ac mains to 12v and dc

Discussion in 'General Electronics Chat' started by embed68k, Jul 21, 2012.

  1. embed68k

    Thread Starter New Member

    Jul 21, 2012
    17
    0
    I am endeavouring to build myself a vintage like radio. I am very nervous building anything that directly involves AC voltages. I would very much like to use a 2.1 channel power amplifier kit in my radio. I am building my own board for the rest and this kit would work perfect and I kind of want the power amp to be on a small board anyway. But this runs on double 12V AC and a 0 ground. The rest of my project is going to be mainly 3.3V dc voltages. How can I design a circuit to step the voltage to dc(say 20 volts 1.5 amps) and the required ac voltage at the same time?
     
  2. MrHam

    New Member

    Jul 20, 2012
    10
    0
    A question or two, what is the "double 12V AC," does it require 2 12V inputs? Why do you want 20V DC? Assuming just 12V AC use a transformer from AC mains to 12 V center tapped. this will give you the 12V AC for the one module and a couple of rectifier diodes will give you 6V DC. You should be able to find a single chip regulator to get the 3.3V from the 6V. The filtering is left as an exercise for the reader:)
     
  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,003
    745
    do you mean 12v ac with centre tapped 0v, 12 0 12vac , or 0v-12, 0v-12v??
     
    Last edited: Jul 21, 2012
  4. mkbutan

    Senior Member

    Sep 30, 2008
    270
    16
    The Radio can work on 1.5v DC depend on the ckt. you r using /making.

    now
    which ckt. r u using for 2.1 Ch. audio Amp. which uses 12v AC? or it have in
    built AC/DC (rectifier) on the Amp. ckt.?:confused:
     
  5. embed68k

    Thread Starter New Member

    Jul 21, 2012
    17
    0
    yes. the kit i am going to use if i can still order it is based around a tda2030A power amplifier. It specifies double 12Vac(12v-0-12v). I have found what I think would work, a Radio shack #273-1511. 12.6v CT 3.0A transformer. I assume with 3 amps I can sufficiently power the speaker power amp kit and my DC power as well. Question is will that transformer do what I want? And is there any thing i should do to filter the AC?
    The basic project is going to be a vintage looking radio that plays Old time radio mp3s(archive.org) from a harddrive using a ALFAT Soc processor(aka usbwiz chipset) from a harddrive(hence why I think a 3A supply is a good estimate) and will use a SIL1735 AM/FM/SW/LW chip to provide radio. I am going to using a parallax propeller for the micro and as long as it feeds the mp3 decoder chip fast enough the project shouldn't be too hard to complete. I just always shy away from AC line voltages and have very little understanding of them. Now that I need AC line voltages I want to use a figure 8 mains line cord instead of a wall wart as this will add to the vintage look. These guts are going to placed into a custom wood cabinet with a drawer and some shelves(for mail maybe). I need the power amp as I am using two 5 1/4" mid range speakers from a car(new in box). 8ohm 40W RMS. and a salvaged sub speaker from a home audio component. The CS4334 ic stereo dac hooked up to the mp3 decoder only outputs .5W so I need to amplify it. The power amp kit will provide tons more amplification than i need, but I intend to clip it to a maximum level.
     
  6. #12

    Expert

    Nov 30, 2010
    16,349
    6,840
    You need a 25.2V center tapped transformer with a full wave bridge rectifier.
    Each rectifier diode must carry an average current of 1/2 the load. Over-do these. Rectifiers are cheap.
    With this transformer, you need to filter the rectified AC down to 2 volts of ripple.
    1.414 C Er F = I
    for 3 amps that's about 8400 microfarads on each output of the rectifiers (because the rectified AC will be at 120 Hz).
    Then again, a transformer will not provide its labeled current due to peak rectified current, peak rectified voltage, and a bunch of other crap.
    The current you can have is (rating/1.23) amps.
    At the end of this, you should have 14 volts minimum (plus 2 volts of peak to peak ripple voltage) headed for the 7812 and 7912 voltage regulators.

