# Gaussmeter dc bias

#### gjoo

May 22, 2013
63
Can someone explain the equation for vr2 in this diagram. An op amp with an 100 ohm output receiver which is fed from an hall sensor I'm not sure how to calculate the adjustable 10k resistor with 5v input into this circuit. I know it is a dc bias. Any help appreciated.

#### gjoo

May 22, 2013
63
I meant to write that the dc bias is biasing the output from the op amp, but how do we calculate how much it is biasing

#### gjoo

May 22, 2013
63
I've posted the circuit below

Last edited:

#### gjoo

May 22, 2013
63
Here is a picture of the circuit

#### Attachments

• Screenshot_20230611-215841.png
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#### gjoo

May 22, 2013
63
I think this pic is clearer

#### Attachments

• Screenshot_20230611-220343.png
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#### davenn

Moderator
Sep 5, 2009
14,215
Hi
sorry I cant help, just wanted to say hi and hope some one else can help you out

#### kellys_eye

Jun 25, 2010
6,229
What's to calculate? It's a pot (presumably linear law) across a 5V supply therefore the voltage at the wiper will be directly proportional to its position. Assuming a 10 turn pot (common) you get 0.5V per revolution all the way from 0 to 5V.

Or am I missing something?

#### gjoo

May 22, 2013
63
How does the output of the pot combine with the output of the op amp? Would we treat us at a voltage divider 5v(0 to 10000)(100)/100+(0to10,000)

#### kellys_eye

Jun 25, 2010
6,229
In this case the op-amp output would only have an effect if/when it exceeded the DC level set by the pot - thus a bias.

#### gjoo

May 22, 2013
63
So, when the op amp exceeds the dc bias, all we do is subtract the bias from the op amp output?

May 22, 2013
63

#### kellys_eye

Jun 25, 2010
6,229
Not here - this isn't a summation circuit. Once the op-amp output exceeds the voltage at the wiper of the pot the op-amp voltage becomes the 'dominant' signal. It's more like a 'squelch' setting in that the pot level sets the point at which the op-amp output has to exceed in order for a change to be effected.

#### gjoo

May 22, 2013
63
Not here - this isn't a summation circuit. Once the op-amp output exceeds the voltage at the wiper of the pot the op-amp voltage becomes the 'dominant' signal. It's more like a 'squelch' setting in that the pot level sets the point at which the op-amp output has to exceed in order for a change to be effected.
Thanks for all the help. I understand it now.

#### gjoo

May 22, 2013
63
Just one last question, how does the 100 ohm resistor affect that bias?

#### Delta Prime

Jul 29, 2020
1,642
Just one last question, how does the 100 ohm resistor affect that bias?

I hope this helps.

#### Attachments

• sloa011b.pdf
898.9 KB · Views: 3

#### gjoo

May 22, 2013
63
How would the resistive load equation look for this circuit?

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