power supply - Page 1
Forum section: electronics / beginners

author: Unknown
author_rank: Unknown
timestamp: October 10, 2013, 10:32:08 pm
content: Hello! im building a bench power supply with some voltage regulators ..the problem is how can make the diagram to work...i need for this power supply 5 volts(7805) -5volts(7905) 12 volts(7812 ) and -12 volts(7912)..I have many transformers so i dont have any problem with that but..lets say that I use 2 transformers .one for the -12 an +12and one for -5 and +5..can some one make a diagram for these four voltage regulators ..Thx

author: tehmeme
author_rank: Regular Contributor
timestamp: October 10, 2013, 11:31:46 pm
content: Quote from: zorkwan on October 10, 2013, 10:32:08 pmYou will only need one transformer. The transformer rating needs to be able to handle the current you will be drawing from both your supplys.The 78xx series datasheet has the circuits you need in firgure 16http://www.fairchildsemi.com/ds/LM/LM7805.pdfThe 79xx series datasheet has a similar circuit in figure 8:http://www.fairchildsemi.com/ds/LM/LM7905.pdfYou can use your 7812 & 7912 to regulate (instead of the 7815/7915 in the diagram)You can then use the same circuit but using the 7805/7905the input voltage in the diagram states +/-20V, but for the 7812/7912 you will need a minimum of +/-14V (since it has a 2Vdrop)14V input + rectifier Vdrop (2V for 2x1n400x) = 16V peak (pre-rectifier)To get Vrms = 16Vpeak / 1.414 = 11.3Vrmsso a 2x12V transformer will do. (24V centre tapped)google it :http://lmgtfy.com/?q=dual+supply+%207812+7912This was the first image google threw at me, includes transformer and rectifier:http://streampowers.blogspot.co.uk/2013/01/simple-dual-voltage-power-supply-12-volt.htmlYou could use the same transformer for both 12v and 5v if it has multiple output pairs.Even if it doesn't you can connect them in parallel from the output of the rectifier, but your 7805/7905 are going to be dissipating more power due to the output - input voltage differential(see figure 3 in lm78xx datasheet)Edit:The transformer will need to be centre tapped of course.There are other ways to do this, it really depends what and how you need to use it for.Edit 2corrected transformer voltage

author: mariush
author_rank: Super Contributor
timestamp: October 11, 2013, 12:55:34 am
content: A single transformer is enough, if it has a center tap, meaning it has 3 wires/terminals on the secondary side, 0 volts, middle , and the maximum voltage the transformer is rated for. For example, if you get a 24v AC CT (Center Tap) transformer, it will have 0 - 12v - 24v which is the same as saying - 12v - 0v - 12v , as the difference between the two terminals on the outside is still 24v.If you don't have such a transformer with a center tap, you can simply get two transformers and connect one of the terminals from each transformer together T1-1 ----- T1-2 + T2-1 ------- T2-2Voltage regulators like 78xx and 79xx (replace xx with 05, 09, 12) need about 1-1.2v above the output voltage to give you a stable output voltage. So in order to get 12v DC, the 7812 regulator needs to receive about 13.5v.Let's say you go with a 24v AC CT transformer (2 x 12v AC windings tied together in the middle) or two 12v AC transformers.Transformers are rated in AC rms, meaning the peak AC voltage will be about 1.414 x Voltage, so for a 12v winding, you have a peak of 17v. But, when you convert this AC voltage to DC and use a bridge rectifier (or four diodes), there's a voltage drop on the two diodes of about 0.7-1v on each. So, you have a peak of about 15-15.5 volts.... but just a peak, you'll still have the voltage going down a bit, then going back up, so you need to add a capacitor to make the voltage stay as much as possible close to 15-15.5v.Since you're aiming for at least 13.5v (otherwise the 7812 linear regulator may not give you 12v at the output), this means you can size the capacitor so that the voltage will always stay between 13.5v and 15-15.5v.You can use a simple formula to estimate how big such a capacitor would be : C = Current / 2 x AC Frequency x Allowed Voltage SagThe Current is how much you think you'd pull from 12v - the 7812 can do about 1.5A but let's say a maximum of 1A to make math simple. AC Frequency is 50 Hz or 60 Hz (US and some countries). I'll use 60 Hz here. Voltage sag is in your case 15.5v- 13.5v = 2v but let's be safe and lower this to 1.5vSo C = 1 / 2x60x1.5 = 0.0055555 Farads or about 5550 uF which doesn't exist, so go for the next standard size... so probably 2 x 3300 uF or a single 5600uF or a 6800uF capacitor would be suitable. The voltage rating for the capacitor would have to be at least 25v, since 15.5v is too close to 16v for comfort.so now you have transformer - bridge rectifier - capacitor - linear regulator 7812 - 12vYou can use this 12v output as the 7809 input to get 9v, you can use the 9v output as input for 7805 and get 7805 but then you'd have to keep in mind the 7812 will give power to the other two smaller regulators, so 12v + 9v + 5v together would only be able to give you 1A or the maximum the 7812 can output.The negative side is basically the same, just have to be careful about the polarity of the capacitors

author: Greyersting
author_rank: Regular Contributor
timestamp: October 11, 2013, 03:06:50 am
content: Sorry to be off topic but what does a power supply do?

