7809 output increase

Question

I’ve been searching the web for ways to increase outputs on voltage regulators recently and have seen an example using a 7805 with two 0.1uf capacitors on each side while also having two zener diodes (1N4148) on the ground. I’m new to learning but this example states the zener diodes increase the output by 0.7v per diode allowing for a 6.4v output. I’m curious to double check if this would work and/or if the capacitor sizes are correct for said function.

I’m looking to apply the same effect to a 7809 regulator to have an output of 10.4v but I’m in over my head trying to figure out capacitor sizes and the worry of it not working has me hesitant to commit to any designs.

Answer 1

1N4148 is not a Zener diode, just a regular diode.

It will work just as well with any 78XX regulator, there is nothing 7805 specific about it.

Adding diodes does not also change the capacitor values in that regard.

On the other hand, if you need to fine tune the output of a fixed regulator, perhaps the right thing to do is to directly use an adjustable regulator.

Answer 2

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. a) 5 V regulator. b) Boosted regulator.

The 1N4148 isn't a Zener diode - it's a regular small-signal diode made of silicon and capable of about 100 mA or so. It has a forward voltage drop of 0.7 V (capital V for the symbol).

The 7805 regulator works by adjusting the output voltage to 5 V above the voltage on the "GND" pin. That means that if we can hold the GND pin at some other voltage then we can adjust the output voltage.

The diode works because the regulator passes a few milliamps out on the GND pin. Thi is shown as 4 mA on the simulation of Figure 1a. If we run this current through one or more diodes the effect will be to raise the GND voltage by the diode's forward voltage drop at that current - 0.645 V according to the simulation.

VM4 shows that the GND voltage has been raised to 1.29 V by the addition of D1 and D2. This results in the regulator's output being raised to 5 + 1.29 = 6.29 V.

All of the 78xx series regulators work in a simular fashion so the 7809 output voltage can be raised in a similar fashion.

^{simulate this circuit}

Figure 2. Using a Zener diode to raise the output voltage.

Note the different symbol for the Zener diode and its reverse connection.

The capacitors are for stability and prevent oscillation of the regulator. Typically 10 nF will do the job. I've shown a 10 μF on the input but a smaller one can be used if the input supply is stable.