Electronic – how does this power supply work

power supplytransistorszener

I was reading the datasheet for the LT1161, and dicovered the below construction in an example application toward the end of the document (Page 11). It appeared to be powering some Logic ICs.

This clearly seems to be clamping the supply voltage to something the logic IC can handle, so how does this work, and how is it better than just using a zener with a resistor?

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simulate this circuit – Schematic created using CircuitLab


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Best Answer

This is quite easy. Remember that BE voltage of a bjt is around 0.6V when active. The Zener has a 5.6V drop.

So, the zener sets the base voltage of the transistor to be 5.6V, this makes the Emitter voltage 5V.

As long as the Zener is active and the BJT is active, the emitter voltage will be nearly 5V(depending on tolerances). This is regardless of all other conditions. So, regardless of whatever else the circuit is doing, we have a stable voltage point of 5V. We just need to make sure the zener is active and BJT is active, which requires setting R1 low enough(mainly for the BJT).

The reason why it is better than just the zener, is that it has very little output impedance. The zener would be R1, by itself. With the BJT, it is much lower and can drive much larger loads, depending only on the BJT's current carrying capacity/internal resistance.

One can add a cap to the base and ground to reduce any fluctuation further stabilizing the emitter voltage. The cap would be smaller than using the same on the emitter to stabilize.

So, such sources are good because they provide a cheap(time, cost, real estate, etc) way to produce a different rail voltage with a low internal resistance. If one just used the zener and resistor, and the circuit required a large amount of power for some event, it would be under supplied creating anomalous behavior.