Dual Secondary Transformer with multiple regulator question - Page 1
Forum section: electronics / beginners

author: advark
author_rank: Contributor
timestamp: September 09, 2016, 03:59:29 am
content: Hello everyone,I need to build a small 3 rail PSU with the following requirements:24V @ ~ 100mA (OpAmps supply)12V @ ~ 200mA (2x Cooling fans)5V @ ~ 500mA (Logic and other circuitry).I have a transformer 15-0-15 transformer that can be wired either in serie or in parallel. I have a feeling that if I power up the attached design, magic smoke will show up, if not worse. My concern, here, is the grounds are not tied to the same point on the transformer. Is my first impression correct?I could drive the 7812 from the 7824, but it will quadruple the heat dissipation from ~2.1W to ~8.4W. I want to avoid using a switch mode regulator to avoid noise on the opamps because I need as much precision as possible out of those. Any suggestion for improvements?Many thanksEDIT:Filter caps were intentionally left out for clarity. -- Thanks MosherIV for pointing that out.The OpAmps inputs are fed with a 0-20V signal. I'm not sure how to deal with it if I use a split ±12V supply, as many have suggested. The OpAmps are LM324 and unless I missed something in the datasheet, the input cannot exceed the power rails. Ideally, no current should be drawn from the input signal, although I can live with a few microamps.

author: MosherIV
author_rank: Super Contributor
timestamp: September 09, 2016, 08:27:31 am
content: HiI assume that you have deliberately left all the capacitors out in the schematic (at least I hope you have!).The design will not work because you have tied the 0V/GND together and the way you have wired the transformer output will not allow that.You can tie the 0V of 12V and 5V but not to the 24V

author: Zero999
author_rank: Super Contributor
timestamp: September 09, 2016, 09:13:57 am
content: Why not use one bridge rectifier and use the centre tap to power the 5V regulator?By the way, the LM7824 is only rated to a maximum input voltage of 40V. A 30V transformer will give a peak output voltage of 42V, even more when unloaded. Use a 12V - 0V - 12V transformer.

author: tatus1969
author_rank: Super Contributor
timestamp: September 09, 2016, 09:16:54 am
content: I assume that your opamps will normally require symmetrical +-12v. Then you can connect the +5 LDO in parallel to the +12.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 09, 2016, 01:03:09 pm
content: To get a +- supply from the split transformer you only need one rectifier. The center tap also provides the center for DC voltage. As the voltage might be to high for a 7824, one might use a 7912 and the 7812 to make it a +-12 V supply. This might be the better choice for the OPs anyway.

author: advark
author_rank: Contributor
timestamp: September 09, 2016, 04:06:24 pm
content: Quote from: Hero999 on September 09, 2016, 09:13:57 amHeat dissipation was my concern. It would have added an extra 1.5W on 7805. Thanks for the info. I always forget about that 0.707 peak.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 09, 2016, 04:49:58 pm
content: To reduce the power for the 7805 one could use a series resistor between the rectifier and filter cap for the 7805. Some of the power is transferred to the resistor. In addition the power factor can be better and thus the total power loss a little lower. In a way you reduce that extra factor of 1.4 you normally get from the rectifier and filter cap.

author: Zero999
author_rank: Super Contributor
timestamp: September 09, 2016, 05:30:57 pm
content: Why not use 24V fans?

author: tatus1969
author_rank: Super Contributor
timestamp: September 09, 2016, 06:30:56 pm
content: Quote from: advark on September 09, 2016, 04:06:24 pmAs your opamps will only draw little current, you could connect the 5V LDO to the output of the 12V LDO. This would distribute the dissipation more evenly.

author: advark
author_rank: Contributor
timestamp: September 09, 2016, 08:12:36 pm
content: Quote from: Hero999 on September 09, 2016, 05:30:57 pmActually I have a few of 12V handy, so I started the design using these. I could use 24V fans an get rid of one regulator, indeed.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 09, 2016, 09:12:47 pm
content: As the voltage is too high for the normal 7824, two 12 V regulators like 7812+7912 might be a good option instead. One could still have the two fans in series.

