 ### Network # Meg ohm resistors , are opens circuits?

#### Davewalker5

Sep 20, 2014
125
When looking at a schematic that has Meg ohm resistors in series, is this like an open circuit when a resistor in the meg ohms is in series or in parallel?

Why would you want to use resistors in Meg ohms in series?

I have seen resistors in Meg ohms in parallel for impedances of the inputs or outputs

#### davenn

Moderator
Sep 5, 2009
14,016
no its not an open circuit ... just high resistance

regardless of their values, resistors are often used to set the impedance of an input or output of
a section, stage, in a circuit

In my RF activities, I will often use a 50 Ohm or 75 Ohm resistor on an input / output to set that impedance value
other circuits require different values.
Another use it for dropping of a very high voltage to something much lower that measuring equip can handle --- one case where there may be several in series
Yet another reason for a really hi value on the input to a measuring system is so that the unit doing the measuring doesn't affect the circuit being measured

Dave

#### Davewalker5

Sep 20, 2014
125
But when you see meg ohm resistors in series this does what to a circuit?

Meg ohm resistors in parallel set the impedance not meg ohm resistors in series

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
If you're really keen you can purchase resistors that have resistances in the gigaohm range.

Here is a link to a 500GΩ resistor that digikey don't currently have in stock. There's a datasheet there, perhaps that has examples of uses.

Here is a 5GΩ 1W resistor that the y DO have in stock. That's like 5000 1MΩ resistors in series.

A little exotic? Yes. But they do have their uses. Meters capable of measuring these sort of resistances usually do so on a "conductance" range, not a resistance range.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
But when you see meg ohm resistors in series this does what to a circuit?

Meg ohm resistors in parallel set the impedance not meg ohm resistors in series

Resistances in series add up. So two 1MΩ resistors in series is the same (resistance-wise) as a single 2MΩ resistor.

A 2MΩ resistance will convert a 0 to 12V voltage into a 0 to 6μA current.

If you make a voltage divider with a 2MΩ resistor and a 2kΩ resistor, then a voltage 0f 0 to 1000V on the divider will become a voltage between 0 and 1V (actually 0 and 0.999V) across the 2kΩ resistor. This effectively attenuates the voltage by a factor of 1000.

If an amplifier had an impedance of 100kΩ and you places a 1MΩ resistance in series with the input signal, the amplifier would see only 1/11th of the signal and it's effective impedance would be above 1MΩ. That might be useful in some cases (oscilloscopes use this method of attenuating signals in x10 and x100 passive probes).

#### Davewalker5

Sep 20, 2014
125
so meg ohm resistors attenuate or set the impedance of the input or output of a circuit?

#### Gryd3

Jun 25, 2014
4,098
so meg ohm resistors attenuate or set the impedance of the input or output of a circuit?
Like others have said before, meg Ohm resistors have their purposes, for example, one is being used on an Opamp to set it's gain.
It's hard to tell you exactly what it is for without seeing a circuit

#### Davewalker5

Sep 20, 2014
125
In general, what you guys have seen on schematics of Meg ohms resistors in series do what?

The ones I see is on the output pin of a op amp, its not the Feedback resistor or Rin resistor to the op amp , its on the output pin in series

#### davenn

Moderator
Sep 5, 2009
14,016
they do all the same things resistors of lower values do, they are not anything exotic one of the output of that Op-amp will likely be setting the output impedance for the following stage to look back at

Dave

#### Davewalker5

Sep 20, 2014
125
one of the output of that Op-amp will likely be setting the output impedance for the following stage to look back at

So Meg ohm resistors in series set the output impedance? i thought only meg ohm resistors in parallel do.

what is the difference from a Meg ohm resistor in series compared to a meg ohm resistor in parallel. They both set up the impedance but what is the difference between the two configurations?

#### davenn

Moderator
Sep 5, 2009
14,016
So Meg ohm resistors in series set the output impedance? i thought only meg ohm resistors in parallel do......

series ones effectively do that as well ... reread Steve's earlier comment.....

If an amplifier had an impedance of 100kΩ and you places a 1MΩ resistance in series with the input signal, the amplifier would see only 1/11th of the signal and it's effective impedance would be above 1MΩ. That might be useful in some cases (oscilloscopes use this method of attenuating signals in x10 and x100 passive probes).

cheers
Dave

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
what is the difference from a Meg ohm resistor in series compared to a meg ohm resistor in parallel

Resistances in series add together. Resistances in parallel result in the addition of conductance. (conductance is 1/resistance)

So in series, the total resistance (Rt) = R1 + R2 + R3 + ...

In parallel the conductances add up. SO the total conductance (Ct) = C1 + C2 + C3 + ...

