# Resistance between Vcc and Gnd.

Hi people.

When I measure the resistance between Vcc (Red probe) and 0V (Black probe) with my DMM there is not any resistance.

But when I change the probes like Vcc (Black probe) and 0V (Red Probe) there is negative resistance.

Is there something wrong with this?

Regards.

In a nutshell, you can only measure the "resistance" of passive devices.

A DMM is going to try to measure resistance by putting a voltage on the probes, measuring the current which flows, and then telling you the ratio of the voltage over the current, as the resistance.

A transistor or diode is going to conduct in one direction only. If you attempt to measure the resistance using your DMM, and you use the probes one way, and then the other way, then you are putting the voltage on in one direction, and then the other. Current only flows through silicon junction devices in one direction, so of course you will get a different answer.

Your arduino device, or it's processor, has millions of silicon junction devices in it. Attempting to measure it's "resistance" is a fundamentally pointless activity.

Thank you michinyon.

Attempting to measure it's "resistance" is a fundamentally pointless activity.

Unless you want to confirm a device is well and truly fuct ( that is "Further Use Completely Terminated" ) by seeing there is no difference in the resistance no matter what direction you measure it.

When I measure the resistance between Vcc (Red probe) and 0V (Black probe) with my DMM there is not any resistance.

On a circuit board (such as an Arduino) you'll normally read something... An open (infinite resistance) or a short (zero Ohms) generally indicates a fault.

But when I change the probes like Vcc (Black probe) and 0V (Red Probe) there is negative resistance.

If the meter actually shows a minus sign, the circuit is powered-up or there is residual voltage from a charged capacitor. When you reverse the leads on certain components (like a transistor or diode) you'll get a different resistance reading, but that's NOT "negative resistance", its different resistance with current flow in the opposite direction.

You can't read resistance with power applied because your DMM measures resistance by supplying a known current and measuring voltage.

You can measure the effective resistance of a board or active circuit by powering-up the circuit with an ammeter in series, measuring current (and measuring voltage if it's not already known) and then calculating resistance (R = V/I). (I is current.)

Normally the "resistance" will appear to change because you are actually charging up
the decoupling capacitors from the multimeter's ohm measuring circuit.

Maybe it’s just my meter, but my meter can almost always find a resistance, though it may be high, between ground and ANY pin on almost any IC. I exploit this to check surface mount components that I attach to “adapter boards”, which I use for almost every project now. I just put one probe on ground and go around the probe every plated through-hole that should have a pin soldered to it. Infinite resistance means check that again under the microscope and be more careful. Also, it’s a great way to check for solder bridges - continuity check every through hole with the next one over.

Like this, I would check this guy out after soldering and before soldering pins on. This IC I had not soldered yet. Looks like I was preparing to do so:

It displays a value meaning that current is able to flow, but it's not really what we would call "a resistance".

Sacha22:
It displays a value meaning that current is able to flow, but it's not really what we would call "a resistance".

Are you relying to me? In most cases it is probably leakage through diodes, resistors, or capacitors, or possibly pullup or pulldown resistors but it is calculated in ohms. Call it something else if you wish.