adequate pull-down resistor value

**r0nd0m** · - 6th February 2009, 22:52

Hi there guys Im new here... Im from Brazil so Im sorry if you guys cant understand my poor english :P

I did some guessing job when finding the components value in this circuit, and I was already expectating some issues.. but Im really noob with electronics and I cant find a way for solving this. Heres my circuit in PDF format:

Its very simple.. you can see 8 lamps that will blink in a frequency given by the PIC. My big problem is when they are NOT blinking... When I turn of my circuit using that switch but keep it connected to the power source (an ATX PC power source) some of the lamps blinks weakly from time to time... its really annoying :P So I realised that transistors were receiving so much ESD for being darlington ones, that made me add that resistor (R12) wich one should grant 0V at the transistors bases..

Sadly there still one lamp that keep blinking..
My question is: How to find the resistor value that I shall use on R12?!! I think that a bigger value would make all the lamps to light again when source turned off, cause there will be no more "grounding" enough to "ground" the transistor base. And I think a lower value would reduce the lamps lighting when source turned off, cause there will be a easier way for the current flow... So whats the corret value?!?!

Its a shame cause its a very simple circuit.. Can you guys please help me solving it??

Plz! Thank you.

*EDIT: Guys, I noticed that my circuit generates some ESD that my television can feel... how can I solve it? I think those ISO especifications will be my next class topic... but can you guys teach me anything about preventing from being a source or a victim of ESD?! Thanks again!

**r0nd0m** · - 8th February 2009, 14:15

Ok I think you guys wont help me if I ask like this, sorry, so let me rephrase:

In this section of circuit I need to find the correct value for this resistor, can you guys tell me if am I right:

So, from the PIC I got 5V when that pin is logically high. I know pic sources at maximum 20/25mA, thats why I got the R1 wich makes arrive just 0,5mA at the TIP122 and thats suficient for connecting my big led of 3W and the ground (cause the gain is 1000).

The led is 3W so when at 12V passes through him 250mA. And thats exactly what I got. According to the datasheet its collector nominal current is 5A (and all Leds current summed is just 2A, so it could hold the whole circuit by himself, wich is pretty good :P but its not gonna happen).

Now my big trouble: When I added that seccond resistor trying to send all that EMI to the ground (what worked pretty well), the current sent by the PIC isnt the same anymore. So if this 2nd resistor is also 10K PIC should send him 0,5mA too, so he would send 0,5+0,5 = 1mA is that right???

Theres anything else that shall I worry about? Plz help!
If thats all and correct I finally understood how to find a pull down resistor :P

Thanks!

**16F628A** · - 8th February 2009, 16:21

Call me crazy, but why not replace pulldown resistor R2 with a small trimpot and simply adjust it until the unwanted behavior disappears?

**mister_e** · - 8th February 2009, 17:07

Clarify that "...all that EMI to the ground..." sentence.... which EMI, Where?

Your base resistor is a tad high (well... for a purist)

Are you aware of TIP122 internal resistors?

R2 shouldn't be needed

**r0nd0m** · - 9th February 2009, 23:26

Ehm guys... thank you but I kinda.. solved the problem.

Thank you anyway.
Cya soon.

**mister_e** · - 9th February 2009, 23:50

Thanks for posting your solution here with as this much details, all forum member and reader will learn a lot from it

**RussMartin** · - 10th February 2009, 00:40

Did you try a capacitor instead of R2?

And how did you solve the problem? Inquiring minds want to know . . .

**r0nd0m** · - 10th February 2009, 19:54