trouble with bipolar powersupply - Page 1
Forum section: electronics / beginners

author: arjepsen
author_rank: Contributor
timestamp: November 07, 2016, 07:03:38 pm
content: Hey.I'm trying to make a bipolar +/- 12V power supply.I'm using a 2x18V transformer - connecting the two secondary windings to make a center tap.I'm using a TS7812 and a L7912CV voltage regulator. I'm attaching a schematic here of how I'm connecting it.My trouble is, that I keep getting only about 5 volts on the positive side, though the negative side seems to output -12V.I've tried another TS7812 to see if the chip was bad, but it's the same result.I know I could do more filtering, but I asume I should still see 12V on both the positive and negative side?Any help?

author: tatus1969
author_rank: Super Contributor
timestamp: November 07, 2016, 07:33:49 pm
content: possibly you mixed up pinout of the 7912? Keep in mind that the two have different pinouts: 7812 in=1 out=3 gnd=2. 7912 in=2 out=3 gnd=1 (in TO220 cases). Your diagram doesn't show your pinning.

author: arjepsen
author_rank: Contributor
timestamp: November 07, 2016, 07:48:54 pm
content: Yeah, I checked that.(A few years back I made a powersupply for some pedals, and couldn't quite understand why the thing got so hot..... Now I know I have to check the pinouts at least 3 times...)(TS7812: 1-in 2-Gnd 3-out, L7912: 1-Gnd 2-in 3-out)

author: tatus1969
author_rank: Super Contributor
timestamp: November 07, 2016, 08:01:08 pm
content: hmmm...do you have a load connected? how did you measure the voltages? nothing obviously wrong so far...

author: arjepsen
author_rank: Contributor
timestamp: November 07, 2016, 08:09:16 pm
content: no - I didn't put a load on it yet - but I presumed that even without a load, the positive and negative side should be about equally close to Gnd?

author: tatus1969
author_rank: Super Contributor
timestamp: November 07, 2016, 08:13:21 pm
content: sure. how did you measure the voltages?

author: arjepsen
author_rank: Contributor
timestamp: November 07, 2016, 08:17:12 pm
content: multimeter - measured the 7812 from plus to Gnd (pin 3 to pin 2)measured 7912 from minus to Gnd (pin 3 to pin 1)

author: tatus1969
author_rank: Super Contributor
timestamp: November 07, 2016, 08:27:16 pm
content: I have no idea, except stupid things like both 7812 broken, bad china fakes, it being an 7805 instead, classic mirroring mistake when looking at the PCB's solder side, ... I assume you have checked both input voltages?

author: singapol
author_rank: Frequent Contributor
timestamp: November 07, 2016, 08:58:59 pm
content: Please cheack transformer voltages and the diodes if you have not done so.

author: arjepsen
author_rank: Contributor
timestamp: November 14, 2016, 08:16:54 pm
content: ok, this is making me quite annoyed now.I just received a new order of parts, just to be certain that I had some brand new components to use, including a new 2x15V trafo, a couple of l7912's and l7812s and a couple of diode bridges.Anyways, I'd start wondering if I'd fried something, so I went to the datasheet for the l7912 and read that it's around 30v max input. Hmm...I've been messing around with the circuit without a load, so I knew the rectified voltage would be a bit high - and it was about 35 +/- to the centertap ground.hmmm.. well good thing I got new components.Anyways, I tried to put a load on the circuit by a 220k resistor from out to ground on each regulator. But the voltages was still high. Hmmm.....finally I strapped some zener diodes from input to ground on each regulator, which brought the input voltages down to around 26V (+/-) on each input.Put a new pair of regulators in there, just to be sure - but still no dice. Voltages on the output of the l7812 is hanging very nicely at 11,6v but the 7912 is at 17V!?!?!?Is this problem showing up becuase of a no-load situation or?

