What kind of range of voltages could I get out of this power supply circuit? - Page 1
Forum section: electronics / beginners

author: dentaku
author_rank: Frequent Contributor
timestamp: February 26, 2014, 04:20:58 am
content: If I was to build thishttp://www.musicfromouterspace.com/analogsynth_new/WALLWARTSUPPLY/WALLWARTSUPPLY.phpwith a 15V VAC 400ma power adaptor, I'm wondering what the practical range of voltages I could get out of it IF I replace the fixed regulators with LM317/LM337 regulators?It seems that I could get between 7V and 17V out of it but I'm not sure how much heat would be created if I go down too low.How big of a range could I expect to reasonably get out of this?I mostly need +-9V, +-12V and +-15V because that's what most DIY synth related circuits use and I'm tired of messing with batteries.

author: mariush
author_rank: Super Contributor
timestamp: February 26, 2014, 06:09:15 am
content: A linear regulator like LM317 will need 2v above the output voltage to output a smooth output voltage. You can easily get anything within 1.25v - output voltage you want.There are linear regulators that need less voltage above the output to work properly, for example **1117 chips (lm1117, nxp1117 etc) will need only 1v above output voltage but can only output 0.8-1A of current, compared to LM317's 2A maximum.Or there's regulators like MIC2941 which only need 0.04v above output voltage to output the desired voltage.The higher the voltage difference between input and output, the more the regulator will heat up, so while you could output 1.25v from a 17v input, at 100mA of power consumption you'll basically dissipate (17v - 1.25v) x 0.1A = 1.575 watts which means your regulator must have a decent heatsink.If you just use the linear regulator without a heatsink, it can probably handle about 0.5-0.75 watt continuously without becoming unstable. Screwed to the pcb like in the picture on the site, it could maybe handle 1 watt.With 17v in, 15v out, you have 2v difference so 1w will be dissipated with just 0.5 amps.So the range of voltages is not a problem, but the current output is.As there's only one diode to provide half wave rectification, if you have a 12v AC wallwart like in the picture, that can output maximum 1A, after the diode you'll have a peak voltage of 12v AC x 1.414 = 17v DC, minus the voltage drop on diode which is about 0.6-1v. So you have a peak DC voltage of about 16v.The current is about 0.28 x Iac so you have a maximum of about 0.28 A. Let's say 0.30A for easier math.Now that 16v is peak dc, so you need capacitors to smooth out the output voltage and get the dc voltage above the minimum voltage required by the regulators.In the picture on that site, they use 7812 which has a dropout voltage of 2v, so the input voltage must be a minimum of 14v all the time.Had we used a bridge rectifier, we could have used the formula Capacitance = Current / 2 x FrequencyAC x Vripple to determine the capacitance required to keep the dc voltage above 14v.With a single diode (half bridge rectification), the formula changes to Capacitance = Current / Frequency AC x Vripple.Vripple is voltage range between minimum voltage you want and the peak voltage you'll get after rectification. You want 16v - 14v = 2v.So for 0.3A which is the maximum you'll get from 12v AC 1A, the capacitance is C = 0.3/ 60x2 = 0.3 / 120 = 0.0025 or about 2500uF. So going with a single 3300uF would probably be enough, but nothing stops you from using more than one.They're using 3 x 3300uF for each regulator and I don't know why, I can only assume he wanted less voltage drop, or he did the math for 1A of current, which is not realistic from a 12v AC 1A wallwart.Also keep in mind with this design, both 12v and -12v have a total of 0.3 amps, if you load one the other will not be able to output much current.My recommendation would be to simply forget about a AC wallwart and go straight to a 24v (or higher) AC transformer with a center tap, which would allow you to use a bridge rectifier on each secondary winding and have better control.For example get one of these transformers :http://www.digikey.com/product-detail/en/PL30-28-130B/MT3135-ND/98232It has two secondary windings, 14v AC each, 30VA , so you have 15VA per winding. Follow picture

author: mariush
author_rank: Super Contributor
timestamp: February 26, 2014, 06:28:44 am
content: Or a more simplified circuit drawing...note: blue, red and black lines aren't connected where they cross over each other. don't join them.

