Column: Circuit drill
The sensitivity of an overvoltage protection circuit
By Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions O
vervoltage protection circuits are an essential part of many systems and appliances. Tey allow voltages across the load to a pre-set maximum,
preventing anything above that from reaching the load. Generally, overvoltage protection
circuits handle gradually-rising input voltages well, but not high, sudden input voltages. Figure 1 shows a common overvoltage
protection circuit, designed to pass 10Vdc, above which transistor Q2 switches off and prevents the electrical current from passing through to the load. Figure 2 shows the voltage applied to the circuit. For representative and accurate
results, the experiment was repeated for input voltages between 11Vdc and 50Vdc. Te load resistance was varied between 10kΩ and 200kΩ.
Experiments It was noticed that, when a sudden high voltage is applied to the overvoltage protection circuit, the circuit responds aſter a delay, which exposes the load to voltages higher than 10Vdc; see Figure 3, where the shaded area shows the time duration the load is subjected to overvoltage. In effect, the load is unprotected during this time. At different input voltages and load
resistances, the time during which the load is exposed to overvoltage is monitored and plotted in Figure 4.
Tis leads to the conclusion that the
overvoltage protection circuit is not very sensitive or responsive to sudden high input voltages. Its sensitivity varies according to the applied input voltage and load resistance. For example, for a load resistance of 200kΩ and a sudden voltage of 50Vdc, the circuit will pass the full 50Vdc for about 10.33µs. Although brief, this time is sufficient to cause major damage to some loads. Hence, it is always recommended to check the response time of the overvoltage protection circuit used and whether its response delay could cause a problem to your load. Overlooking such important parameters during circuit design could cause unnecessary damage to the load and your design.
Figure 1: Overvoltage protection circuit designed for maximum input voltage of 10Vdc
08 March 2022
www.electronicsworld.co.uk
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