Common functioning of a 1M resistor?

[syzygy]

I'm still learning about electornics on the journey to building my first pedal.  I see a lot of 1M resistors on circuit boards.. what are they used for, since their resistance is so high?  Is a very small current allowed to get through for a 9V power source going through a 1M resistor?

Thanks!

[petemoore]

Imagine a cup of water, and the amount of pressure at the bottom of IT being .001V.
 Now Imagine a resevoir full of water, the amount of pressure at the bottom is '1000V'.
 Voltage Potential analogy to water pressure, the more voltage there is the harder it'll 'push'...
 Now take the pressure at the dam [resevoir analogy] and run that through a small tube, say about like a bic pen body...although the pressure is still 1000V [or 1000 lbs. asitwern't] the current going through that teeny tube can only be "X' or so large. So if you have something you want to 'work' with only a very small current, and has the capability to draw large quantities of water, the pressure/voltage [analogy] Drops considerably.
 so voltage is 'pressure'
 and current rate is 'flow'
 If you have another resevoir that the bic pen tube [the plastic of course would be shattered by these pressures] filled up, the voltage or pressure...
 I'm intentionally mixing terms, to brief the analogy to water...this kind of babble helped me grasp V/I and R [voltage current and resistance].
 have you drawn the ohms law circle yet ?

[onboard]

whoa pete

I was just going to say they're typically at both ends to conduct stray DC to ground which can help eliminate popping while switching, and are often used to establish a high input impedance (double duty there for the one up front).

Not sure if that's what you were asking, or more of an Ohm's Law type look at things.

[syzygy]

Yes, those are helpful explanations.     When you add a 1M to ground at the input, do you lose any of your signal that way?  Or just enough to rid the transients without affecting the sound?  Most people couldn't tell by ear, it seems like this would affect the signal nonetheless...

[petemoore]

Yes, those are helpful explanations.     When you add a 1M to ground at the input, do you lose any of your signal that way?  Or just enough to rid the transients without affecting the sound?  Most people couldn't tell by ear, it seems like this would affect the signal nonetheless...

Probably so minute you can't tell.

[Alex C]

You can also use a 2.2M for a pulldown resistor, if you or Eric Johnson think you can tell a difference.  

[syzygy]

Not that I can tell a difference, I'm just curious what's happen happening to the signal.   Transmogrifox used 4.7M resistor to ground at the input of his true bypass circuit.. :shock:

I also see a 100k resistor to ground at the output.  Is this a pulldown resistor, too?  Same deallie-o?

[onboard]

Not sure on that specific schem, but resistors to ground at the input usually help establish input impedance, which can be quite high depending on the circuit. The highest I've seen was 2.2M on a jfet buffer, but 4.7M isn't out of the question.  

Also, keep in mind that an output cap followed by a pulldown resistor to ground together form an RC highpass filter. You can "tune" the corner of the low frequency roll-off by adjusting the value of either.

In which case, that last resistor before the output is doing double duty, DC pulldown and establishing a frequency rolloff.

To be clear, there's always high pass filtering with an output cap followed by a pulldown resistor. But with a typical 0.1uf and 1M, the frequency corner is 1.6Hz which is far below audible range.

[ESPguitar]

Look on page one ot two of the topic, (build your own stompbox).
There was a guy yesterday he asked about learning more about circuits and how each part works..

Let me see if i find the topic..

Explain to me on a 6th grade level how a tubescreamer works
That's the topic heading..


This is a link to GGG

http://www.generalguitargadgets.com/richardo/distortion/index.html

Read! You'll learn a lot.. It's good written and pretty easy to understand..

Good reading..

RB

[syzygy]

Those are frikken great links!  Thanks everyone