Testing IO pins with a DVOM

Can I use my DVOM to test the state(voltage) of the output pins(ATmega168) without damaging the port due to drawing too much amperage? The Atmega is on a device board with several of the pins driving triacs. I want to know if the triac gate is getting power.

I know sounds too simple, VOMs have real high impedence, but I ask because over the years I’ve destroyed way too many devices by accident.

Paul

Since a pin can sink 20 or 30 mA, the almost insignificant current drawn by a Digital meter (on volts that is) won’t be a problem. Just to make sure you could measure the voltage through say a 5k resistor, it shouldn’t make much difference to the reading and will give the arduino board protection against a short circuit if you say tried measuring it on the current setting.

Interesting question, something that also puzzled me when I was starting out. Maybe you know what I saying here, but then at least it can help others who don’t know this.

Even though the microcontroller chip can source and sink current in the milliampere (mA) range you mustnt forget that milliamperes are still big compared to the small currents like mircoamperes (uA) and nanoamperes (nA) you can work with in small signal electronics.

According to the way a veasurement equipment must work:

An ideal voltmeter draws no current at all and therefor has an infinite resistance.

An ideal ammeter will have zero resistance.

We don’t live in an ideal mode, but when they design multimeters they aim is to get as close as possible to having the meter draw no current.

In the old days there were moving coil meters and you still get them, these have to draw small currents to actually move the needle.

Nowadays thanks to digital electronics we have digital multimeters (DMM) which draw almost no current and certainly that is drawn is very very low, I am not sure of the exact current but it will vary across brands. Regardless it will be low enough for almost any DMM/DVOM to be used for digital electronics.

The reason for this is that the DMM’s use FET transistor inputs and FET’s have very high input “resistances”.

You can be sure that the cheaper DVOM’s will probably draw a bit more current than the better quality ones. If you can afford it go for the higher priced ones.

I am relatively new to the Arduino and Atmel land, having come from PIC land. I have never had a problem with PICs even with some of the cheaper meters.

Like pluggy said make sure you don’t measure it on current setting, but thankfully most meters require you to change the socket for the lead when switching to current measuring mode.

I have destroyed a couple of PICs already but accidently shorting pins with the multimeter leads. Even though I am aware of it still happens ;D

Are you driving the triacs directly from the port pin or are you using a driver stage inbetween.

You can only measure the output like this if it is stable. That is if it is a solid unchanging logic one. You can use it if the output is pulsed or rapidly turning on and off like PWM. If you connect the arduino output directly to a triac (without a resistor) you might be drawing too much current for the device. On the other hand look at the data sheet for you triac if might be it needs 20V to turn it on.

One final point it you are driving the gate of a triac IO would go through an opto isolator because the main part of the triac will be switching AC and any error or circuit failing here would drive negative voltages into the Arduino and kill it.