Column: Circuit drill


Using BJT switching circuits in outdoor applications imposes the challenge of keeping them properly functioning at a given ambient temperature, and especially at high temperatures


The reaction of switching BJTs to different ambient temperatures


By Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions Company


B


i-junction transistor (BJT) switching circuits fit into a wide range of applications, suitable for either indoor or outdoor electronic devices. However, before creating such a circuit, it’s wise to check its switching rate and response time for stable performance in the application. For


example, a slow-response switching circuit will be wrong in a high switching application. Figure 1 shows two circuits: (1) a one-BJT switching circuit, and (2) a two-BJT switching circuit.


Why this experiment? Using BJT switching circuits in outdoor applications imposes the challenge of keeping them properly functioning at a given ambient temperature, and especially at high temperatures. It’ll be interesting to determine the impact of such a temperature on the circuit and the limitations it imposes on the circuit’s performance. So, what should electronics circuit designers consider when designing these circuits? For the purposes of this experiment, the ambient temperature was changed from -55o of 25o


C to 125o C, in increments C. The input signals at V3 and V5 are a square pulse of


2µs on and 18µs off. The responses were measured and plotted for comparison.


14 June 2021 www.electronicsworld.co.uk


Experimental assumptions We did the following: • Typical BJT circuits were selected, such as 2N3904 since it is commonly used, and, likewise, the 1N4007 diode (D1, D2) for its universality.


• Same bi-junction transistors and diodes were used in both circuits.


• The two circuits were tested for an ambient temperature range from -55o


C to 150o recommended operating range.


• A switching square-wave signal was used with a 5V amplitude.


• The response time required for the circuits to change state (from on to off) was measured in every scenario for both circuits. Circuit 1 showed a reasonable response delay of 2.4µs C, and 5.4µs at 125o


at -55o C; see Figure 2.


a longer response delay of 5.1µs at -55 o 125o


C. Circuit 2, however, showed C and 15.5µs at


The table and plot in Figure 3 show the response times of Circuit 1 and 2 for different temperatures.


Substantial response The response times show that both circuits responded meaningfully to ambient temperature variations. This is an important finding as the circuit designer has to make sure the circuit is not demanded to change its state (back) before this delay elapses; otherwise, the switched signal will be severely distorted. For example, suppose the input signal chain switches from on to off state before the output signal delay is over; the resultant output signal will not be relevant to the input signal, showing output signal distortion. Another important finding is that the one-BJT


switching circuit behaves much better than the two-BJT switching circuit, since the one-BJT circuit has shorter response time and can handle faster switching demands.


C, as per the BJT manufacturer’s


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