Electronics > Beginners
Diode in GND pin of voltage regulator
tester43:
Let's use an example: LM7805;
Input voltage 12V - output 5V (let's ignore if it makes sense for the moment);
I made an experiment: supplied 2V to GND pin of LM7805;
As expected output vs GND from LM7805 jumped to 7V in reference to gnd - because I gave him a reference of 2V and left 10V as voltage difference so LM7805 has got its space to operate and produce it's 5V output to it's given reference (5V) which is 7V when compared to global gnd.
I found that it's possible to put just a diode in GND to obtain similar result. How does it work with diode between LM7805 and GND?
wraper:
78xx have relatively high bias current of a few mA flowing through GND pin. Thus it creates forward voltage drop across the diode. That's why they suck as adjustable voltage regulator and LM317 with resistive divider should be used instead.
not1xor1:
--- Quote from: wraper on July 03, 2018, 10:13:17 am ---78xx have relatively high bias current of a few mA flowing through GND pin. Thus it creates forward voltage drop across the diode. That's why they suck as adjustable voltage regulator and LM317 with resistive divider should be used instead.
--- End quote ---
There is not much difference. The quiescent current of 78xx is around 4.3mA (typical - ST datasheet) while LM117 is granted to be stable with a load of 5mA and LM317 with 10mAs (although the few I checked work even with as little as 3-4mAs).
If somebody needs an higher voltage and is not concerned by the voltage drift can just put a LED in series with the ground pin to get an increase of few Volts (depending on the forward voltage of the led).
ArthurDent:
As not1xor1 said, LEDs work as low voltage zeners. I used one with a 1.7 VDC drop in series with a 5 VDC supply to get 3.3 VDC at low current for a small circuit I was working with. As a bonus, I also got a visual indication that the circuit was drawing current because the LED was on. The drop isn't as consistant as a real zener but it isn't too bad.
tester43:
Hi, does it mean that on reference pin of lm7805 potential is higher than 0,7V when not grounded (so diode can conduct when it is connected to reference and ground)?
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