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Using an 5V out voltage regulator LM7805/NTE960 with a project that doesn't always need to use the regulator and may run on a battery.

The regulator will always draw some milliamps from the battery even when everything is powered off (I have an MCU on another regulator than can power off components connected to this regulator when not needed).

This will reduce the time the battery can be used. How many milliamps will be used though? (If it's less than 5ma, then it's probably not worth worrying about).

I'd like to have a transistor connecting the GND pin on the regulator to actual ground. Then I think I can turn off the regulator by using the transistor.

I'm using a BC516 transistor, with the emitter connected to actual ground, collector connected to the GND pin on the regulator. When I measure the voltage between the Vout on the regulator to actual ground, I get 10V. When I measure the voltage between Vout and the GND pin, I get 3.4V, when in both cases I need 5V.

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    \$\begingroup\$ I don't think it is a good idea to disconnect the ground from the regulator, you should cut the positive input supply instead. \$\endgroup\$ – alexan_e Feb 24 '14 at 9:09
  • \$\begingroup\$ I would look for a voltage regulator with a shutdown input. \$\endgroup\$ – Uwe Sep 14 '16 at 7:45
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If your ampere requirements are less than 300mA maybe this could work for you: -

enter image description here

It's 5V, low quiescent current and can be turned off/on with a control line.

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    \$\begingroup\$ +1, it's a lot easier to use a screwdriver than spend all day hammering a screw. \$\endgroup\$ – Matt Young Feb 24 '14 at 15:56
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If you open the ground connection on the 7805, the output will equal the input minus a couple of volts. Probably not what you want, and it could damage the load.

You could use a high-side p-channel MOSFET switch in front of the 7805 but it's easier to just use a regulator with an enable input as others have suggested.

Here is a typical circuit from Power Electronics:

The 7805 input would be your load. The input capacitor for the 7805 should not be switched.

enter image description here

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Checking the datasheet of the 7805, the quiescent current is indicated as 5mA, so maybe it's not worth bothering (depending on your battery capacity), also given the cheap price of the 7805 ?

What output current do you need ? For example, if 50mA is enough, there is also the LM2936, which is supposed to have a very low quiescent current. It is however more expensive (~4x) than the 7805.

The Linear chips, with which I am unfamiliar, look interesting but are more expensive apparently (I found for example an LTC1474 with standby mode and a slightly higher output current of 400mA, but it is ~10x more expensive than the 7805).

EDIT/PS : some of these Linear chips have the advantage of being switched supply (contrary to the 7805), so that they will spare some battery power otherwise dissipated by the 7805. The downside is that they require a few more external components.

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