transformer caps - Page 1
Forum section: electronics / beginners

author: josephpate
author_rank: Contributor
timestamp: September 04, 2012, 04:39:57 pm
content: Scored a doorbell transformer from work super cheap now I need to turn it into something usable. The transformer itself claims to output 10V 5A, but after running it through a 4-diode full rectifier I get ~14-15V vmax on my scope. I know I will need smoothing caps to maintain a clean DC voltage out of this thing but i'm unsure of how much capacitance I will actually need. Anyone know of a calculator or something I can use? I stuck 7 470uf caps (what I had lying around) into my breadboard, and the output seemed smooth using a 1k resistor as a dummy load (didn't want to go lower since they are only 1/4W) but I'm unsure if that is overkill or underkill as I can't really put it under a significant load.My ultimate goal is to make a crude adjustable power supply (lm317 probably, this seems to be the easiest method the internet could provide me. Not ready to tackle microcontrollers yet).

author: redtails
author_rank: Contributor
timestamp: September 04, 2012, 05:40:03 pm
content: For smoothing out voltage, it is better to use a voltage regulator such as the 78xx series (7812 for 12v for example), or the adjustable LM317 series of regulators as you wanted toThe regulator will eat away any irregularities in the voltage, though there'll be a tiny bit of ripple always left. Take a look at the datasheethttp://www.ee.buffalo.edu/courses/elab/LM117.pdffor application hints on necessary capacitance. I think they're suggesting 10uFI'll copypaste some other person's post:Quote

author: Kevin.D
author_rank: Frequent Contributor
timestamp: September 04, 2012, 06:11:57 pm
content: What redtails says it correct in that the regulator will remove a lot of the ripple as long as it's not too big .i.e the "ripple voltage lowest point" must not fall below the dropout V of the regulator .see this easy to understand article : -http://www.silicontronics.com/index.php?action=ezportal;sa=page;p=23

author: josephpate
author_rank: Contributor
timestamp: September 04, 2012, 07:56:30 pm
content: I've read the data sheet but wasn't sure if that vin was my rectified ac or some voltage that was already smoothed. Since the rectified AC drops down to 0, wouldn't that hit the dropout voltage of the lm317? Is that 10uf cap enough to keep it running?EDIT: actually it saysQuoteSo I guess I would still need the filter caps.

author: T4P
author_rank: Super Contributor
timestamp: September 04, 2012, 08:20:52 pm
content: For good measure

author: SeanB
author_rank: Super Contributor
timestamp: September 04, 2012, 08:33:34 pm
content: The caps you used will work, though the ripple will be over a volt if you get to about 500mA of current. Provided you only want a output lower than the peak voltage less diode drop and regulator dropout, then you can use this, so you will be able to get a regulated output from 1.2V to just under 10V. More capacitance will help, though you will need a high current bridge rectifier, preferably a shottky diode one, as the lower voltage drop will be an advantage at this low voltage. you can probably use 10000uF or more with no problems, though to get the full 3.5A of DC current out of the transformer will need around 33000uF or more of capacitance to get a low value of ripple.

author: David_AVD
author_rank: Super Contributor
timestamp: September 04, 2012, 09:16:18 pm
content: I once heard someone say that 1000uF per Amp of required load was a vague rule of thumb.You can look up the "ripple rating" of capacitors in their data sheet. This will also give you some guidance in what the capacitor can handle by itself.Beware of going overboard with capacitance though, as I believe it can cause issues with blown transformer fuses (or bridge diodes?) due to the high charge pulses.

author: mariush
author_rank: Super Contributor
timestamp: September 04, 2012, 09:34:38 pm
content: You can check this page out :http://www.falstad.com/circuit/e-index.htmlIt uses java but if you have Java disabled in browser (I do, there's some o-day bugs in Java that can be used to infect your pc nowadays) you can download everything on your pc here:http://www.falstad.com/circuit/circuit.zipBrowse down to bridge rectifier with filtering... it shows the transformer + bridge rectifier + capacitor and a resistive load. You can play with resistance value, capacitor, voltage of transformer etc and you can probe at various places.But basically people already said what you need to take care of. You have 14-15v but at no or little load - at several amps you're probably going to hover at around 11-12v DC.You can get steady voltage using linear regulators - the ones in the style of 7805 need as much as 1.5-2 volts but there are other which need quite less than that.For example, two regulators I have experience with :LT1083 - up to 7.5A , 1v dropout at 5A :http://cds.linear.com/docs/Datasheet/108345fg.pdfMIC2941 - up to 1.25A , about 0.35v dropout at 1A :http://www.micrel.com/_PDF/mic2940.pdfSo as long as your transformer can give you desired output voltage + the dropout voltage + about 5-10% for safety and to factor in heat and other things, and you have the minimum voltage the transformer should provide.... and you have smooth filtered voltage coming out of the linear regulatorps. too much capacitance can cause quite large current pulses in the transformer, which can cause the fuses to pop. That's one of the reasons why you often see 10A fuses on linear power supplies, even though they're only 30v x 3A or something like that.

author: T4P
author_rank: Super Contributor
timestamp: September 04, 2012, 10:41:40 pm
content: Quote from: David_AVD on September 04, 2012, 09:16:18 pmyeah i said that, i too heard someone else say that before i was here, on diystompboxesQuote from: SeanB on September 04, 2012, 08:33:34 pmWe're dealing with regulators here so need not have so much capacitance, caps are only used in regulators to stabilize them but mostly for the 78xx and the 79xx a millifarad/thousand microfarad per amp is a vague rule of thumb but works nicely for me everytime

author: josephpate
author_rank: Contributor
timestamp: September 04, 2012, 11:53:36 pm
content: I bought some good rectifiers and a LM350 regulator (only goes down to ~1.2v but I guess i can live with that) the capacitors are really the only thing I need ..... so if I assume ~2.5A max output I could probably get away with 2500uf? Can't wait to breadboard it up and play with it.

