Can I use the output of an NPN proximity switch as the input to an LM7805 (positive input) voltage regulator? I'm a little confused on PNP vs NPN, but I'm reading as fast as I can. Also I see that they make an LM79xx series of VR's that, I think, accept a negative voltage as the input, would this negative voltage come from an NPN style device?
Thanks, Jim G.
Electrical – PNP vs NPN input to LM7805 VR
voltage-regulator
Related Solutions
If I understand you correctly, you are trying to adjust the output voltage of your 7805 regulator to output 5V - 10V by adjusting the voltage at it's ground pin between 0V and 5V.
If you are doing this, the pin needs to be able to sink current in order to maintain regulation, so whatever the power source used, it needs to be capable of this. Many supplies will only source current, and if this is the case what you are seeing will occur.
A simple resistor divider with potentiometer can be used, or an opamp to drive the 7805 ground pin is maybe the best solution, preferably with a output including its negative rail if you want to go all the way down to 5V. You will find many example adjustable supply circuits on the web, and in the datasheets of the regulators also.
Here are a couple of example circuits:
Pot adjust, from this page with more info:
Opamp adjust, from datasheet pg.24 (don't use a 741 if you can possibly avoid it, as they are obsolete):
If you take the input for the 7805 from the 7809 output, you must include the current drawn from the 7805 into account when determining the power dissipation in the 7809.
You should not use a voltage divider to make 3.3 volts - the voltage on your 3.3 volt line will vary depending on the current drawn from the 3.3 volt line, as that current will have to flow through the "top" resistor of the divider, increasing the voltage drop across it. Look for a dedicated 3.3 volt regulator, or use an LM317 or similar adjustable regulator.
Depending on the currents you need from each supply, it may be best to use a switching regulator to get from the 15 volt supply to 5 volts.
Make sure you have adequate heatsinks on the linear regulators!!
Best Answer
You want a PNP type. That type sources current.
If you need to use an existing NPN type you can add a high side switch, but at that point you'd perhaps be be better off using a regulator with an enable input as @Ignatio suggests.
Here is one way to do the former:
simulate this circuit – Schematic created using CircuitLab
The proper choice of R1/R2/Q2 depends on the current you intend to draw from U1.