Six considerations for better circuit protection

In an electronic system where the power source can be connected the wrong way round, protection should be considered to prevent damage to the circuitry. Most electronic circuits don’t like the power supply being reversed. However, there are other eventualities you could protect your circuit or system from temperature, humidity, excess current or excess loading, for example. In each case, you not only need to sense the undesirable event you also need to decide what to do about it. For example, with an overvoltage input power supply you could disconnect from your circuitry or (more commonly) short out the incoming supply with a “crowbar” circuit. With over-temperature, you may simply shut down the circuitry causing the heat, or shut down the system if the heat source is external (e.g. ambient temperature). However, a shutdown might need to be orderly depending on the system, not simply disconnecting the power.

Temperature
You could be monitoring heatsink temperature, inner case temperature, ambient temperature or the temperature of a specific chip. Power ICs can have temperature shutdown built in, or temperature monitoring. If shutdown is built in and there is no signal to say it has shutdown due to over-temperature, you may need to detect the shutdown by detecting a voltage, for example. I have discussed temperature sensing before and there are plenty of devices out there to suit a wide range of interfaces and mounting methods. For systems with a microcontroller and general inner case temperature sensing I have used the Microchip MCP9800 series sensors. If you have no microcontroller you could look at something like the Microchip MCP950x series which are switches with a preset temperature.

You could use the output to disconnect power or to shut off a part that is generating heat. You need to consider what will happen when the temperature drops below the threshold – do you want the circuit to restart? Another possible issue could be devices which don’t like to be too cold. LCDs can be bought with built-in heaters for operation at low temperatures. I have designed a system with heaters to preheat the circuitry before applying power where there are devices which will not function to specification at very low temperatures.

Overvoltage
Protection from an overvoltage power supply is often done with a “crowbar” circuit rather than a series device to disconnect the power. A crowbar is “lossless” so even with a high current you will not waste any power but it relies on having a fuse or other overcurrent protection device on the power input. That overcurrent device can itself introduce a small voltage drop. A crowbar circuit monitors the supply voltage and shorts it out if it exceeds a threshold. Unfortunately, devices such as the On Semiconductor MC3423 crowbar IC are now obsolete so you are likely to have to design your own circuit unless you can find a power supervisory chip with the required function. You will need a voltage reference, comparator and some form of power switch such as an SCR or MOSFET. Below is the MC3423 block diagram which shows the principle.

The output would drive an SCR.

Littelfuse Polyzen devices provide a crowbar with PTC (positive temperature coefficient) series protection so they are one chip solutions, but the lowest voltage is 5.6V nominal so they cannot be used to protect devices running from 3.3V supplies for example. Series resistances can be as low as 0.04 ohms. You need to take note of the maximum voltage you can tolerate. Some newer 5V Linear Technology chips, for example, have 5.5V for the absolute maximum voltage and the can be damaged very quickly (a few µs). Older 5V devices tend to have 7V or 6.5V absolute maximum voltages.

Overcurrent
The simple overcurrent device is a fuse. However, they are slow to blow depending on the level of overcurrent. Even a fast blow fuse can take 100ms to blow at twice the rated current. Below is a sample of the type of time to blow response of the Littelfuse 672 series fuses. As you can see, for a very high overload fuses are fairly fast.

PTC devices are often used but they are even slower than a fuse – you are relying on a device which has to heat up in order to increase its resistance and therefore reduce the current. Also, bear in mind that PTC devices don’t break the circuit completely — they increase in resistance so current still flows and if the current drops below the “holding current”, the device will cool down enough for the resistance to decrease.  If you want a fast response without a massive current overload then you may need to design your own overcurrent circuit. Bear in mind that a small current peak may be acceptable though, such as when a motor starts or power is switched to a circuit with a high input capacitance.

ESD protection
Wires connecting signals externally to a system may need extra ESD protection. While most active devices have built-in ESD protection, that is usually intended to protect against accidental static discharge from a person during assembly. It is not usually intended for the sort of spikes that might occur when using equipment in an electrically noisy environment. Devices intended for connection to the outside world can often be selected with a higher level of ESD protection. For example, the Maxim Integrated RS232 line driver/receiver such as the MAX3322E is protected to 15kV (human body model/IEC1000-4-2) whereas devices are often only rated to 2kV (assuming they specify the actual level). For extra protection for sensitive devices, you can add ESD protection diodes which are also available in arrays for ease of protecting multiple lines. Devices rated for automotive use tend to have higher levels of protection as they need to work in environments with discharge interference from ignition systems.

Humidity
Protection from humidity is normally physical. If you think you will have problems with humidity causing condensation on your circuitry you would seal it or use a conformal coating to keep out the moisture. In extreme cases, you would either add a desiccant and/or fill with dry nitrogen. That relies on a hermetically sealed system. Some systems have removable desiccant holders so they can be periodically replaced where some air leakage is anticipated.

Excess Loading
A problem of excess loading, such as drawing too much current from a motor drive or other high current output, is similar to the problem of power supply overcurrent. The difference is that you will probably only shut down the problem drive rather than the whole power supply.