Now you're interested in resistance, not conductance, so we can convert: 1/Rt = 1/R1 + 1/R2 + 1/R3 + ....
Or Rt = 1/( 1/R1 + 1/R2 + 1/R3 + ...)

Whether the value is milliohms, ohms, kilo-ohms, megohms, or gigohms, there is no difference in the relationship.

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
But when you see meg ohm resistors in series this does what to a circuit?
Meg ohm resistors in parallel set the impedance not meg ohm resistors in series
Connecting resistors in parallel reduces the resistance as Steve just described. Connecting resistors in series increases the resistance by simple addition. A 10M resistor in series with a 4M7 resistor has a total resistance of 14.7 MΩ.

Resistors above 10 MΩ are not available in standard ranges. Sometimes it is simpler and cheaper to use two readily available resistors in series, rather than trying to find a special high-value resistor from a different manufacturer.

Also as Dave mentioned, it can be done because of voltage drop specifications. If a single resistor is rated for, say, 300V DC maximum across it, and you connect two of them (the same value) in series, you now have a resistor that's rated for 600V maximum across it. Assuming you're careful with the circuit board layout.

#### davenn

Moderator
Sep 5, 2009
14,016
If a single resistor is rated for, say, 300V DC maximum across it, and you connect two of them (the same value) in series, you now have a resistor that's rated for 600V maximum across it.

ahhh indeed ... remember some of those old TV's we were discussing Kris, with 2 or 3 blue bodied hi voltage hi resistance resistors in series
in the EHT sections #### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
Yes, in the focus and A1 circuits. We used those chunky blue resistors for all that high-voltage stuff, and I never saw a failure. Those were high-quality high-voltage Philips metal film resistors, and you could use two or three in series to drop several kV IIRC.

It's funny that lesser-quality high-voltage resistors are still a common cause of problems in cheap switching supplies (failure to start up), despite the fact that this problem has been known for decades now. Saving a few cents there seems like false economy to me... unless they actually want their products to fail after a few years :-/

#### Davewalker5

Sep 20, 2014
125
R110 ( 2 meg ohm resistor ) blocks the voltage and current of output U12 and goes to another output point

R113 ( 4.99 Meg ohm resistor ) blocks like an open switch from current and voltage looking back

These resistors are called isolation resistors

#### Gryd3

Jun 25, 2014
4,098
R110 ( 2 meg ohm resistor ) blocks the voltage and current of output U12 and goes to another output point

R113 ( 4.99 Meg ohm resistor ) blocks like an open switch from current and voltage looking back

These resistors are called isolation resistors
c
I'd be careful with your terms not to confuse the Op.
Those resistors will only 'seem' like an open circuit if the remaining circuit in series has a much lower resistance. It does not 'block' the voltage or current, but significantly reduces it, again, based on the rest of the circuit.
A 1MegΩ resistor connected in series to a 100KΩ will have 91% of the applied voltage present across itself.. that means that if you attempt to 'block' the voltage from say... 220V AC with a 1MegΩ resistor, the rest of the circuit past the resistor will still see 20V. The higher the resistance of the remaining circuit, the less voltage it will 'block'. Again, this is entirely dependant on the rest of the circuit.
The pics you posted, and referring to 'looking back' is a great reason to use higher value resistors, as they are typically used for feedback. These portions of the circuit must remain mostly stable, and larger values resistors for this will help keep the expected readings in a range that varies very little as the remainder of the circuit changes.. Much like a multi-meter.
When measuring voltage with a multi-meter, you end up putting a MegΩ or so in parallel with the circuit you are measuring, if these did indeed 'block' things or act as open circuits, technicians would not be taught to be able to measure and compensate for the internal resistances of the meters they use. This can usually be omitted because the change is very slight, but again, it's good to know as there may be a situation where the circuit you are dealing with will not treat a MegΩ like an 'open circuit'

#### Davewalker5

Sep 20, 2014
125
So what does these Meg ohm resistors do? they just reduce the feedback voltage or current from other parallel branches or parallel paths? but why reduce it?

The Meg ohm resistors stablize the feedback voltage from drifting out of tolerance?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Your question is poorly stated and despite our repeated attempts to explain, you are going nowhere.

You seem to be fixated on these resistors, and I have no idea why.

They're just resistors, ok! We can't tell you what they're doing unless you show us a circuit diagram. And any answer we give is specific to that circuit.

edit: Ah, I now see the circuit. U13 appears to be an integrator. U3 appears to change the resistance in the lower leg of a voltage divider by a factor of 10 (from about 5M to about 500k). A lot of the rest of the circuit cannot be seen.

What is this circuit for?

Last edited:
• KrisBlueNZ