7805/7905 circuit recommendation

dev/null

Hi

I am building a voltmeter (using the ICL7107 chip) which needs +5V and -5V supply. The meter will draw 200mA max.

First question: What is negative voltage? I am really confused about this. Can someone explain the concept behind that?

Second question: The recommended circuit for the supply is as follows:


That seems pretty simple.

Another example (for the plus side):


Not to bad either.

But then I came across this. Now what is the reason for having such a complex setup, and which is the most reliable?


Thanks.

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Don't worry. Be happy

Mike Green

Voltage is always relative. +5V is a five Volt supply connected so that the wire marked +5V is positive by five Volts relative to what's marked as ground. -5V is a five Volt supply connected so that the wire marked -5V is negative by five Volts relative to what's marked as ground. If you measure voltage relative to the wire marked +5V, the wire marked as ground is -5V and the wire marked as -5V is -10V.

The 7805 and 7905 regulators are designed to provide a regulated 5V with the "free" output either +5V (for the 7805) or -5V (for the 7905). It's possible to wire each of these backwards so the +5V output is provided by the 7905 and the -5V output is provided by the 7805, but it's more convenient to use them as they're designed, partly because the heatsink tab is the ground terminal and can be directly mounted to a grounded chassis for better heat dissipation if you respect how they're designed.

Your recommended circuit is actually somewhat wasteful of power since the 18V supply will always be supplying the 2K of resistance which will absorb about 9mA and dissipate about 160mW as heat. The sole function of the two 1K resistors is to provide a "virtual ground" midway between the positive and negative voltages of the 18V power source (see en.wikipedia.org/wiki/Virtual_ground).

dev/null

Very informative Mike. But I am stil confused with minus voltage. Does it in practice mean that the electrons go the opposite direction from a plus potential?

Any recommendations on how I can prevent the power loss from the resistors?

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Don't worry. Be happy