Laptop Multimeter Tests - Checking Voltage and Short Circuits

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The Laptop Repair Workbook

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Copyright 2010 by Morris Rosenthal

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Copyright 2010 by Morris Rosenthal -All Rights Reserved contact info

The printable eBook version of The Laptop Repair Workbook is now available for download anywhere in the world.
Laptops aren't resistors, which is the short way of saying you have to think about what you see when you try to diagnose a laptop with a multimeter. One of the more useful tests you can do is to measure the resistance between the positive pin of the power input (usually the center pin of the connector) and ground. The outer shell of the connector is ground, but it's not usually possible to get both multimeter probes into the port without touching each other. Any exposed metal shielding on the outside of the laptop, such as the metal around USB ports, the video output, etc, should be connected to ground, which you can test separately. When you do find a good ground, the input resistance for a healthy laptop may be anywhere from a few hundred Ohms on up. Measuring on the 20K Ohm scale, this particular laptop read 6.48K Ohms. If you get a reading of just a few ohms or less, there's a short circuit. checking laptop input impedance
checking laptop power regulator board resistance To the left I'm testing the input resistance of a power regulator board. The connector on the bottom edge of the board married this daughter board to the laptop motherboard in the same plane. The connector to the left went directly to the battery bay connects. The input impedance of the board, reading 918 Ohms, was the same when the board was installed in the laptop, with the battery. Power regulation boards go for as little as $20 on eBay as pulls, and they can often be purchased new from Internet based liquidators on reasonable terms. The power board includes a couple of fuses that I'll get to on another page. If you don't own a multimeter, I just had a look at the ones on Amazon, and here's a middle of the road model for $24.99.

Testing the output voltage of an AC adapter is pretty simple, providing it's a standard barrel connect so you can get the positive (red) probe inside the barrel and use the ground probe on the outside, without taking any chances of touching the two probes together and shorting the output. The problem is, laptop AC adapters are switching power supplies, and they may require a load to start generating a voltage. It doesn't need to be a perfect load, and the multimeter may be enough. But, if you observed that the status LED on the AC adapter was lit when it was plugged into the laptop, and now that you've borrowed a meter and are seeing zero voltage, it's not lit, it's because it's not seeing enough load to fire up. Plug it back into the laptop for a moment, the LED will probably come on, and then stay on when you remove it from the laptop. The voltage should read a little higher than the voltage on the label. Testing laptop AC adapter voltage
Length hazard of replacement DC connector The reading above is 19.8 Volts DC on a 19.0 Volt labeled AC adapter. If you've replaced the DC end on your AC adapter, be very careful when checking voltage. As mentioned above laptop AC adapters is that they are switching mode power supplies. While this gives them great advantages in low weight, low cost and flexibility on the input voltage, they may put out some audible high-frequency noise when not attached to a load, such as when the battery is charged and the laptop is turned off. As you can see in the picture to the left, the multi-meter probe is long enough to travel all the way up the inside of the barrel connector and possibly connect the ground. Unless you want to experiment with low voltage welding (AC adapter destruction) you don't want to create a short.
Manufactured (molded) cable ends are much less likely to be open, but there's no need to jam the multimeter probe all the way up the connector when checking the voltage. Another test you can do with your multimeter is to unplug the AC adapter from it's power source, the 110 V to 240 V wall socket, and look at the impedance at both ends. From the AC input, measuring between the recessed pins where the AC power cord would be plugged into the brick if it were powered up, you should see hundreds of kilohms (K Ohms), which means you have to switch to the megaohm scale to get a reading. If you get a beep on the continuity scale or a value less than an ohm on the 200 Ohm scale, it's a short circuit, and it shouldn't be plugged into live power. If you look at the impedance on the DC output side (this is still with no power), you should see a reading that keeps moving, as the capacitor charges up and the resistance increases. Input impedance of laptop AC adapter shows capacitor charging to open
If you start on a high scale, 200 K Ohms or more, the reading might start by dropping and then stabilize at some number of K OHms, but if you start on a lower scale, you should just see the capacitor charge up and the circuit look like an open (over scale or a "1" on many meters). Again, if there's no resistance, it a short you can't plug it in until you resolve it. A short on the DC connector end is likely in the connector. And remember that shorting the output, even for a moment, will often fry the brick, so be careful with your probes!
The printable eBook version of The Laptop Repair Workbook is now available for download anywhere in the world.