Voltage switch - selection of component values

Could someone help me choose the value of the elements in this key? I am total begginer and I don't know how to start with it.
I want the 5V power supply to turn on when the voltage on the battery will be below 3.3V.

Hi, where will you place your load? Should Q1 be a Low-Side switch?

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... further 3.3 V - 5 V = - 1.7 V. For me the intended function is not clear :/

I need a simple scheme that has functions:

1. If there is 0V at the input, then 3V3 at the output of the system

2. If the base input is 3V3, then the output is 0V

I don't know how to design it

Is scheme below will be ok?

Hi,

Do you mean that:

A) When 3.3V battery is at 3.3V, it powers the load and the 5V supply is off.
And
B) When the 3.3V battery falls under 3.3V, that load path should close/turn off and the 5V supply be on.

The PNP may need a pull-up resistor.

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Show the whole circuit you are talking about, please.

are you talking about getting the (voltage) signal alone at the output
or to pass a powersupply ?

Sounds to me like a enhancement P-channel MOSFET/PMOST application.

Source attached to 3.3 V and the Drain to the load. If the Gate voltage is 3.3 V than the PMOST turns off. If the Gate is 0 V, the PMOST is turned on, thus 3.3 V are availabel at the Drain.
This would be a possible solution if the device should be used to switch a power supply. If you are interest in a data-signal, there are other possebilities.

greets

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BTW, I'm a little bit confused as your initial post mentioned volatge levels of 3.3 V and 5 V.

Hi,

Looking at picture in post #3, and possible meaning of definition/objective... 3.3V battery present = pnp ON and powering something. Load and load current would be nice to know. You know rule-of-thumb rule of Ic/Ib = 10, so make base resistor current ten times smaller than collector current (load current)? To turn on a pnp when a positive voltage is detected requires signal inversion, hence the npn inverter driving pnp base.

Probably a logic level PMOS + NMOS pair might be nicer than using bjts, but if you're not familiar with datasheets it might be easier with bjts.

This showed current spikes in the simulation at the load point Vout, by the way, but it would do to turn on and off a pnp:

stenzer said:

Sounds to me like a enhancement P-channel MOSFET/PMOST application.
Source attached to 3.3 V and the Drain to the load. If the Gate voltage is 3.3 V than the PMOST turns off. If the Gate is 0 V, the PMOST is turned on, thus 3.3 V are availabel at the Drain.
This would be a possible solution if the device should be used to switch a power supply. If you are interest in a data-signal, there are other possebilities.

Click to expand...

greets

Hi,

Had to rush last night, so today did better version of yesterday's switch circuit.

Also, for fun and in case of use, 2nd picture is to show a BJT AND gate with a positive and negative input for enabling and disabling 3.3V supply when both it and 5V supplies are present but enabling 5V with an NPN.

T1 and T3 work as the AND gate. T4 enables 5V supply.
When 3.3V is present and 5V is not, 3.3V supply is enabled.
When 3.3V is present and 5V is present, 3.3V supply is disabled.
When only 5V supply is present, that is enabled.

Not great, but functional circuits for such a basic function if comparators and logic gates are not available. As can be seen, bulk capacitance doesn't hold up Vout as well as desired and current to load drops quickly, and BC547/557 vce is pretty high and 'steals' voltage from the load, plus are only capable of 100mA.

If not what you wanted, sorry, tried...
Guess

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Picture 1: pnp 'power path' switch



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Picture 2: BJT AND and NPN 'power path' switch separated



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Picture 3: BJT AND and NPN 'power path' switch together



I'm not so good at this stuff and still have a lot to learn...