    Any questions?
     
    Last edited: Jul 22, 2012
  7. embed68k

    Thread Starter New Member

    Jul 21, 2012
    17
    0
    so what you are saying is using a 25.2V CT transformer with a bridge rectifier will convert 120V ac into 14V DC? I still need 12Vac somewhere in addition to the DC.
     
  8. #12

    Expert

    Nov 30, 2010
    16,349
    6,840
    You can not trust a transformer to give exactly what the label says because the 120 volt power line is different from place to place and from time to time. (It's 125 volts AC at my house, today.) That is why you use a voltage regulator to get the DC 12 volts.

    The transformer I named will provide 12.6 volts AC (twice) if the 120 volt power line is correct and stable. (Yes, they sell those at Radio Shack.) It is unlikely that an AC driven device will be bothered by an extra or a shortage of .6 volts, so you can get the 12 VAC from the same transformer. The problem to consider is that the AC voltage available will be connected to a "ground" or "common" point of the 12VDC circuits and that could cause trouble with the ground for the 12VAC circuits. This is the point where we need to use drawings to describe the circuits.

    I suspect that the rectifiers, filters, and regulators are inside one of your kits, but you did not name the kit or provide a drawing. If it really needs 12VAC, my previous answer was wrong and you should be using a transformer labeled, "24V, CT"

    If the kit is designed as I expect, the center tap of the transformer will be the ground point of the 12 VDC circuits. If the circuits needing 12 VAC can use the same gound point, you can use one transformer to supply all your needs.

    What we need now is drawings of the guts of both parts of the project to see how to connect them and not let the smoke out. (That's a joke.)

    ps, you don't filter AC. You filter the rectified DC part.
     
    Last edited: Jul 26, 2012
  9. embed68k

    Thread Starter New Member

    Jul 21, 2012
    17
    0
  10. #12

    Expert

    Nov 30, 2010
    16,349
    6,840
    Last edited: Jul 24, 2012
  11. embed68k

    Thread Starter New Member

    Jul 21, 2012
    17
    0
    ok. very good thank you. I see what you said about AC not being filtered, but the rectified DC. I'm good with the DC part, and I'm glad i asked for help on this. I would never have figured not to count on the transformer secondary rating. But just from a theory standpoint, I still don't understand the "2 volts of ripple".
     
  12. #12

    Expert

    Nov 30, 2010
    16,349
    6,840
    When you rectify AC the result is pulses of power. The voltage ranges from zero to the peak voltage of the wave. You wouldn't want a power supply that goes to zero volts 120 times a second would you? So, you add a capacitor to keep the voltage up between pulses. Zero capacitor, zero volts between pulses. Infinite capacitor, no sag in the voltage between pulses. Well, infinite capacitors cost infinite money so we calculate what size of capacitor will sustain an acceptable voltage between power pulses. I showed you the math in post #6 and the answer is: 1492 uf per amp.
     
  13. embed68k

    Thread Starter New Member

    Jul 21, 2012
    17
    0
    i think i'm just about comfortable with these concepts. So the 2 volts of ripple is dependent on the value of the capacitor value for the total amps. So in effect we are smooting out the dc so the dc voltage doesn't go below 2 volts of our target?
     
  14. #12

    Expert

    Nov 30, 2010
    16,349
    6,840
    No..so the ripple never goes below the target voltage at all. With the combination I described, the peak voltage is 2 volts more than necessary so we can afford 2 volts of sag between pulses of energy.
     
  15. embed68k

    Thread Starter New Member

    Jul 21, 2012
    17
    0
    so for the bridge rectifier something like this: digikey - 3KBP02M-E4/45-ND .
    so I need two of these. they have 4 diodes a piece according to the datasheet. 200V 3A is the rating. So on these 4 pin packages I only need + and - and disreqard the ~~ pins? So in a setup of two bridge recifiers like this we are spreading the load by 8 diodes? (btw i don't think this package is even in stock at digikey. I need to order the cs4334 stereo dac from there so i want to get other stuff at digi-key)
     
  16. embed68k

    Thread Starter New Member

    Jul 21, 2012
    17
    0
    ok. looked again. the ~ ~ pins are for the secondary winding and the + - is the recified DC(aka load) so I only need one bridge recifier and it divides the load by 4? and on another note. is there any real safety reason why I shouldn't mount the transformer directly to the wood inside the cabinet? I believe the radio shack ones I looked at already had soldered leads and are probably insulated at the terminals.
     