author: bktemp
author_rank: Super Contributor
timestamp: November 14, 2016, 08:57:27 pm
content: 78/79 regulators should regulate the correct voltage without any load.Try adding diodes at the output of the regulators, because those regulators start to behave strange when the output gets pulled negative (or positive for the negative version).If it does not work, post a picture of your circuit. It is easy to get the pinout wrong, therefor let others verify if everything is wired correctly.

author: _Wim_
author_rank: Super Contributor
timestamp: November 14, 2016, 09:07:22 pm
content: Just to be sure, the ground of center tap of the transfo is connected to the gnd of the 7812 and 7912?

author: janoc
author_rank: Super Contributor
timestamp: November 14, 2016, 09:12:40 pm
content: Quote from: bktemp on November 14, 2016, 08:57:27 pmErr no - they don't. The datasheet specifies the output voltage for output current from 5mA up or so, with no load it is unspecified and depends on the part you get.I have encountered a few of these regulators where the output voltage was out by almost a volt with no load and they started to regulate properly only when some minimal load was connected.

author: bktemp
author_rank: Super Contributor
timestamp: November 14, 2016, 09:27:52 pm
content: Quote from: janoc on November 14, 2016, 09:12:40 pmYou are right, there is no specification for <5mA. But I have never seen a 78/79 regulator with no load outputting a much too high voltage at room temperature (I always build the power supply first and verify the voltage before populating other ics and the voltage is spot on for most regulators). Therefore if I would encounter one outputting a significantly higher voltage, I wouldn't use it, because there is something wrong if it behaves different than most other regulators. The specification is valid for the full operating temperature (up to 125°C), therefore the minimum current is probably there because of some increased leakage currents at high junction temperature.

author: arjepsen
author_rank: Contributor
timestamp: November 14, 2016, 09:41:53 pm
content: Yes, the Center tap is connected to the grounds of the regulators.Here's a link to the trafo producers page:http://www.hahn-trafo.com/english/pcb-transformers-ei30t1.phpI've connected pin 7 and 9 together to form the center tap/ground (there's no pin.I wonder if the chip is instantly ruined, if the voltage on the input pin goes above/below 30V? Doesn't seem like it with the l7812 which just happily keeps putting out the correct voltage.And yeah, this is off by much more than a few volts.A 220k resistor from output pin to ground should work ok as a load, right?

author: bktemp
author_rank: Super Contributor
timestamp: November 14, 2016, 09:56:08 pm
content: Try connecting 1kohms to make sure there is a load drawing >5mA.The absolute max voltage is 35V, so 30V shouldn't do any damage.

author: Zero999
author_rank: Super Contributor
timestamp: November 14, 2016, 10:04:13 pm
content: That's odd. I couldn't view the PDF in Firefox, so I had to use LibreOffice.It should work. Have you mounted the L7912 & L7812 on a heat sink? Are both of the tabs electrically isolated from one another?What load are you planning to draw? The main filter capacitors may need to be significantly larger than 470µF.The maximum rated input voltage for the L7812 40V and the L7912 is 35V so the input voltage shouldn't be a problem.http://www.st.com/content/ccc/resource/technical/document/datasheet/c9/16/86/41/c7/2b/45/f2/CD00000450.pdf/files/CD00000450.pdf/jcr:content/translations/en.CD00000450.pdfhttp://www.hobbytronics.co.uk/datasheets/L7812.pdf

author: arjepsen
author_rank: Contributor
timestamp: November 14, 2016, 10:15:09 pm
content: ok, here's a few picks of the current setup

author: Audioguru
author_rank: Super Contributor
timestamp: November 15, 2016, 05:15:04 am
content: 2 problems:1) Intermittent contacts on the solderless breadboard. Solder everything together properly!2) Who is TS? It is Taiwan Semiconductor Company. Use a reputable manufacturer like Texas Instruments.

author: tatus1969
author_rank: Super Contributor
timestamp: November 15, 2016, 06:27:32 am
content: did you try with different output capacitors? maybe the ones you use are all old and dryed out?

