I am trying to use a STMicroelectronics L7805CV to increase voltage from 7.5v to about 12v, but I can't get it work. I've placed 470 ohm resistors between out and common and between common and ground, which should give me about 10v, but instead it only gives about 7.5v. It's like the 7805 can't handle any higher, when I put a second 7805 in parallel, the voltage rises to 8v.
I've made the following observations:
- 100 ohm resistor between out and common, commond connected to ground; The voltage is a nice 5.0v.
- 470 ohm resistor between out and commond, common connected to ground; the voltage drops to around 3v, this I find very strange. The load is lower so it should be easy to maintain 5v?
- 100 ohm resistor between out and common, and a 100 ohm
resistor between common and ground; the voltage is around 8v. - 470 ohm resistor between out and common, and a 470 ohm
resistor between common and ground, the voltage is around 7.5v.
My setup is similar to http://www.rason.org/Projects/regulator/regulator.htm
Any ideas on why this isn't working? According to the datasheet, you can use the device this way.
Thanks in advance!
edit:
If you're reading this and are trying the same. This is not the way, try this: Step up DC/DC 5V -> 6V
If you want to do this you must have a current limiting resistor. Without it, your circuit essentially shorts the output of the 7805 regulator to the ground.
Best Answer
The link recommends a fairly moronic way to increase the voltage from a 7805, and incorrectly equates the 78xx parts with the LM317.
One of the differences is that the internal currents (a few mA) that the regulator uses goes out the output of an LM317 and out the GND of a 7805. The ADJ terminal of an LM317 typically sources 50uA of current, not 5-8mA, so a resistor divider is reasonable. The LM317, OTOH, has a requirement for a minimum output current to keep the output in regulation. The 7805, since it dumps the internal current out the GND pin works fine down to no load.
If, for some reason, you want to increase the output voltage of a 7805, use a zener diode between the GND terminal and ground, 6.2V will give you 11.2V out.
The dropout voltage of a 7805 is around 2V typical at 1A (somewhat less at lower current, can be worse over temperature), so if you want 12V out, you need to give it 14 or 15VDC in.