author: dentaku
author_rank: Frequent Contributor
timestamp: February 26, 2014, 02:19:53 pm
content: Quote from: mariush on February 26, 2014, 06:28:44 amI was hoping to get away with using a wallwart but considering the ones I have are rated for 15VAC 400mA they might not be terribly useful especially after seeing your calculations.Late last night after posting this I realised that the MFOS schematic only does half-wave rectification (at least I know that much) so it's not really the best anyway.I happen to have TWO identical adaptors like this.I wonder if I could use both of them together in some way?I just finished reading thishttp://learn.adafruit.com/power-supplies/transformer-based-ac-slash-dc-convertersand it's quite good for questions beginners might have.

author: dentaku
author_rank: Frequent Contributor
timestamp: February 26, 2014, 04:01:16 pm
content: I just found that I DO have a center tapped transformer that I believe came from an old stereo of some sort.My meter measures 12.3 to 12.4V AC and my old scope measures around 17.5V peak which I guess make sense considering this calculationhttp://www.wolframalpha.com/input/?i=RMS+voltage+calculator&f1=12.35&f=ACVoltage.vrms_12.35&f2=60+Hz&f=ACVoltage.f%5Cu005f60+Hz&a=*FP.ACVoltage.SignalShape-_sine&a=*FVarOpt.1-_***ACVoltage.vrms--.***ACVoltage.vpp--.**ACVoltage.vp---.**ACVoltage.f---I wish it measured a bit higher though, because that doesn't give me enough room to get +15V and -15V DC out of it.Oh well, I think I'm going to see if I can breadboard myself a 9 to 12V power supply out of it just to learn more about this stuff.

author: mariush
author_rank: Super Contributor
timestamp: February 26, 2014, 04:17:40 pm
content: In regards to your two transformers .. yes you can get them working together. Connect the primary wires in parallel , 1-1 , 2-2 , connect secondary wires in series 1 , 2-3 , 4 2-3 becomes center tap.As for our center tap transformer, judging on the size of the transformer it's probably only good for about 10VA ... so 24v AC 10VA , about 0.4 Amps. You'll probably get 150-200mA DC voltage in total on both taps.That small transformer also can have about +5-15% output with no load, as you measure on the transformer - when you actually pull some power from it, the ac voltage may drop to about 11v so keep that in mind. A cheap way to test this would be using a simple 12v lightbulb that you can buy in any gas station or car parts store.But the point is when you rectify the voltage you'll get closer to peaks of 15.5-16v, not 17v. Should still be good for a +/- 3-12v power supply.

author: dentaku
author_rank: Frequent Contributor
timestamp: February 26, 2014, 05:02:22 pm
content: Quote from: mariush on February 26, 2014, 04:17:40 pmSo you mean I can safely connect the tips of two 15V AC 400mA wallwarts together like this? (the way I connect two 9V batteries all the time)This would let me use it as if I had a center tapped transformer.edit...OOps, that image should be using 15V, not 12.3V but the concept is the same.

author: mariush
author_rank: Super Contributor
timestamp: February 26, 2014, 05:15:58 pm
content: I mean like this:

author: dentaku
author_rank: Frequent Contributor
timestamp: February 26, 2014, 09:47:09 pm
content: OK.The "primaries" of wallwarts are connected to the prongs that go into the 120V wall outlet and if you plug them both into a power strip they're definitely connected together like your image if they're both inserted in the same orientation. These are two identical AC Adaptors so that's helpful.The "secondaries" part of the image would represent the tip that goes into the barrel jack so you would connect the center of one to the outside (sleeve) of the other.I've always been curious if you could do this with wallwarts/plugpacks just like I do with 9V batteries all the time to give me +/- 9V.

author: dentaku
author_rank: Frequent Contributor
timestamp: March 06, 2014, 04:06:11 am
content: Just an updateI took my two identical 15V AC wallwarts and connected them together like mariush's diagram, used a 3N257 bridge rectifier and two 1800uF caps and it works. It gives me a bit over +/- 24V DC with no load.I also tried it with my little 12.66,0,12.66 center tapped transformer and it worked well too. After rectification I get almost +/- 16V from it which is a shame because it doesn't give me enough to make a +/- 15V power if I put some regulators in the circuit.I've never built anything that directly involved 120V AC from the wall before so this was a good learning experience.