author: Brumby
author_rank: Unknown
timestamp: September 10, 2016, 01:42:41 am
content: Quote from: advark on September 09, 2016, 03:59:29 amFrom the thread title, original diagram and this statement, It seems you have a transformer with two, separate, identical secondary windings. If that is the case, then there is a very simple approach.Just make two complete, but separate, circuits for 12V and then connect to +ve of one to the -ve of the other**. This will be your 0V reference. The remaining +ve will be +12V and the other -ve will be your -12V. You can then throw in the +5V on the +12V side.** Use a switch and you can connect them at will, giving you a split supply or two floating supplies.

author: tatus1969
author_rank: Super Contributor
timestamp: September 10, 2016, 06:07:55 am
content: Quote from: Brumby on September 10, 2016, 01:42:41 amAlready proposed by some of us including me

author: Brumby
author_rank: Unknown
timestamp: September 10, 2016, 07:18:53 am
content: Quote from: tatus1969 on September 10, 2016, 06:07:55 amI didn't pick up on that in this thread.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 10, 2016, 09:05:23 am
content: The solution with two separate 12 V supplies is quite good, as one can use two 7812 regulators and the extra drop at the rectifier does not matter with a rather high transformer voltage.

author: David Hess
author_rank: Super Contributor
timestamp: September 11, 2016, 02:41:08 am
content: Stacking the +12 volt regulated outputs for 0-12-24 or +/-12 is probably a better idea simply to avoid using a 7824 with its input voltage limitation however there is a solution to the original problem.Configure the secondary as shown and use one bridge rectifier between the ends to produce the unregulated 24 volt supply. Tie the negative output to ground. Now the center tap by itself becomes the unregulated 12 volt output and you can put the bulk input filtering capacitor between the center tap and ground.If this is hard to visualize, then imagine instead using the bridge rectifier and described to produce unregulated +/-12 volts with the center tap grounded. Now move ground from the center tap to the -12 volt unregulated output. The transformer secondary is floating so it does not care where the ground connection is as long as there is only one.

author: advark
author_rank: Contributor
timestamp: September 11, 2016, 05:53:23 pm
content: Quote from: David Hess on September 11, 2016, 02:41:08 amThat's great. Makes a lot of sense. Here is what I understand (filter caps omitted).

author: MosherIV
author_rank: Super Contributor
timestamp: September 11, 2016, 10:06:40 pm
content: HiNope, I do not think this is what was suggested.Each secondary should have a rectifier, which goes to a LM7812 but DO NOT join the 0V together. Join the output of one LM7812 to the 0V of the other, you will get 24V across both.The LM7805 shloud be on the outpt of the first/lower LM7812 so that it is referenced to 0VHope this helps (sorry typing this in my tablet so I cannot draw a schematic right now).

author: David Hess
author_rank: Super Contributor
timestamp: September 11, 2016, 11:17:58 pm
content: Here is what I suggested. I know it looks weird but the centertap becomes a positive full wave rectified output through the two diodes to ground in the bridge rectifier.

author: advark
author_rank: Contributor
timestamp: September 12, 2016, 01:54:40 am
content: Quote from: David Hess on September 11, 2016, 11:17:58 pmWell, a little bit for a newbie. That would have never crossed my mindWhat confuses me to most is that U2 input comes directly from the transformer...Anyway, I wil run some tests next weekend as I'm out of town for the week (vacations are over) I'll keep you posted with the results.Thanks a lot.

author: Brumby
author_rank: Unknown
timestamp: September 12, 2016, 03:34:08 am
content: This is what I had in mind, but without caps in place (obviously) This is only showing the two 12V sections. Just add the 5V into the top one.NOTE: This is only possible if the two secondaries 6-7 and 9-10 are completely separate windings.Sorry - both regulators are labelled U2. I missed that.

author: Brumby
author_rank: Unknown
timestamp: September 12, 2016, 03:52:13 am
content: IF not, there's the classic split rail design you can find in a hundred places.Here's one example:

author: David Hess
author_rank: Super Contributor
timestamp: September 12, 2016, 04:06:48 am
content: Quote from: advark on September 12, 2016, 01:54:40 amIf you remove the regulators and move ground to the centertap, then it becomes the split rail design Brumby posted. It just looks weird because it is another way to draw it.Here is an example where it is used in the Tektronix 465 oscilloscope.