Short Circuit Protection: How to Protect a Circuit from Exploding

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Electronics have a learning curve that does not seem to be leveling off. We’ve all smoked a few components on breadboards due to a misplaced wire or incorrect power calculation, but things only get more sensitive as they get smaller and more complex. An expensive IC can release its magic blue smoke because of something as simple as noise on the DC power line, and a switching power regulator can get hot enough to damage neighboring components. It’s a minefield out there, but these five short circuit protection methods will at least let you protect your system from the most common destructive forces.

1. Inrush Fuse Circuit Protection

A fuse on the input line is often a subject of debate between engineers and customers. They tend to be expensive relative to other components on a small board, and it is a component you hope to never really need. Ultimately, it’s up to you to decide if it’s necessary for your production board, but for prototyping, my advice is just do it.  It will protect all circuitry downstream from catastrophic amounts of current. In a breadboard a glass/wire fuse in a holder is a great tool, because you can clearly see when it has blown.

On a PCB a PTC resettable fuse is a very common choice because they can be wave soldered down with the rest of the surface mount components and does not require replacement if activated. Keep in mind that by definition, they are very sensitive to temperature. As the temperature rises, whether due to too much current through the fuse or neighboring components getting warm, the resistance of the fuse increases and eventually leads to the fuse being an open circuit. Always check the datasheet to verify the fuse you select will pass the current you need at your maximum expected operating temperature.

2. Zener Diode as a Voltage Regulator

Virtually all ICs have a maximum input voltage. If there is any chance of fluctuations on the input power line even briefly exceeding the absolute maximum voltage the IC can handle, you need to clip the voltage with a Zener diode. They are designed to operate backwards and tend to have a higher breakdown voltage than typical diodes. Zeners act like a regular diode with an exceptionally high forward voltage – they will only conduct once a certain voltage is applied. If you are running a system off 12V and the IC downstream can tolerate a maximum of 15Vin, it would be wise to put a Zener diode with a breakdown voltage of 15Vin across the input rails. It will likely never be used, but if the input voltage rises or spikes for any reason, the IC will never see more than 15V. Any power above 15V will be dissipated through the Zener, so this is not typically a recommended way to consistently drop down a voltage.

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3. Current Limiting IC

Though common in LED designs, current limiting ICs are otherwise widely underutilized. Many designers use resistors to limit current, but that equation depends entirely on the voltage. If the input voltage spikes, the resistor will pass a proportional amount of current that may be dangerous for the components downstream. ON Semiconductor has a line of regulators that regulate current within a tight margin regardless of the voltage applied. They offer parts that limit from 10mA to 100mA in a package no bigger than a typical diode.  

0616 Dont Explode IMAGE 2

Larger packages can be used to regulate up to 500mA, and the devices can be put in parallel to increase the current limit. Some of the parts can be adjusted with an external resistor, making them perfect for prototyping.

4. 1000uF Capacitor: DC Line Shock Absorber

Every circuit has a weak link when it comes to power tolerance. Sometimes it’s a sensitive IC that will die if the input voltage wobbles even a bit. Sometimes it’s a power supply that will choke if a current surge is demanded. Prevent conflict with a big capacitor across your DC input. A 1000uF electrolytic capacitor across power and ground will absorb any instantaneous current demands or surges and keep the peace in your circuitry. You can do the math for your particular circuit and expected current peaks, but a 1000uF capacitor rated to at least your maximum possible voltage peak is sufficient for first draft prototyping.

5. NTC Thermistor with an LED: Visual Temperature Indicator

Circuit protection is not just about preventing damage within a circuit – it means preventing damage to you, too! An IC running too hot can cause unpredictable results before burning up, and you can accidentally burn yourself while debugging. It is always good to have a temperature sensor on hand, whether infrared or thermocouple-based, but the right tools are not always within arm’s reach. Get ahead of your own reluctance to dig a thermometer out of your toolbox with a simple NTC thermistor and LED combo. Choose a thermistor that provides just a trickle of current to an LED at ambient temperature so that if the board starts to heat up, the thermistor will decrease in resistance and the LED will glow brighter due to more current.  

For example, if you are running off a 5V rail, use a thermistor like Panasonic’s ERT-j0EA151J that gives a resistance of 150ohm at 25degC, but only 75ohm at 100degC. That will allow about 13mA through the LED at room temperature, but nearly 30mA when the area is too hot to touch. If you use an LED designed to run at 20mA, you will definitely notice an increase in brightness as the temperature increases and can cut power before the chips (or your fingers) take over-temperature damage. 

Power Loss in Wires: Tests to Run

Wires and traces are the sneaky power-sucks that designers tend to forget about when powering up that first board. We hook the power pins up directly to our power supply with the giant 12AWG cables.  We make the traces on our first proto board huge because we want to have space to test, forgetting that the actual board (and traces!) needs to be half that size. When evaluating a board, use the correct gauge and length of wire to hook it up to get accurate results and account for any issues if necessary.

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