What determines the voltage when converting currents?  - Page 1
Forum section: electronics / beginners

author: Syllith
author_rank: Regular Contributor
timestamp: October 19, 2016, 05:46:51 am
content: I took a transformer that took 120VAC and stepped it down to about 60VAC. I then added a wave rectifier to keep only the positive voltages. Now my meter was reading oscillating 0-13VDC and 6 VAC on the same line. I'm a little confused as to where exactly the 6VAC is bleeding in from. Is it the diodes? After adding a couple caps (I don't know the values off hand right now), it then read 0VAC and 21.5 DC under no load. I'd like my original 13VDC but the caps have increased the final voltage so high I cannot use it.I have a few questions about that. I just added the beefiest caps I had on hand, so I'm sure that's why my voltage increased so high. I'd like to know exactly how it came up with that number though. If I wanted it to have a high capacitance but remain at 13VDC cleanly, what would I need to do? Where does it get that 13VDC from anyway? Can somebody explain exactly how 60 was translated into 13? Does the quality/type of diode have anything to do with it? Is it an RMS value of something?I'm using this to bench test car amplifiers and other 12VDC things so I'd like it to be able to push at least a few amps or so just to see if it powers on and can maybe play just for a second. How reasonable is it to do this with just caps? Would I be better off adding a small rechargeable battery just to ensure a clean DC signal? I realize this can probably just be done with a PC power supply but hey, I'm not here to do it the easy way. I want to learn why this is behaving the way it is. Thanks for any tips.I've attached an image very early "prototype" just so you can see what I'm referring to. (Ignore the mess with the switches, I removed them later on.)

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: October 19, 2016, 06:45:36 am
content: When you measure AC with a multimeter, you're measuring "rms" voltage. To convert to "0 to peak", you would multiply that by sqrt(2) (since it's a pure sine wave): 60V rms is almost 85V pk. So when you run that through your rectifier, you'll have something going from 0 to around 84V (because of diode voltage drop.) Diodes don't turn a sine wave into flat DC. If you try to measure this rectified AC with a DC multimeter, you're going to get a garbage result.Once you added capacitors, they should have charged up to 80-something Volts and stayed there (since there was no load to drain them between the peaks.) That you're measuring 21.5V suggests something is not working correctly.I think Transformer + rectifier + capacitors is doable for what you want since automotive 12V is notoriously noisy anyway (and varies from 11.5 - 15V) But not with a 120->60V transformer.In your picture, I see the transformer and what I believe to be a fuse (which will also lower the voltage a bit once current is flowing.) I don't see the rectifier or the capacitors. I'm also not sure what I'm looking at in terms of transformer wiring. Schematic?

author: Whales
author_rank: Super Contributor
timestamp: October 19, 2016, 06:52:44 am
content: Your transformer will be spitting out a voltage that somewhat resembles a sine wave in shape:If you are using a single diode to rectify your AC, then you will be getting half-wave rectified AC on the other side of your diode:I presume this is what you are measuring when you say your multimeter fluctuates.The fluctuation is a hint: the multimeter does not know what it's doing. You don't have 13VDC. You don't have 6VAC. They're both lies.Most multimeters can only reliably measure a constant voltage (not a fluctuating one) when in DC volts mode. Similarly many multimeters incorrectly measure in AC volts mode when you give them a waveform other than a pure sine wave. More expensive meters features capabilities such as 'true RMS' that can give you more meaningful measurements in these situations, but you need to know what's going on before you assign any meaning to the readings your equipment gives you. Measurement equipment can't read your mindQuoteIs this higher voltage something more like 40 to 100V? Eg closer to 60V?If your '60V' transformer is not outputting anything evennear60VDC after rectification and then capacitor smoothing, then something is amiss. Perhaps it assumes a different input voltage, perhaps your measuring equipment is faulty, perhaps the transformer is faulty, etc. I'm not sure what's gone wrong.

author: Syllith
author_rank: Regular Contributor
timestamp: October 19, 2016, 02:21:20 pm
content: Huh, all that makes perfect sense. What doesn't is why I'm getting 60VAC going into the diodes and 13 coming out.http://www.diotec-usa.com/B75PF.PDFThese are the diodes that are press fit into the transformers heatsink.They don't seem to behave different than any other diode though. Could they just be bad? It did come from a blown up charger after all.Tomorrow I will be heading to an electronics store in St. Louis to build a proper one of these from the ground up. I did try mocking up another simpler one without the transformer following some basic schematic online. I assembled a full wave rectifier using 4 6A diodes and the same 2 caps I used for the previous project. When I plugged it in, pop went the breaker. I'm guessing this is because with those big caps on there, when I plugged it in, it presented far too big of a load and flipped the breaker? If that's the case, what do I need to do to get it to not draw some much current but still be efficient for my purposes? Just smaller caps?When I go to pick up the parts tomorrow, can you guys point me in the right direction on what I should need? I'm going to get a new transformer and I'm assuming it needs to drop 120VAC down to 14VAC? This way when I run it through the diodes it'll leave me around 13 something VDC. What kind of caps will I need to make this thing pretty beefy? Maybe capable of drawing 5-10 amps cleanly? Any other suggestions before I do this would be greatly appreciated. You guys have been so helpful already.

author: BobsURuncle
author_rank: Regular Contributor
timestamp: October 19, 2016, 04:48:50 pm
content: We can only guess as to how you are hooking this up and how you are measuring the outputs. First off, as was already mentioned some of those measurements are unreliable because you are using the meter to measure signals it was not designed to measure - so ignore them. The 60V unloaded AC output and a DC -rectified filtered output - it can measure. If you did it correctly you should get approximately 80V DC unloaded. How many and how exactly did you hook up the capacitors and where did you connect the test leads? How many diodes are in the bridge and do you know if it is a full wave or half wave rectifier? Schematic?

author: retrolefty
author_rank: Super Contributor
timestamp: October 19, 2016, 06:01:32 pm
content: Yes, without an actual schematic and what the objective voltage and maximum current output required, it's hard to help you with any precision. Certainly not be able to tell you component values to purchase.

author: chriswebb
author_rank: Unknown
timestamp: October 19, 2016, 07:45:00 pm
content: OP what specs are you looking for with your 12VDC power supply? Have you tried looking for relatively cheap solutions that are already built, or are you using this purely for a learning exercise? This supply seems to have good reviews on amazon, buthttps://www.amazon.com/gp/product/B01FF5X3SU/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1