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Forum Home > Tips, Tricks and Tutorials! - Builder's Guides for GuitarPCB Boards! > Sticky: DMM (Digital Multimeter) How, What and Why.

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In regards to measuring Voltages using your DMM here are some basic tips.

When using the meter it is possible to follow a number of simple steps:

1. Turn the meter on and Get into the right mode!

There are often two seperate modes for AC and DC voltage.

Both will have a V but one will have two lines, one dashed and one solid (DC) and one with have a wave next to it (AC).

For Pedals here we want DC!!

2. Insert the probes into the correct connections - this is required because there may be a number of different connections that can be used.

3. Set switch to the correct measurement type and range for the measurement to be made. When selecting the range, ensure that the maximum range is above that anticipated. The range on the DMM can then be reduced as necessary. However by selecting a range that is too high, it prevents the meter being overloaded.

4. Optimize the range for the best reading. If possible enable all the leading digits to not read zero, and in this way the greatest number of significant digits can be read.


Testing Voltages of Transistors is very helpful in Debugging a build it looks like this:

Any Ground plane will do for the Black Probe then start taking Voltage readings and report them to us. This applies to Transistors as well as IC Chips.

Google Datasheets to report the proper Pin Numbers or Letters with your reading.

Example 1: Google "2N3904 Datasheet" then you will know which leg is C, B or E.

Example 2: Google "MPF102 Datasheet" to know which leg is D,S or G   etc....

6. Then post a Forum Topic in the Support section complete with Pictures and your Voltage Readings!

7. Get or Build an Audio Probe! - This will save you a lot of time and guessing!

Also in regards to Audio Probes you can read this helpful link from Tips and Tricks:

Using an Audio Probe to Debug Builds


An explanation from Tonmann on DMM's:

Continuity testers that are built into DMMs work on the following principle:

The resistance between the two probes is measured, if the resistance is below a certain level (usually 40 -50 Ω;) a buzzer will sound.



The meter in the link above doesn't have a continuity tester so the only way to test is to set the meter to the 200 Ω range and read off the display. The main disadvantage here is that you have to look away from what you are measuring to read the display. Hands up all those people who have had to look away from what they are measuring only to have one of the probes slip or short circuit against a neighboring component !!!!! I bet I'm not the only person with my hand in the air.



So, yes you can test cold joints with this DMM.



Before you buy a DMM you should be considering the following:



How often are you going to use the DMM ? If you are only building one or two circuits and you have no other uses for a DMM then buy a cheap one.



How many functions do you need ? All DMMs can measure DC voltage and resistance, the two parameters you need for circuit fault finding. Extra functions such as continuity tester diode tester, BJT transistor tester, capacitance measurement etc cost more money, again if you are an infrequent builder or are just a "voltage and resistance measurement" person, then buy a cheap DMM.

How accurate do your measurements have to be ? Cheap DMMs are not accurate (that's why they are cheap). Fortunately voltage and resistance measurements in pedal building don't need to be all that accurate - 1.7V on one DMM is just as good as 1.745 V on a more expensive DMM. If you need to be accurate for capacitance measurement, diode forward voltage measurement, transistor hfe, then accuracy costs a bit more money.



Luxury Extras - these include auto ranging (the DMM selects the correct measurement range), measurement hold functions (hold the current value, hold the maximum value, hold the minimum value) data export (stores all of you measurements to your computer) etc. These are rarely  needed by anybody except hard core designers and people who have a lot of money to waste, although the most useful of these functions (auto ranging) can be had on most reasonably priced DMMs - I'm looking at one for $40.



So basically it is for you to decide - buy a cheap one for just basic measurements - accuracy and luxury cost extra.



Lastly few words on resistance measurements and continuity testing for cold joints.

Never take resistance or continuity measurements with power applied to the circuit - one sure way of destroying the DMM. I always test my solder joints once I've finished soldering before power is applied and not when a fault occurs. It takes five minutes to continuity test the circuit, how long does it take to find a fault ?



You can't measure resistance accurately once the resistors are soldered to the circuit board - unless you know how to read a schematic, find the parallel resistance branches (if any) connected to your point of measurement and then do the maths to calculate what the resistance should be. Resistors should be measured before you solder them - something I still do to every resistor and that's after thirty years "in the biz".



Always test for continuity (and measure voltage for that matter) on the top side of the board on the component leads. If you try to test on the solder joint, the pressure of your probe could make a cold solder joint look good until you take the probe away and are left with no connection between the solder and the component lead.  The only exception I can think of is a box metal film / electrolytic capacitor where the component sits tight to the board and the leads aren't visible on the top side.


Also an article from Tonmann regarding Measuring resistors when in a Circuit with a DMM and why we can't do that!

Let's have a look at what goes on when you measure a resistor out of circuit with a DMM:

When you clip your red and black probes across a resistor and select "Resistance", the DMM applies a DC voltage across the resistor which causes a current to flow through the resistor (the green arrow). The DMM measures the amount of current flowing through the resistor and then calculates the resistance - the DMM knows how much voltage it has supplied, it has measured the current, and by Ohm's Law (R = V/I) it can calculate the resistance - clever things these DMMs.

This is what your "no other components between the leads" diagram shows.

When you put the resistor in a circuit and then try to measure it's resistance you can end up with a totally different resistance reading. By the way, if you are measuring resistors "in circuit", you must have the power supply disconnected.

In the next diagram I've taken only the relevant parts of the circuit when measuring R4 and/or R5:

Applying the probes across R4 causes a current to flow through R4, it also causes a current to flow through the drain-source of Q2 and Q1, through R2, through R5 via the ground connection and back into the DMM.

The DMM is now measuring more current than it did when the resistor was out of circuit. As far as the DMM is concerned, more current must mean there is less resistance. - remember the DMM "knows" how much voltage it applied, has measured more current and calculates the resistance, R = V/I.

In the two right-hand diagrams, I've made the circuit (hopefully) easier to understand. As I said in my last post, what we have here is a series/parallel circuit. I've replaced the JFETs with their drain-source resistances - a few hundred Ohms each; add the resistances of Q2, Q1, R2 and R5 together, that's the series part of the circuit which we can call Rs and then calculate the parallel part of the circuit - (R4 * Rs) / (R4 + Rs).

I hope this explains everything for those not interested in the techie part, and has given some insight on circuit calculations for those who are interested.

June 2, 2011 at 9:27 AM Flag Quote & Reply

Site Owner
Posts: 8793

You can get a real cheap DMM from Harbor Freight for example that will do the job for most of you here:

Personally though I recomend this bad body for a few bucks more!!!

I like it because it  measures Capacitors, Transistors and Diodes and I find it very reliable and easy to read:

June 2, 2011 at 9:43 AM Flag Quote & Reply

Posts: 633

Nice video introduction to DMMs


Happiness is a warm soldering iron

February 3, 2014 at 2:07 PM Flag Quote & Reply

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