# Gigaohm rheostat? => Controlling duty cycle of oscillator

#### xox

Joined Sep 8, 2017
800
I've been looking around for potentiometers up to the gigaohm range but haven't yet had much luck. Mouser for example only seems to give results for ~100K and lower. Maybe I'm using the wrong search terms?

#### kubeek

Joined Sep 20, 2005
5,785
I have seen 1Meg i think. Why would you need such a thing?

#### xox

Joined Sep 8, 2017
800
I have seen 1Meg i think. Why would you need such a thing?
In this case it just allows for the adjustment of the circuit within the fullest possible range. Dropping that maximum down to 1M the output signal only achieves about half of what would be possible otherwise.

#### MrChips

Joined Oct 2, 2009
28,525
With a 1kΩ or 1MΩ potentiometer you can still adjust a signal from 0 to 100%.

Show us your circuit that you are trying to adjust.

#### sparky 1

Joined Nov 3, 2018
734
A series of 100M resistors with a selector and a trimming potentiometer might work

#### xox

Joined Sep 8, 2017
800
Thanks everyone! I could actually drop it down to a lower maximum resistance but that would then require lowering the values of some of the constant resistors within the circuit which in turn would raise the overall power consumption to potentially undesirable levels.

Worse case scenario I suppose I could buy (or even perhaps make) a liquid rheostat.

#### xox

Joined Sep 8, 2017
800
A series of 100M resistors with a selector and a trimming potentiometer might work
Unfortunately the series resistance (or otherwise the lowest setting on the rheostat) needs to be no more than 10K ohms or so.

#### strantor

Joined Oct 3, 2010
6,468
Think how hard it would be to manufacture a potentiometer of that value. What would you make it out of? How far apart would the leads need to be, so that the air around it doesn't skew the value? And what if the air is really humid? You could probably make a 1Gohm pot by moving wires closer or further away in air - assuming controlled humidity.

Pretty sure I had a 1 Gohm decade resistor once upon a time. Would something like that work for you? Or does it need to be a permanent part of your circuit? Here's one on ebay (not mine)

#### MisterBill2

Joined Jan 23, 2018
14,714
For a circuit of that high a resistance you can just use a series resistor, or a string of multi-megohm resistors.

#### Audioguru again

Joined Oct 21, 2019
5,821
You do not say what circuit you have needs an extremely high variable resistance that is not available.
A transistor, a current-sensing resistor and an opamp can do it easily with an ordinary reasonably low resistance potentiometer.

#### BobTPH

Joined Jun 5, 2013
6,527
Unfortunately the series resistance (or otherwise the lowest setting on the rheostat) needs to be no more than 10K ohms or so.
I don't think any potentiometer could be set to a value that is 1/10000 of the full scale value.

Tell us what you are trying to do. There is likely a better way.

Bob

#### DickCappels

Joined Aug 21, 2008
9,873
I have 1 meg and lower pots.

Once I got around the need for a very high impedance pot by using an adjustable current source, employing a JFET as the output device.

xox

#### MisterBill2

Joined Jan 23, 2018
14,714
According to a Bob Pease comment, that great a resistance is unlikely in an electronic assembly. And he understood very well about very high impedance circuits. So the question stands, what are you hoping to achieve? And what sort of circuit needs that great a resistance?Really, ten megohms is the highest value for any normal electronics. You could use PCB material to create a hundred position rotary switch, and then wire in 100 ten meg resistors.

#### Beau Schwabe

Joined Nov 7, 2019
133
The value of the pot isn't so much the problem as the impedance will be for what the pot is connected to. It will most likely be susceptible to noise and bring the noise floor up to unwanted levels in your circuit. If lowering the values introduces unwanted current draws, then there are ways to apply energy management to your design. So tell us more information as far as what it is you are trying to accomplish.

#### MisterBill2

Joined Jan 23, 2018
14,714
Given that the TS is a student it may be for a theoretical application, or possibly an impossible creation.

#### xox

Joined Sep 8, 2017
800
So this is the isolated subcircuit (attached to an LED for demonstration purposes).

It's just a simple "single-supply" oscillator really. And yes I do realize that it is somewhat crude!

Resistors marked Rk are fixed at 1M. Cf tunes the frequency. Rd controls the duty cycle (more or less anyway).

Now I did notice that by driving Rd up to about 10M it does seem to diminish the duty cycle value well enough but as someone has already mentioned here I may still have a bit of a struggle finding a potentiometer that could go from as high as that down to say 10K in any case.

So perhaps Audioguru's suggestion is the best route to go here?

#### MrChips

Joined Oct 2, 2009
28,525
If I understand your goal correctly, you wish to control the duty cycle of a square wave oscillator from 0% to 100%.
This is not the way to do it.

#### BobTPH

Joined Jun 5, 2013
6,527
It is kind of hard to make something oscillate with 0% or 100% duty cycle! But, as MrChips hints, there are other ways of controlling duty cycle.

Bob