author: Asim
author_rank: Regular Contributor
timestamp: October 11, 2013, 03:11:30 am
content: Quote from: Greyersting on October 11, 2013, 03:06:50 amsupplies power O.o

author: Greyersting
author_rank: Regular Contributor
timestamp: October 11, 2013, 03:19:03 am
content: Quote from: Asim on October 11, 2013, 03:11:30 amWhat does it supply power to?

author: Wytnucls
author_rank: Super Contributor
timestamp: October 11, 2013, 03:37:09 am
content: This:

author: roach969
author_rank: Newbie
timestamp: October 11, 2013, 04:18:14 am
content: I have that in my closet!

author: codeboy2k
author_rank: Super Contributor
timestamp: October 11, 2013, 07:50:13 am
content: Quote from: roach969 on October 11, 2013, 04:18:14 amhah! kids these days... we we're lucky ta' even 'ave a closet !

author: tehmeme
author_rank: Regular Contributor
timestamp: October 11, 2013, 12:23:32 pm
content: Quote from: Greyersting on October 11, 2013, 03:19:03 amAnything that needs power.https://en.wikipedia.org/wiki/Power_supply

author: Greyersting
author_rank: Regular Contributor
timestamp: October 11, 2013, 10:36:47 pm
content: The stuff I have used so far you can just plug in to the wall. What kind of stuff requires it?

author: mariush
author_rank: Super Contributor
timestamp: October 11, 2013, 10:47:06 pm
content: I have a feeling you're just a troll, but I'm going to amuse you.The things you plug in the wall have power supplies inside. If you want to make something you need to power that something with a power source, which can be a battery or instead of a battery, a power supply can generate the voltage needed by converting the AC power from the socket on your wall.As I see you asked in a post about arduino, the Arduino needs a power supply to run. It can either use the power coming from the USB port (which is generated by the power supply in the computer), or you can power it from a power adapter... that power adapter is a power supply.As you go on and learn about circuits and chips and opamps you'll find out it's useful to have around a power supply you can adjust to give you voltages you need, like -5v, -9v which are used by opamps for example. The USB can only give you 5v, power adapters are fixed to a particular voltage, like 7.5v or 9v.Later, when you make your own stuff (let's say you make something with a microcontroller and leds and whatever) you may be interested to optimize your circuit to use as little power as possible - in this case you may want to have a power supply you can finely adjust from let's say 1.25v to 5v, because maybe for example you'd want to see how much power the microcontroller uses at 2.5v instead of 5v (2.5v is basically 2 AA batteries)There's lots and lots of reasons for why you'd want to make your own power supply, besides just learning. Someone had to learn and make those "things you plug in to the wall".

author: Greyersting
author_rank: Regular Contributor
timestamp: October 12, 2013, 01:27:40 am
content: Thanks. Also, no I'm not a troll. I'm just a super beginner and have no idea about any of this stuff.

author: Shark Bate
author_rank: Newbie
timestamp: October 12, 2013, 05:58:49 am
content: I'm building the classic LM317/337 based adjustable power supply and I've also included a 5V out in it using 7805 and for this I used two transformers in parallel to mains. One was center-tapped -15V 0V 15V and the other just about 10V.Can you guys comment on this if this is a good practice?Thanks.

author: homebrew
author_rank: Frequent Contributor
timestamp: October 12, 2013, 06:31:27 am
content: From my point of view that looks ok. The main advantage you get out of using two transformers (or two separate windings) is that the +-15V and the adjustable part don't share a common ground which makes your supply more versatile.But please play safe when using mains voltage. While using your PSU and measuring your circuits you will touch the output leads if your supply very often. Thus your life depends on a safe separation between the line and low-voltage part.Incomplete list of critical points and hints (even if some might be more than required by law):1) If your case has any exposed metal parts, ground them properly !!!2) If your transformer has an exposed metal core, ground that, too.3) If you have the choice, use transformers with separate winding chambers for high and low voltage part.4) Have a fuse before and after your transformer(s) with appropriate rating.5) Use double insulation on all line wires and heat shrink all exposed connectors/joints etc. (Make it safe to handle even when the case is opened)6) Make it mechanical robust. Tie down all live wirings to the case with cableties.7) Never run cables from low-voltage side together with cables from the line voltage side. At best ensure that they simply can't get into contact at all.I trust my life on my PSU and I would never ever use crapy/cheap wallplug (even worse with those SMPS types) PSU for any experimentations...Cheers,Peter