author: tatus1969
author_rank: Super Contributor
timestamp: November 15, 2016, 06:44:38 am
content: why have you chosen a 2x18v transformer? a 2x12 would have been better in terms of efficiency. but you still should never have more than +-26v (with load)after your rectifier. if you measure that, then the problem already starts before the regulators. is your transformer very small?EDIT.. your transformer *is* very small. This means that its no-load voltage will be way above its rated voltage. This is because transformer makers normally specify the output voltage at nominal output current. But these tiny transformers have significant winding resistance. What the manufacturer does to compensate this, is to add more turns to the secondary. At full load, your new transformer will deliver +-15V, but at no load it will be way above that. And that is what is probably killing your regulators. My suggestion here would be to- use a transformer with a smaller output voltage. +-12V is more than enough, remember that the rectified voltage is +-12V * 1.41 = +-16.9V. This is more than enough drop for the regulators.- add zener diodes to both regulator inputs, 1W / 27V types should be fine.

author: arjepsen
author_rank: Contributor
timestamp: November 15, 2016, 02:27:08 pm
content: Thanks for the info :-)I wonder though - wouldn't a 2x12V trafo be to low on the output to supply the regulators with high enough voltage, when under load?What confuses me is that the l7812 consistantly seems to be fine, and delivers about 11,5 volts.However, even when I used zener diodes on the inputs and made sure that there only was about 26 volts going in on both regulators, AND put in a brand new 7912, the output voltage was around 17 volts - no difference between a brand new, and an "used" regulator.I can understand that there can be less trust in the taiwanese products, but I presume that would more be one defective chip here and there - not ALL bad...?

author: CJay
author_rank: Super Contributor
timestamp: November 15, 2016, 03:36:30 pm
content: 11.5V from a 7812 off load is not fine, it really should be 12V.For the life of me I cannot see how you're getting more than 30V input to either regulator if you're measuring from the centre tap of the transformer and yikes, mains on a breadboard??Power it all off and measure continuity from the centre tap to the ground pin of both regulators, measure continuity from centre tap to the shared connection of the main capacitors.Are you sure you've got a connection between the secondaries, it looks like you've got the link between them in adjacent holes and both wires in one hole would be a tight fit.

author: Lee Leduc
author_rank: Regular Contributor
timestamp: November 15, 2016, 04:41:37 pm
content: Does your circuit have protection diodes from the output rails toground? If the negative regulator "comes up" quicker than thepositive regulator and pulls the positive regulator's output below ground, theregulator can latch up. It's rare nowadays but I have seen it happen with some brands.See Fig.16 from the attached Fairchild data sheet for a dual supply with output protection diodes.

author: janoc
author_rank: Super Contributor
timestamp: November 15, 2016, 05:34:12 pm
content: Quote from: bktemp on November 14, 2016, 09:27:52 pmI have seen this with some breadboard power supplies (this style:http://artofcircuits.com/wp-content/uploads/2014/04/MB102-3V3-5V-3.jpg). Those are most often using Chinese clones of the regulators, so it is well possible the circuit inside isn't exactly the same as the original LM78xx. The supply I have will give 3.5-3.8V without load, if I recall right, once loaded the output voltage is correct.I have seen this also on another ocassion in a non-Chinese device - again the regulator wasn't working right until loaded properly. So it isn't behaviour I would rely on.

author: janoc
author_rank: Super Contributor
timestamp: November 15, 2016, 05:39:08 pm
content: Quote from: CJay on November 15, 2016, 03:36:30 pmThere, fixed it for you. That is seriously scary stuff. Breadboards are definitelly not rated above 50V and many have metal plate backing - with the only thing insulating the contacts from it being paint and/or a thin layer of doublesided tape ...

author: arjepsen
author_rank: Contributor
timestamp: November 15, 2016, 07:08:56 pm
content: Yeah, I'm aware that it's not the most safe setup - I am being reeeeeally carefull :-)Quote from: CJay on November 15, 2016, 03:36:30 pmUnloaded I get 25,6 volt AC between each side and CT. With nothing but the first pair of caps after the rectifier, I get 35,7 volt DC between each side and CT.And yes, both wires for the CT are in the same hole.Quote from: Lee Leduc on November 15, 2016, 04:41:37 pmJust tried with that and zeners. Still 11,6 on the 7812 and 17,5 on the 7912.One question about the datasheet though. In firgure 16, the negative rail has bigger caps than the positive rail. Would that make any difference? (though of course I'm already using bigger caps than in that figure).Another question: shouldn't it be enough load to put a 220k resistor from each output to ground?

