5 ways to detect short circuits in electronic boards

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5 ways to detect short circuits in electronic boards

Short circuits are one of the most common causes of failures in electronic boards. Almost any component is susceptible to short circuiting. Even the printed circuit, or the welds. In this article I am going to tell you five ways to detect short circuits on a board. Let’s see them.

1. Locate a short by measuring resistance

Obviously a short circuit is a zero ohm resistance at a point where it should be much higher. But in reality, the resistance is never equal to zero ohms. There is always a little resistance, no matter how low. With a little understanding of physics and basic electronics,

we will know that resistance depends on length. Without going into maths, the longer a conductor is, the more resistance it has. In a short circuit, the closer we measure, the lower the resistance. And the further we go on the plate,

the higher the resistance. So far it seems logical and easy. We simply measure the resistance at different points on the track, focusing on the points with a lower value. There will be the component, track or solder that causes the short circuit.

The problem is that this is very easy when we measure a few metres of cable. But on a plate, everything is very close. Therefore, the difference in resistance is minimal. We are talking about millions. How do we measure such low values with the multimeter?

Well, it’s difficult, unless we have a very sensitive team. The normal thing here would be to use a milliohmmeter. Basically, it is a special equipment to measure very low resistances, even in microohms (μΩ).

2. Locate short circuits with thermal imaging cameras

 A thermograph is a thermal image similar to a photograph, but with colours proportional to temperature. In this way, the hot areas have a very contrasting colour with the cold ones. A short circuit causes the current to be very high in the affected area.

This is why it is sometimes really easy to locate a short circuit with a thermal imager. Although it is also possible that a specific point is not marked, but an entire area, or several tracks of the printed circuit in which there are many components.

To differentiate which element is the cause, you have to “see” the path that the current is following from the power supply. This way you will be able to deduce which element “downstream” causes the overload “upstream”.

3. Locate a short circuit with the evaporation of a liquid

 As we have seen, a short circuit causes overheating in an area of the board. This means that if there is a liquid on the plate, it will evaporate faster in the hotter areas. So it seems very logical to wet the plate, and then apply tension to see which area dries out first.

Although, on second thought, mixing the words “wet” and “tension” in the same sentence does not make so much sense. Most liquids reduce insulation, or can cause electrolytic effects. So care must be taken when using liquids on plates under tension.

I have done it with isopropyl alcohol, on digital boards, where the voltage is usually about 5V. Also, because it’s so volatile, you can quickly see where there are hot spots just by turning on the power. As the alcohol evaporates in a very short time,

it hardly affects the operation of the hob. Of course, never use liquids with plates that work at mains voltage, much less if they work with high voltage. I have seen lightning travel through a wet plate, in corona effect equipment, or diesel burners,

which work with voltages greater than 3kV. To avoid conductivity problems, oils can be used, which with high temperatures, typical of a short circuit, generate quite visible white smoke.

The problem is to clean after the oily remains of the plate. Ideally, use a special electronics cooling spray. It cools a lot, and leaves little residue, so you have to use it quickly and with more care. 

4. Locate a short circuit by touching components with your fingers 

4. Locate a short circuit by touching components with your fingers 

This is the least recommended technique. But I’d be lying if I said I’ve never applied for it. On boards with very low voltages, 12V or less, it is sometimes enough to touch the components to discover abnormal temperatures.

I will officially tell you not to do it, and to use the above methods. But since I know that few will listen to me, I will tell you to be especially careful. If a part is very hot, it can cause burns. Even your skin can be left as a covering for some resistance if you are not careful.

Touch very lightly, before resting a finger on a component. And, of course, it is VERY IMPORTANT that you make sure that the board is isolated from the network. Many appliances have transformerless power supplies,

so touching any component can put you in contact with the mains. So before using this method, make sure you have fully assessed the risks. Remember that when we talk about electric current, any mistake can be the last. 

5. Use a laboratory power supply

Surely in one of the previous points you have already asked yourself this question: How do I measure the board with voltage, if the short circuit causes the power supply to be protected or the fuse to blow? It’s a great question,

I congratulate you for coming up with it. You can apply a voltage to the area of the board affected by the short circuit. Of course, this stress can cause further damage, such as burning the copper tracks. That is why the ideal is to use a laboratory power supply, which allows you to adjust the voltage and current.

If you are measuring, for example, the 5V supply of a section of digital electronics, you can connect the supply with 4V and 50mA. This means a power of about 20mW, which is bearable by most components. In this way you can see if any component heats up, following the methods already explained.

If nothing gets hot, you can slightly increase the current, for example to 100mA. I don’t recommend going much higher unless you’re feeding a power section. Normally this value already generates enough heat in the affected area, especially if we are using a cooling spray.

Of course, be careful with refrigerant aerosols, because they can suddenly contract materials and crack them, such as ceramic diodes and capacitors. 

What technique do you use to locate short circuits in electronic circuits? 

I have told you five techniques. They are not the only ones, there are many more. Which one did you like or surprised the most?

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