author: JackOfVA
author_rank: Unknown
timestamp: September 05, 2012, 12:22:36 am
content: With a bit of simple math, you can derive the following approximations for required filter capacitor, given a known current load and target voltage drop:50 Hz power mains with full wave bridge:C = 10000 x I/ delta V60 Hz power mains with full wave bridge:C = 8333 x I / delta VC is the capacitance required in uF.I is the current load in amperesdelta V is the change in voltage from peak voltage to the point where the rectifier begins conducting on the next half cycle.Example ... 50 Hz mains frequency, want no more than a 2V sag from peak during the period when the rectifiers are not conducting. Current is 250 mA.C is 10000 x 0.25 / 2 = 1250 uF. In practice one would likely use the next largest standard value, say 1500 uF or 2200 uF.There are a bunch of approximations and assumptions in this simple formula, but it's a decent starting point.

author: mariush
author_rank: Super Contributor
timestamp: September 05, 2012, 12:23:16 am
content: Make sure you're also using a heatsink on that regulator. For breadboard tests it can work without but forget about 1A+ output without one.The difference in voltage is dissipated as heat... so for example if you want 3.3v out at 1 A and you have 10v input, you're dissipating (10v-3.3v) x 1A = 6.7 watts ... the temperature junction-case is about 2.5-3C per Watt so it heats up pretty fast without heatsinkYou'll need a 0.1-0.47 uF capacitor at the input and 10uF or better (33uF, 47uF electrolytics are cheap and doesn't hurt to go over) on the output.. but it says so in the datasheet.

author: sorin
author_rank: Frequent Contributor
timestamp: September 05, 2012, 04:08:24 am
content: @JackOfVAyou dont have considered the serial inductance and capacitance of the transformer

author: amyk
author_rank: Super Contributor
timestamp: September 05, 2012, 08:21:17 am
content: Quote from: David_AVD on September 04, 2012, 09:16:18 pmThat's what the inrush current limiter is for.

author: David_AVD
author_rank: Super Contributor
timestamp: September 05, 2012, 08:47:51 am
content: Quote from: amyk on September 05, 2012, 08:21:17 amIs that to stop the primary fuse blowing or protect the diode bridge though? How many linear power supplies have inrush limiting anyway?

author: JackOfVA
author_rank: Unknown
timestamp: September 05, 2012, 10:31:59 am
content: Quote from: sorin on September 05, 2012, 04:08:24 amAs I said there are many simplifications and assumptions. One also has to consider diode drop, wire resistance, transformer resistance, change in resistance with temperature and many other things. But if you want a quick estimate of a starting point, my simple technique is adequate.

author: David_AVD
author_rank: Super Contributor
timestamp: September 05, 2012, 10:38:56 am
content: Agreed. There is a lot over over thinking going on here.

author: josephpate
author_rank: Contributor
timestamp: September 05, 2012, 02:26:45 pm
content: Alright, one more question:Would you guys suggest sticking with the half-dozen or so 470uf caps that I currently have? or should I buy one or two huge filter caps to use? the smaller ones are "free" since i've already paid for them but they can be kind of cumbersome.

author: mariush
author_rank: Super Contributor
timestamp: September 05, 2012, 02:35:08 pm
content: If they're 470uF with 16v or less voltage rating, I'd suggest just buying new capacitors.Your transformer is 10VAC but at idle it's 14-15v like you say, which is very close to a 16v maximum rating, and 470uf/16v capacitors are also not that great at tolerating ripple.I think more than 10.000 uF is not needed. If those are hard to find, I'd suggest going for 2 x 1500-3300uF / 25-35v capacitors, in parallel.

author: josephpate
author_rank: Contributor
timestamp: September 05, 2012, 02:36:15 pm
content: the ones i have are rated at 50v, but I just checked and can get 1000uf caps for 22 cents from mouser so i'll be buying those.

author: mariush
author_rank: Super Contributor
timestamp: September 05, 2012, 02:50:35 pm
content: Just get a bunch of these, better value for money:http://uk.mouser.com/ProductDetail/Nichicon/UVR1E332MHD1AA/?qs=sGAEpiMZZMsh%252b1woXyUXj8Jx5z%2fmxZu6kSLwqOKAMK0%3dIf you want longer life/higher temp rating go for these:http://uk.mouser.com/ProductDetail/United-Chemi-Con/EKY-250ELL332MLN3S/?qs=sGAEpiMZZMsh%252b1woXyUXjxQaTr%252bq21uXx9oJtzfinow%3dor look for Panasonic FM/FR or Nichicon HE, HMWith 1000uF, you'd still need to use 3-5 in parallel to get enough capacity to get some usable voltage range for the linear regulator you put after.... it's easier to just use 2-3x3300 and they handle ripple better.Your transformer was only 10VAC so a 25v voltage rating would be OK, but if you want you can go for 35v for a few more pennies.

author: T4P
author_rank: Super Contributor
timestamp: September 05, 2012, 03:18:58 pm
content: Quote from: mariush on September 05, 2012, 12:23:16 amWell that's junction-to-case if heatsinked ... if ambient the story changes, less than even 1A at 2V drop and it will go into thermal shutdown