turned off from quite a far distance. :).



hmm... i dont think its the internal batt resistance thats limiting it here. the microcontroller pins are very well current limited (to the extent that you could put an LED on there without a resistor and it would be just fine) somewhere between 20ma-25ma

adafruit
 
Posts: 12151
Joined: Thu Apr 06, 2006 4:21 pm
Location: nyc

Resistor values

by gmg on Fri Dec 21, 2007 1:39 pm

I wondered about that too.
Atmel datasheet says 40 ma max at each I/O pin.
And I'd think you'd saturate the 3904 with far less than what 120 ohms provides.
gmg
 
Posts: 6
Joined: Wed Jan 17, 2007 1:26 pm
Location: NYC

by caladan on Fri Dec 21, 2007 2:58 pm

But it may turn on/off slower, with bigger resistors. This may be the case. It depends on Cbe...
caladan
 
Posts: 19
Joined: Mon Dec 17, 2007 12:16 pm

by maltman23 on Sat Dec 22, 2007 6:18 am

Well, my bench tests show that the light hitting an IR receiver from several feet away is brighter using four 47 ohm resistors than when I used four 120 ohm resistors. Try it out, and see how it works for you.

Another thing I've tried (and I'm doing this with the TV-B-Gone Pro SHP) is to use an emitter follower on the output of the micro, and then I use it to drive 8 other transistors (these 8 are set up like the TV-B-Gone Kit). This works really well!

Mitch.

maltman23
 
Posts: 134
Joined: Fri Jul 13, 2007 4:54 pm
Location: San Francisco

Please be positive and constructive with your questions and comments.