Hi all
i am making a circuit to feed 4-5 volts to my arduino mega 2560. i have a 12 volt supply coming from machine, i am using an led with 1k resistor to show the power is coming from machine, i am also using a diode to avoid any back signal from controller to machine(as machine card is sensitive). i am using a capacitor to absorb any sudden voltage surges. i have designed a circuit in protius can u ppl have a look at it and tell me is it okay or not?

 

no need. if you really want the diode then you must place a load >1mA between pin 2-3
put cap 10uF between 2 and 3.

 

no need. if you really want the diode then you must place a load >1mA between pin 2-3
put cap 10uF between 2 and 3.

when i upload a program in arduino it makes all its pins to high for a few mili seconds, that sends a back signal, i have used the diode to avoid that signal going back.
and kindly tell me whats the purpose of capactior between 2-3 pic?

 

Hi, have you read the datasheet for the 7805? Use Google to find it. Google "LM7805 pdf".

 

D1 should be on the input side of the 7805. It should be the first thing after the 12 V input connection. You should not ave a diode on the 7805 output, as that will ower the output voltage and make the regulation worse.

ak

 

yes i am getting the same problem, my circuit is giving me 8 volt output, but when i removed diode, i got 5 volts.
why is that so?

 

D1 should be on the input side of the 7805. It should be the first thing after the 12 V input connection. You should not ave a diode on the 7805 output, as that will ower the output voltage and make the regulation worse.

ak

yes i am facing the same problem, my circuit is giving me 8 volt output, but when i removed diode, i got 5 volts.
why is that so?

 

yes i am facing the same problem, my circuit is giving me 8 volt output, but when i removed diode, i got 5 volts.
why is that so?

Maybe the input 12 volts is not very clean, and consists of pulses? Just a guess. The 7805 might be passing high current pulses to try to get its output to 5 volts. Because of D1, there could actually be 8 volts at the output for a while, with 5 volts at the anode side of the diode. You could put a capacitor from the 7805 output to ground, just 10 uF, and also a (say) 2k resistor in parallel with that, as a diagnostic measure. If that stabilizes your output voltage, then you have a good lead on the cause. However, I agree with Analog Kid that D1 is in the wrong place to begin with.

 

If your output load is less than 5ma, output voltage specs are not guaranteed.

Why not put the led at the 5v output?

It would provide a minimum load and also confirm you have voltage in and out.

Also, please review the data sheet, it will provide answers to your issues.

Be aware of how much current your entire project may require. Just connecting a arduino and a couple of outputs will not cause excessive heating of the 7805 Tab, but as you require more and more current, a heatsink may be needed.

 

If you had the diode at the output with no output capacitor, then the circuit may be oscillating and causing the high output voltage you observed.