Can 3 zener diodes and 1 resistor in series make many voltages?

[Nigel Goodwin]

My goal here is to be able to produce 14, 12 and 5V for the vpp pin using two digital inputs.

I used a voltage regulator on left to provide 15V. then I use diodes and transistors to select a voltage. What may have confused people was that the 7805 output was connected to VCC but that VCC is actually the +ve power going INTO my circuit.I used my arrangement to not overheat the 7805.

No, what confused us is that the 'circuit' is nonsense, and wouldn't work - look at post #13, which uses an adjustable 'super zener' - you can find them (TL341's) in many switch-mode PSU's, such as those used in VCR's etc.

 

[mik3ca]

Can I still be able to use BJT transistors here? I don't own a TL431 but luckily I can get one nearby. and if I start buying mosfets, then my bill will go up. Now what's confusing me is that three names are thrown at TL431. You (Nigel) calls it a "super zener", texas instruments calls it an "adjustable precision shunt regulator", and the local store calls it an adjustable voltage regulator.

If someone was able to make a battery charger with auto shut-off feature using a transistor connected to the LM317 regulator, couldn't I somehow pull something like that off as well?

 

[audioguru]

Your 7805 with a 15V input and a 1A output will heat with 10W. The heatsink must be huge and expensive, mounted outside the circuit's enclosure.

 

[mik3ca]

Your 7805 with a 15V input and a 1A output will heat with 10W. The heatsink must be huge and expensive, mounted outside the circuit's enclosure.

I was kind of thinking that after posting. I could chain up a few regulators but then again, 12V to 5V is a 7 volt difference which is 7W power. I'm gonna see if a special IC is available for my situation.

 

[audioguru]

A switching (buck) regulator circuit produces very little heat. If you buy 10 from ebay then probably one of them will work properly.

 

[mik3ca]

Ok, I might as well go with a commercial DC converter from 12 to 5V. the box with nothing in it represents just that. One at the local store accepts up to 16V input.

Now did I wire this up right? I learned from talkingelectronics.com that not connecting 7805 GND pin to ground will make the output voltage higher than 5. Is it true with the 7812 as well that if GND pin is not grounded then its voltage is higher than 12? That's what I'm trying to do with the two resistors and left-most transistor... change between 12 and 14V.

The second transistor's purpose is to allow me to choose between 5V or 12/14V. I forgot but I think I will include a resistor in the second collector as well to prevent a short.

But am I at least on the right track? because I don't want to have to keep plugging in nd unplugging my circuit with different voltage adapters for each byte that I want to program and/or erase.

 

[audioguru]

The voltage increasing circuit and its formula is in the datasheet that you never look at.
Your regulators are missing the important input and output capacitors shown on the datasheet.
The transistor in your circuit on the right might blow up when it instantly tries to charge the regulator's output capacitor with unlimited current and will be damaged by the reverse emitter-base voltage when it turns off.