# How to measure resistance of resistor in PCB? [closed]

I have a resistor 1MOhm soldered in a PCB that looks like that And the schematic is looking like that: When I measure the resistance by touching the a multimeter on the two ends of the resistor the measured value is around 500KOhm. When the resistor is not soldered the value is the correct one 1MOhm measured by the same multimeter in the same way. Why is that and what is the resistance value seen from the resistor "input"?

If we make the equivalent circuit of the measurement it looks like that, doens't it?:

simulate this circuit – Schematic created using CircuitLab

• Instead of asking for examples how this could look like you should rather use the example you already have. That is, show us the schematic Sep 1, 2015 at 7:58
• I'll do that, but it won't help. Sep 1, 2015 at 8:06
• Your schematic won't help because you do not include everything. You assume it is the part shown but if the circuit was only the part you show then this cannot explain measuring 500 kohms instead of 1 Mohms. Are you measuring R205 of 910 kohm ?? It looks like it's floating doesn't it ? Is it ? No it is not, there's a node called Iref, where does that go ? I do not see it in the schematic, thus the schematic is incomplete. Sep 1, 2015 at 9:04
• It won't help, not because I haven't included everything, but because the resistor is connected to a display pin. The display is not shown since it's not part of the design. Inside the display there is an integrated controller SSD1322 for which I don't have schematic. You can look into the datasheet if you wan't but this question is already finished. Sep 1, 2015 at 9:16
• It will always help since the schematics is all what is needed to analyse it at the level needed. But nobody forces you to do it, so nobody can exactly explain your case but only speculate about the general mechanism, I.e. "some resistances or similar things in parallel" Sep 1, 2015 at 10:02

• If the resistor is placed in a circuit. Then you'll measure the equivalent resistance value of the resistor parallel with the rest of the connected circuit. Just like here every component has it's own static or dynamic resistance, a diode, transistor, ic... If you now measure the resistance of a component none of them will show it's own resistance but only the circuit resistance.

• It can also be that you've made a bad soldering connection and short the other end also to the ground-plane.

• Another option is maybe a residue of soldering flux around your component shorting it's terminals. You can remove this with a cotton-tip and the right product.