Back to Basics: TVS Diodes

Electro Static Discharge (ESD) is one of the most critical things to guard against a mass-produced product. They can easily damage your product, especially in areas where someone can potentially touch it like exposed pads, USB connectors, screens etc. To protect against this issue, you use a Transient Voltage Suppressor(TVS) or ESD diode on the input side of your circuit. TVS diodes and ESD diodes are technically more or less the same as TVS diodes capable of protecting against larger surge currents too. If you are only worried about ESD, then use the smaller ESD diodes.

You connect ESD diodes right at the input(In parallel, across the rest of the input), usually very close to the part which is exposed. There is a common misconception that the ESD diode will absorb all the transient energy just because you put it somewhere. Well, that’s not how it works. ESD diodes help shunt away the high voltage back to the source, hence it’s imperative that you choose a diode that is fast responding and is placed as close to the input as possible so that the energy put in can return back faster thereby protecting circuits downstream. Never use Zener or other types of diodes for this as it’s usually slow to react to the pulse of an ESD.

IEC 61000-4-2 is the testing standard applicable for ESD before releasing your product on the market. Your product will be tested with an ESD gun capable of injecting a voltage of the order of kilovolts.

They are usually of 2 types, Air Discharge(That is when you bring your hand close to a device but not touch it. Think sparks from fingertips due to charge build-up) and Contact discharges(Direct touch). There are different levels from Level 1 to Level 4 increasing the amount of protection your device is ensured against. For example, Level 4 protects against +-15kV(Air Discharge ) and +-8kV(Contact Discharge) There are lots of considerations in choosing diodes and types of them. Can’t fit them on a single post. If there is some interest, will probably do a detailed series later on.

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Back to Basics: PTC Fuses

PTC Fuses

Resettable fuses are one of the commonly used components in an electrical design. If you have never used them, consider starting to use them if your device will be connecting to an external power source. In case of an accidental short, it’s a live saver for your circuits. These are components that will sacrifice themselves temporarily(Reusable) to protect circuits that follow them. There are a few things to consider when you are selecting fuses in your design and its important for any new designer to know what they are

Trip Current: Minimum current needed to “trip” or stop the current flow at around 25C. This is the minimum current needed to activate your fuse, so make sure that your circuit current draw is less than this. This is the upper threshold and is dependent on temperature. Check the datasheet for graphs. Usually, this value will drop for larger ambient temperatures.

PTC Fuse
PTC Fuses

Hold Current: It’s the maximum current that the fuse can allow to go through it over a long time at around 25C. This number (or lower) should be what you should be designing your circuit input current for.

Trip Time: It’s the time taken by the fuse to activate when the trip current starts to flow. Lower the better.

Rated Voltage: Operating voltage of the fuse. Make sure your circuit voltages are below this number.

Always remember that PTC fuses are resistive based, meaning there will always be some I^2R power losses on the fuse during its normal use. There will be a voltage drop too as it’s connected in series. So try to select the fuses with lower resistance and always account for this voltage drop so that it doesn’t affect the circuit downstream. A classic example of this mistake might be that you are powering high power LEDs with constant voltage power input.

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