  17. #12

    Expert

    Nov 30, 2010
    16,349
    6,840
    You lost me. Divide the load among the diodes? A bridge rectifier has 4 diodes but you only need 2...look at the drawing. Wrong part. Get a couple of single, fat diodes in the 200 volt range. Try a couple of MUR420G @ 49 cents each.

    and transformers don't care how you bolt them down. Just don't drill a hole through the middle of it.

    Edit: I feel bedtime approching. Had to get up at 6:30 this morning. C U 2morrow.
     
    Last edited: Jul 25, 2012
  18. embed68k

    Thread Starter New Member

    Jul 21, 2012
    17
    0
    ok. sorry. just trying to make sure i'm understanding this before attempting to stick my finger in the outlet. the smoothing capacitor is supposed to have a working voltage higher than the diodes at idle. So am I to assume that I really should have a 200V rated capacitor that is huge and costs bucks or a smaller one since we should already be close to 12V after the diodes. If this is the case a 50V cap should be good? So if we take a 3A rated transformer and divide by 1.23 and multiply by 1492uF we get about 3700uF. I just can't see the need for a massive filter cap, and all the 200WVdc caps this large are massively expensive. So I guess I am asking for clarification on the working volts in this setup. And I assume we are talking about polorized aluminum caps here. Every other schematic i've seen has polorized smoothing caps.
     
  19. #12

    Expert

    Nov 30, 2010
    16,349
    6,840
    *blink*...*blink*...must have coffee....

    Oh. There you are. I see I have more 'splainin' to do.
    The diodes have a high voltage rating because they have to hold the 17 something volts in the capacitor and survive the transformer trying to jerk the voltage the other way by 17 volts, plus variations of the power line voltage, plus whatever happens when lightning hits nearby and tries to get in your house. I bet you're glad I didn't choose 1000 volt diodes!

    So...the storage capacitor has to survive 17 volts. Anything higher will work. 25 or 35 volt rated capacitors would do this job. I think the next higher size (from 3700 uf) is 4700 uf....or is it 3900? That's all. Just choose the next bigger capacitance than the calculated minimum microfarads, with a voltage rating higher than the expected maximum voltage.

    and while we're at this, get a fuse holder and put a fuse in the 120 volt power to the transformer. One amp should do. As a beginner, you are likely to make a mistake. Better blow a fuse than a transformer or the rectifiers.

    ps, you would use the 4 diode, bridge rectifiers, if you were going to build +12 and -12 volt supplies. I haven't heard you say anything needs a negative voltage power supply, so I'm just showing you how to do positive only. The kit you named has the dual polarity supply. I can tell by counting the parts. Problem is, you can't leach off the kit supply to run a hard drive. It isn't built with the excess capacity that would run "extras".

    Edit: Here's a cool reference sheet gerty put up today.www.hammondmfg.com/pdf/5c007.pdf
     
    Last edited: Jul 26, 2012
  20. embed68k

    Thread Starter New Member

    Jul 21, 2012
    17
    0
    ok. i was just going by the description on the kit. the little I know about op amps I should have guessed there was a negative voltage as well. Well maybe two transformers are in order. I still think I want to go this route with the kit and building a circuit board for the rest. There are other kits or schematics that I can get ahold of to do the amplifing job, but right now I'm sticking to this idea. But I am going to need some additional current. I'm driving a 2.8" tft screen, harddrive and several ic's. So even bringing two transformers in wouldn't really hurt(except my wallet) a 25.2V CT 3 amp was like $28.
     
Loading...