Hello people,
Im working In a project using an Arduino Mini Pro and I built a 5.1V source using zener diodes, shown Below. I am also using a RFID module RC522 wich needs 3.3V, I read on internet that i just add a resistor and then a zener diode that works with 3.3V. The whole circuit is shown below.
How good is that circuit? Any suggestions for it?
Please Im new in this world so I will apreciate any help.
Thank you so much.
You are messing with mains which is dangerous if you don't know what you do. And you clearly don't.
Just drop the whole schematic and get yourself a nice and save USB charger. And for the RFID, just grab a 3v3 regulator.
Ok thank you so much. Why is that dangerous? Im curious about. Can I regulate voltage with a potenciometer?
Yes, you can absolutely set voltage with a potentiometer, if that potentiometer is part of a safe adjustable AC- or DC-DC converter that supports to be set by a resistor.
Since you probably are rather speaking of something like a voltage divider: That just goes back to septillion's answer: You don't know what you do and you are going to kill yourself.
Why is that dangerous?
Because AC acts on your neuronal system, which can mess up your heart rate. Or contract your muscles to a point that your bones break. Just as examples. It might also burn you directly - or indirectly, after it set your place of living on fire while you sleep.
Get a 24V power supply (5-12V is also fine, but 24V leaves you more options for playing in the safe range, if you want to play) and an MP1584 DC-DC converter module.
Simply because mains voltage can kill you 
Ok thank you so much. Why is that dangerous? Im curious about.
Althought there's ~5VDC between the Arduino's ground and Vcc, the Arduino's ground can end-up at about 170V* relative to earth ground and that can kill you.
Be smart and use a step-down transformer (which provides 100% electrical isolation from the power line) or buy power supply (which will be isolated).
P.S.
Plus, a brute-force Zener regulator is a lousy and inefficient power supply design!!!!
*120VAC RMS is about 170V peak.
As others have said, don't make your own power supply - nice isolated power supplies are dirt cheap and available at any corner store in the form of USB chargers, and they give you 5v, which is just what you want.
Zener regulators are just fine for some purposes - at lowish current, when it's not running on battery or anything, it's a perfectly good way to get a cheapo regulator - I do it when i've got something that needs an mA or two at 5v and all I've got handy is 12, stuff like that.
It's not ideal though, I wouldn't use it for an RFID reader, for example :-P. 3.3v LDO regulators, and the caps they need to be happy, are pretty damned cheap (under a buck even in small lots) - and DC-DC converters can be had on ebay for only slightly more (they start at around a buck)...
The current limiting resistors for the zeners don't look correct. Post the datasheets for the zeners.
@raschemmel:
Although you are "electrical-wise" right -> I wouldn't encourage the PO to get back to the fatal path again ...
@MeletChirino:
If you want more details how and why you will be killed, pursuing your initial approach, read the following links about the fatal impact of mains voltage/current to human bodies:
How much electricity can you take on before you die
and
https://www.physics.ohio-state.edu/~p616/safety/fatal_current.html
which says:
the only reasonable conclusion that can be drawn is that 75 volts are just as lethal as 750 volts
Ok thank you so much. Why is that dangerous? Im curious about. Can I regulate voltage with a potenciometer?
In the simplest terms I would say that it is being implied that if you have to post for guidance on a circuit that uses mains voltage then you probably should not be messing with mains voltage. It is almost impossible to completely train someone to work with mains voltage safely in a few forum posts. You are therefore being advised to buy a factory made power supply.
Hello there,
Your original power supply design is an offline power supply. These power supplies are usually used in small equipment that dont require much power. The design however requires some serious cautions about the enclosure and controls and other points. Because of the risk of serious shock, it is never recommended for anyone other than an experience designer/user that knows all the dangers and has taken all the precautions to make sure none of those can ever affect the end user.
For example, if you are standing on a concrete basement floor and touch any part of that circuit such as any component lead, you can get a very bad shock by current that goes through a large section of your body.
A place where you might see this kind of circuit is in a lamp dimmer, or an LED night light. Cost is important so the designers try to keep it down.
In the hobby place, cost isnt as important as safety. Wall warts can be had these days pretty cheap and they are much safer because they have to be double isolated to meet a certain safety standard. A good choice for hobby stuff.
If you really have to use an offline supply then you have to take the necessary precautions. For example, if you use a potentiometer in the circuit you should use one that has a plastic shaft not a metal shaft. That helps to keep the isolation between the user and the circuit in the event that the knob breaks off and the users still operates the equipment. It also helps if there is a small amount of damage to the enclosure where something inside shorts to the shaft but it's not visible from the outside.
Microchip has an app note about these kinds of supplies but you'll have to look it up as i dont remember the app number. I am sure there are other notes on the web too.
I got a 12v wall wart online for 3 dollars USD. It was either Electric Goldmine or Banggood. They both sell stuff for good prices.
Would his cuircuit even work? There is a lot of current going over those Zeners, so I guess they are not really 5.1V in his application.
Look at current limiting resistors
You mean R1, R2 and C1? Yeah, I an absolute resistance of 2.6 kOhm with those, so around 60mA. Over 2 Zeners gives 30mA. OK, may work. Seem to have had an error in my calculation before giving 100mA per Zener.
Well of course the 5V zener would blow immediately on power up because there is no resistor but the 3.3V has a 10 k ohm resistor.
120V /56 ohms = 2.1 A
Now you lost me, I have to admit. Where did you get the 56Ohm to define the current? I see that 2x for R1 and R2, so that would be 112 Ohm, but there is also C1, which will do most of the current limiting, wouldn't it? There would possibly be quite considerable switching peaks, though.
you're right, ElCaron, the impedance of C1 is about 2.6k à 60Hz
Z = 1/(23.1415926535960 Hz * 1 E-6) = 2652.582 ohms + 56 ohms+56 ohms = 2764.58 ohms
120V / 2764.58 = 43 mA.
Pzener = 5.1 V * 0.043 A = 0.2193 W
so I suppose the 5.1 V zener will be ok. ( I forgot to calculate the impedance of the 1 uF cap)
MeletChirino:
How good is that circuit? Any suggestions for it?
Yeah..... better not do this one yourself, since you first need to understand the dangers of working with mains voltage. I recommend googling "dangers of working with mains voltage", and also "isolation transformer".
The 1uF capacitor is the vital component in this supply design, its limiting the current non-dissipatively to less
than 100mA
