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Topic: What kind of capacitors and diodes for AC/DC 12v converter?  (Read 895 times) previous topic - next topic

tadobibale

I'm trying to build this based on a instructable:




BUT it is designed to deliver about  5 VDC and I need it to be 12 VDC. I need help deciding if the capacitors and diodes can be the same. I'm fairly new, just yesterday I discovered what a diode is, so I'm full of guesses and would appreciate any confirmation or rectification.

The parts they give for the 5 VDC output are:

Diodes: "1N4001, 1N4004, 1N4007 will all work fine"

Capacitors: "2 220-470uF electrolytic, 2 100nF ceramic disc". They just say to "be sure the Voltage in the capacitor is higher than the Voltages they will experience". I'm guessing the 220-470 is 220V already, if that's so, does it have to be that high? or will a 16 V do it too? I'm also guessing the 100nF ceramic disc just needs to be something around 15 V. 

I guessed the resistor has to be a 7812 (since the original is a 7805) and the transformer has to deliver 15 VAC to compensate for the regulator.

Any input appreciated! Also any thoughts on the converter too! Thanks!

DVDdoug

Do you have any idea how much current you need?   ...The 78xx series is rated for 1A but at that much current it generally needs a heatsink, and if you're dropping a lot of voltage across it you may be power-limited and you may not get that much current (without the regulator shutting-down or going unstable).

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Diodes: "1N4001, 1N4004, 1N4007 will all work fine"
Here is the datasheet for the diodes.  The 1N400x series are all rated for 1A (that's the maximum current) and only the voltage (also the maximum rating) is the only difference.  Since they are cheap and there isn't much price difference, I usually buy extras and I buy the higher-voltage versions because I'm not sure what I'll be using diodes for in the future.

There is also something called a bridge diode which has all 4 diodes in one package and is easier to wire-up.

The capacitors are all on the low-voltage side of the transformer.   16V is fine for the output-side of the voltage regulator, but the input-side will be higher.    Note that the peak voltage is about 1.4 times the RMS, so the rectified & filtered AC from a 12V transformer will be about 17V.    And, a "12V" transformer will put-out a little more with no load, and the output of a transformer is unregulated and proportional to the power-line voltage, and the power line voltage can vary a bit.    So, it's a good idea to use capacitors rated for double the expected voltage.  

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I guessed the resistor has to be a 7812 (since the original is a 7805)
Correct.

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and the transformer has to deliver 15 VAC to compensate for the regulator.
That would work (if you can actually find a 15V transformer).  But remember the 1.4 factor, and note that the more voltage you "drop" across the regulator, the hotter it will get.  


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will all work fine..
:D  In electronics (or any other engineering) it's often the case that components are not critical...  There are lots of situations where any resistor between 1K and 10K will work, etc.    But, the trick is knowing what's critical and what's not and that only comes with learning and experience.   

tadobibale

Thanks for all the info, I think I'll go with a 12V transformer then.

Do you have any idea how much current you need?     
I think I'll need 0,5 A maximum. but was thinking making it 1 A just in case later on I need to plug something that requires more amperage.


tinman13kup

Working with mains as a first project with little knowledge of electric or electronics can spell disaster. Mains power can kill you if you are careless.

 Use a GFCI protected power strip to plug your project into.
 Turn off the power when attaching wires
 Make sure your breadboard is secure on the CLEAR table.
 If possible, find an enclosed transformer pack. I have several I salvaged from old home security systems. Some are 24VAC, some 16VAC.

 It's not a big deal to send a LDO regulator up in smoke, or blow a capacitor because it was too low a voltage or backward polarity. It stinks bad, but a quick flip of the power button makes it all better. Roasting the cat because it jumped onto your bare mains wires might be amusing to some, but should still be avoided.

Double check your stuff.
Tom
It's not a hobby if you're not having fun doing it. Step back and breathe

Jiggy-Ninja

:D  In electronics (or any other engineering) it's often the case that components are not critical...  There are lots of situations where any resistor between 1K and 10K will work, etc.    But, the trick is knowing what's critical and what's not and that only comes with learning and experience.   

I would just like to add to this that what properties are important is extremely dependent on context.

For example, when picking a diode for AC power rectification, things like the reverse current, reverse recovery time, and junction capacitance are going to be on the bottom of the list in terms of their importance. Your key specs for that are going to be the average forward current, maximum reverse voltage, and (if it's a particularly high power use) thermal ratings.

It's a very different story if you're picking out a diode to do transient voltage suppression (TVS) of a high-speed signal path. The capacitance the signal is exposed to has a huge impact on its bandwidth, so the reverse capacitance becomes a critical spec that you need to minimize to avoid degrading the signal.

There's really no way to generalize it to a short list. That's why forums like this exist.
Hackaday: https://hackaday.io/MarkRD
Advanced C++ Techniques: https://forum.arduino.cc/index.php?topic=493075.0

tadobibale

Working with mains as a first project with little knowledge of electric or electronics can spell disaster. Mains power can kill you if you are careless.

 Use a GFCI protected power strip to plug your project into.
 Turn off the power when attaching wires
 Make sure your breadboard is secure on the CLEAR table.
 If possible, find an enclosed transformer pack. I have several I salvaged from old home security systems. Some are 24VAC, some 16VAC.

 It's not a big deal to send a LDO regulator up in smoke, or blow a capacitor because it was too low a voltage or backward polarity. It stinks bad, but a quick flip of the power button makes it all better. Roasting the cat because it jumped onto your bare mains wires might be amusing to some, but should still be avoided.

Double check your stuff.
Yeah, I'll definitely do that, thanks for the suggestions. Any extra safety advice would be appreciated too! Would a fuse make sense in this project?

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