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Power supply Answered

Last time I tried to do my own homemade project I failed utterly, I think mostly because I had no idea what anything was (if you saw my last topic on a distortion petal, you know what I'm talking about). I decided instead to buy a book (Tab Electronics Guide to Understanding Electricity and Electronics by G. Randy Slone) which has projects in it, the first one being a power supply. I haven't started building it yet (waiting until I get back to the city) but I've started looking up parts. The thing is, I can't find a 4400-uF 50-WVdc capacitor anywhere! So my question is, can I just put two 2200-uF 50-WVdc capacitors in parallel to do the job of one of the 4400's? That isn't my only question, just the first which I'll ask later in this thread. Thank you (PS: Has anyone had a problem with using Chrome and entering text into these fields? I had to switch to Firefox to get it to work)

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Sandisk1duo (author)2008-12-28

the value of the capacitor doesn't have to be exact..

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NachoMahma (author)2008-12-26

. If that's a ripple filtering cap, then uF is not critical. The closest standard size should work. If it's not a ripple filter, I dunno. . Or your two 2200uF's in parallel will work. As long as the WVdc (working voltage DC) is the same or higher. And they are all the same type.

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gohuskies (author)NachoMahma2008-12-26

It appears then that I'll have to use the 2200's in parallel since the 4700's available are rated at only 16 and 25 WVdc while the 2200's are rated at 50.

Ok, next question.
I also will be needing a 24-volt 2-amp transformer (120 volt primary) for this project, but I can't find that at my school's store (http://www.ee.washington.edu/stores/) or at Radio Shack. Can you guys find them there? Or something else that would work?
That part doesn't seem to be very common since it's in neither location as far as I can tell.

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purduecer (author)gohuskies2008-12-27

Radio shack sells a 25.2V Transformer. That may be close enough for government work (or hobby electronics work, as the case may be here ;-P)

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gohuskies (author)purduecer2008-12-28

Do you really think that is close enough? Would it not skew work further on in the work?

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gmoon (author)gohuskies2008-12-28

Are you using voltage regulators in your power supply? If so, then the 25.2 transformer is fine. Remember, 24V AC doesn't equal 24V DC anyway. AC voltage is measured as the average voltage of the waveform, not the peak voltage. When the RMS AC voltage is rectified and filtered for DC, the conversion factor is approx. 1.414. So 24V AC yields a bit over 33V DC; 25.2V AC over 35V DC (the 1.414 conversion is theoretical, and doesn't include the voltage drop of the rectifier diodes, etc. Your values will be slightly less...) If 35V DC (or half of that, if it's a bipolar supply) is within the range the voltage regulator can handle, you're cool...

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11010010110 (author)gohuskies2008-12-26

wire 2 magnetic halogen lamp transformers with outputs in series i think its possible - never tried - maybe hack a computer power supply to add this output (but it'll be like +12 V / -12 V to ground and not 0 / +24 V or floating)

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Sandisk1duo (author)110100101102008-12-27

no, i don't think power supplies would work, you can't isolate the ground, so what you would be doing is shorting one PSU's positive terminal to the other PSU's negative terminal. PSUs are grounded, at least computer power supplies

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11010010110 (author)Sandisk1duo2008-12-27

i mean another hack inside there is the big transformer with 2 wires that provide each voltage. from each there is a diode to the correct voltage (like +12 V and + 5 V). the diodes (sometimes combined in pairs in a single component) are located on the second cooler if you copy this part of the circuit and put the diodes and capacitors the opposite way i think you'll get -12 V or -5 V supply +12 V and -12 V together give 24 V (the supply has -12 V but its for small currents only)

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Sandisk1duo (author)110100101102008-12-27

take it one step further, extract the transformer and the fuse, note where the input is then just hook the fuse to the input, then Mains AC to the fuse+transformer, add a voltage regulator (or a couple, in a series) and you've got high-power! then do the same to the other PSU, hook both PSUs in series with a diode

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11010010110 (author)Sandisk1duo2008-12-27

the xformer is meant to work at high frequency and not 50 - 60 hz it needs the entire circuit of the psu

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westfw (author)2008-12-26

If I was building a power supply "from scratch" these days, I would be EXTREMELY tempted to replace the whole AC->DC section (Transformer, rectifiers, filter caps) with a surplus or "found" switch-mode "wall wart" style supply of the sort used for laptops and such...

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11010010110 (author)2008-12-26

they'll work fine for ripple filter caps you can usually use everything larger than the needed capacitor. it does not harm (untill you oversized it to the extent where it blows fuses with its inrush current) connecting 1 or more 1uF or more polyester caps in parallel with the electrolytic cap makes the filter way more responsive and may have a good effect in audio circuits and the like. (its the same effect as what the big 12 V cap in car amplifier gives). it happens cause polyester capacitors have less resistance than electrolytic and respond more quickly to spikes in voltage electrolytic caps like high voltage. if it says 50 V it'll work best at 30 - 40 V and somewhat less good (usually small and not noticable difference) at low voltages like 12 V

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Goodhart (author)2008-12-26

I concur with Nacho, a capacitor in a psu will probably be a filter cap and so you can use the closest thing you can find OR you can parallel two 2200's of the same or hight working voltage (two electrolytic caps).

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