author: Zero999
author_rank: Super Contributor
timestamp: November 15, 2016, 07:22:19 pm
content: Why not get yourself some strip board?Don't use it for mains. The safe way to do this to solder the neutral of the mains cable directly to the transformer's primary, connect phase conductor via a 100mA fuse in a suitable holder.

author: tatus1969
author_rank: Super Contributor
timestamp: November 15, 2016, 07:30:09 pm
content: Quote from: arjepsen on November 15, 2016, 02:27:08 pmThe transformer will deliver 2x12V AC at its rated (sinusodial) current. After the bridge rectifier, this will result in roughly +-17V DC (keep in mind that the capacitors charge to the PEAK voltage, whereas that 12VAC is an RMS value), minus one diode drop for each rail. Plus the current is not sinusodial but pulsed, but I would assume that you'll still get +-15V, which is enough for the 7812/7912.As I explained, for such small transformers the secondary voltage can be way higher when there is no load, this is why you measure more than +-30V without load. You say you added zeners to both regulator inputs, so that should limit that voltage. Did you check their temperatures? Also, are your electrolytic capacitors new and fresh?

author: tatus1969
author_rank: Super Contributor
timestamp: November 15, 2016, 07:40:52 pm
content: Quote from: arjepsen on November 15, 2016, 07:08:56 pm220k is definitely not enough to provide a base load, the current would just be 12V/220k = 55 microamps. Try a lower resistor value, for a 0.25W leaded part -> 680R.

author: tatus1969
author_rank: Super Contributor
timestamp: November 15, 2016, 07:47:49 pm
content: Quote from: arjepsen on November 15, 2016, 07:08:56 pmShould be okay as you do it.+1 to lift off the primary from the breadboard and see if that makes a difference (plus avoiding possible real danger).

author: arjepsen
author_rank: Contributor
timestamp: November 15, 2016, 08:14:04 pm
content: Man, this is getting weird.First of all, yes the caps are new. Tried different ones - same result.And I did check very carefully, to make sure the mains leads arent having a continuity to anything.Anyways, I just put a couple of 470R 1W resistors from each output to ground.Now the 7912 shows a nice -11.8 Happy happy.However, now the 7812 only shows 0.7The resistor across the 7912 does get a bit warm, whereas the one across the 7812 stays cool.EDIT:Just changed the 7812 again, and it turns out that was a bad one. Now the 7912 is around 11,8 and the 7812 is around 11,6 under load.However unloaded they are right back at -17,8 and +11,7

author: tatus1969
author_rank: Super Contributor
timestamp: November 15, 2016, 08:43:25 pm
content: congrats, looks like you have won this battle! just one last question: what specific type are your output caps? many medieval linear regulators have problems with low ESR types and tend to overshoot. maybe that is what you are experiencing, you would be able to verify that with an oscilloscope. but maybe it is bad chinese replica that is causing your priblems. anyway, minimum load is a must, maybe you turn these into some nice indicator LEDs

author: arjepsen
author_rank: Contributor
timestamp: November 15, 2016, 08:54:37 pm
content: Caps are panasonic fc's. They should be fine, right?

author: tatus1969
author_rank: Super Contributor
timestamp: November 15, 2016, 09:40:06 pm
content: Quote from: arjepsen on November 15, 2016, 08:54:37 pmhard to say. the 100uf 50v part has only 0.162 ohms. the 78/79 family was not designed for such low ESR caps back then, simply because they didn't exist. you can try adding 1 ohm in series with them and check if it makes a difference. modern LDOs mostly specify the ESR range that is allowed, the better one are "ceramic stable" meaning the ESR can be very low.