# Turn on voltage by using smaller voltage

I'm wondering if there's a way to create a current controlled voltage source. At the moment I have a mosfet (NPN, PN2222) connected in series with a positive 5V terminal and ground. I control that mosfet using a ~2.5 V input voltage. What I wanted to do was create a 5V output when the mosfet is on. However, mosfets can only be connected in series, with the voltage on one end being necessarily higher than the other for it to turn on. Is there a way to output a voltage using only a mosfet, A 5V source, 3 volt source to turn on the mosfet, and ground? Basically, if the 3 Volt source is connected, output 5 volts, otherwise 0 V. I only have 2 diodes and 2 mosfets and a handful of resistors to use.

Something like that ^. Obviously that doesn't work because there's no voltage difference and so the mosfet doesn't turn on even when 3V is applied to it, and that's where I'm stuck.

• Am I the only one who got brain damage after reading this? Please, rephrase it. You want to switch 5V output with 3V signal? – Ilya Feb 19 at 8:32
• Yup, hope I didn't damage it too much lol – Omar Farag Feb 19 at 8:46
• A PN2222 is an NPN bipolar transistor, not a MOSFET. – Peter Bennett Feb 19 at 17:05
• You need to learn the basics first. – copper.hat Feb 19 at 18:08

If I understand you right, you mean something like this:

Where Q1 is a transistor, which opens or closes 5V supply to the load. R1 is 1k-100k (standard is 10k, but it doesn't matter which one, it's a pullup for the gate). Q1 can be a PNP, whose base is similarly pulled up by R1 so that it doesn't conduct in normal condition. If you use NPN or PNP, don't forget base resistors.

ON/OFF is your 3V signal. When it's HIGH (3V), T1 conducts and pulls gate/base of Q1 low, powering the load with juicy 5V. When ON/OFF is LOW (0V), T1 doesn't conduct, Q1's gate or base is pulled up via R1 (as if T1 simply doesn't exist) and the Q1 does NOT conduct.

Hope I guessed your intentions right

• Oops, my bad, will change title. – Omar Farag Feb 19 at 8:48
• you won't get a better reply than this, I think, because this is how they do it everywhere in phones and laptops too (T1 can be an N-channel MOSFET too, so that there is less current waste). Make sure you have logic level MOSFETs, for which 3V Vgs would be enough to start conducting (refer to datasheets) – Ilya Feb 19 at 8:51
• A PN2222 is a BJT not a mosfet. – Kartman Feb 19 at 9:39
• you can use NPN (it's a BJT), which needs CURRENT through base to conduct between collector and emitter. OR you can use MOSFET, which cares about applied voltage, it doesn't need current. BJTs are PNP and NPN; MOSFETs are P-channel and N-channel (PMOS and NMOS). BJTs are controlled by current through base. MOSFETs are controlled by voltage. And they're ALL transistors. Google some transistor basics, BJTs and MOSFETs are always the first and the easiest to be discussed. – Ilya Feb 19 at 10:04
• up-voting @Ilya clear answer. speaking of base current, wouldn't be interesting to mention that a series base resistor may be in order to limit it and protect the be junction? – devnull Feb 19 at 10:40