Resistor value help

[Kelly Davis]

Hi, I'm trying to do a project that I found on the net for a laser receiver.
http://www.ews.uiuc.edu/~nshin/PICTS/laser_layout.gif

First of all I have had some electronics training, but that was years ago.
I'm just fiddling around with some old parts out of VCR's and the like and
just doing this for entertainment. My question is about the 100M resistor on
the base of the phototransistor. I can't find any surface mount resistors of
this value anywhere. I did see one 100M resistor on a component supply site,
but it was wire wound and about the same dimensions of a broom stick. I know
I can add in series to get this but that's a lot of resistors. e.g. 100,
1Mohm or 10, 10Mohm. Seems like overkill to me. I thought maybe I was
reading it wrong, or the schematic was wrong, but the parts list says 100Meg
resistor also. I realize that if my electronics theory was up to snuff I
could just calculate the appropriate value, but it's not and right now I
don't care about theory, I just want to build the dang thing and tinker
around with it whether it works or not. That being said, am I reading it
wrong or is the value wrong or is it just a big hoax to get me to spend a
lot of time chasing my tail looking for a non existing part. TIA for helping
out.

 

[Tom MacIntyre]

On Sun, 03 Aug 2003 23:41:52 GMT, Jonathan Kirwan
<jkirwan@easystreet.com> wrote:

On Sun, 03 Aug 2003 22:39:22 GMT, "Kelly Davis"
kdavis2002@mchsi.com> wrote:

Hi, I'm trying to do a project that I found on the net for a laser receiver.
http://www.ews.uiuc.edu/~nshin/PICTS/laser_layout.gif

First of all I have had some electronics training, but that was years ago.
I'm just fiddling around with some old parts out of VCR's and the like and
just doing this for entertainment. My question is about the 100M resistor on
the base of the phototransistor. I can't find any surface mount resistors of
this value anywhere. I did see one 100M resistor on a component supply site,
but it was wire wound and about the same dimensions of a broom stick. I know
I can add in series to get this but that's a lot of resistors. e.g. 100,
1Mohm or 10, 10Mohm. Seems like overkill to me. I thought maybe I was
reading it wrong, or the schematic was wrong, but the parts list says 100Meg
resistor also. I realize that if my electronics theory was up to snuff I
could just calculate the appropriate value, but it's not and right now I
don't care about theory, I just want to build the dang thing and tinker
around with it whether it works or not. That being said, am I reading it
wrong or is the value wrong or is it just a big hoax to get me to spend a
lot of time chasing my tail looking for a non existing part. TIA for helping
out.

You need to be "VERY CAREFUL" with 100M ohm resistor values. So
much as a finger print or solder flux or just about anything
else will be a problem. Keep your boards *very* clean and dry.
Isolation guard rings may be needed as well,

Might check with Vishay or Ohmite. You can use two 50M ohm in
series, for example, too. So that's a way, too. Digikey sells
some 50M Ohmite surface mount type (chip) resistors, I think.

That's right; since they're surface mount, and such high resistance,
they are so small that their value can be changed by the touch of ones
hand.

Tom

Jon