Please check my schematic? - Page 1
Forum section: electronics / beginners

author: r00n
author_rank: Contributor
timestamp: February 11, 2024, 10:44:56 pm
content: Would you mind having a look at my power supply schematic as attached please?I am using a power transformer that is a 30V with a centre tap so that i can get +/- 15V. I keep frying the diodes in the full bridge rectifier.

author: retiredfeline
author_rank: Frequent Contributor
timestamp: February 11, 2024, 11:07:39 pm
content: You shouldn't be labelling the secondary pins of the transformer with 15V and -15V. They have no polarity, it's AC at that point. And in a PCB design the -15V would get connected to the -15V output from the negative regulator.Otherwise I can't see anything obvious. Check the polarity of the diodes on your breadboard.

author: iMo
author_rank: Super Contributor
timestamp: February 11, 2024, 11:13:03 pm
content: Doublecheck wiring of your transformer. Perhaps you messed up the leads..

author: Benta
author_rank: Super Contributor
timestamp: February 11, 2024, 11:16:45 pm
content: Same comment as @retiredfeline.But why keep the transformer and the output connector floating at all?Just connect them. Much simpler.

author: scuzzyTerminator
author_rank: Regular Contributor
timestamp: February 11, 2024, 11:53:02 pm
content: Inrush current ?https://en.wikipedia.org/wiki/Inrush_current

author: vk6zgo
author_rank: Super Contributor
timestamp: February 12, 2024, 12:14:03 am
content: Quote from: r00n on February 11, 2024, 10:44:56 pmYour diodesare not & shouldn't be configured asa bridge rectifier.They are just a pair of old style "full wave rectifiers" used with a transformer with a centre tapped secondary.You can actually redraw them as two separate full wave rectifiers, imagining for the purpose of illustration, that they are using separate transformers.Because most people are so familiar with bridge rectifiers it is perilously easy to inadvertently wire them up in that configuration.

author: pqass
author_rank: Super Contributor
timestamp: February 12, 2024, 12:31:03 am
content: C3 and C10 are not needed; may cause issues. TI recommends only 0.1uF on the output.Seedatasheetand relevant section attached below.

author: Majorassburn
author_rank: Regular Contributor
timestamp: February 12, 2024, 10:48:29 pm
content: Quote from: scuzzyTerminator on February 11, 2024, 11:53:02 pmThat's what's wrong. 1N4007 diodes are 1Amp diodes. When you switch on the power, it may take several milliseconds to fully charge your input filter caps. That time period may exceed the 1N4007's transient overload capability so they see a short-circuit and die. Inrush current at cold power-on can reach 10's of amps, depending on the cap characteristics!Also, your 3-terminal regulators may draw up to 2+ amps when short-circuited or overloaded. The 1N4007's overload rating is for very short transients, not continuous current. So, they can overheat and short/open.Use minimum 3Amp diodes or a modular 3+Amp FW Bridge. I build that schematic (with some modifications) all the time with no problems.There is no problem with the capacitors, as drawn.Also, fuse your hot input lead to your transformer (after the on/off switch not shown) for safety.

author: srb1954
author_rank: Super Contributor
timestamp: February 13, 2024, 08:48:06 am
content: Quote from: Majorassburn on February 12, 2024, 10:48:29 pmThe inrush current is more dependent on the transformer characteristics than the capacitor characteristics. It is the effective winding resistance and, to a lesser extent, the leakage inductance of the transformer that limits the inrush current to the capacitor and rectifier diodes. With modern electrolytic capacitors their ESR is low compared to the transformer resistance and has negligible effect at limiting the inrush current; unless the transformer is ridiculously oversized for the application.QuoteIf either regulator is run at its full 1A load or the output is shorted the 1N4007s will definitely overheat and won't last long.

author: srb1954
author_rank: Super Contributor
timestamp: February 13, 2024, 09:01:59 am
content: Quote from: r00n on February 11, 2024, 10:44:56 pmWhat is the total output current from each of your regulators?If your are trying to get a full 1A from each of your regulators the 1N4007s will not be adequate for the job and you will need to upgrade to a higher current diode. It should be remembered that the diodes see more than just the DC output current as they also carry the ripple current to the capacitor. Since this ripple current is quite peaky the diodes can dissipate quite a bit more power than might be expected from the DC current alone.It should also be remembered that diode current ratings are often specified with the diodes mounted onto a PCB with a specified lead length as most of the heat is coupled out of the diode via the leads and dissipated in the PCB and surrounds. Using the diodes with their full lead lengths such as in a breadboard will necessitate the diode current to be derated significantly.

author: r00n
author_rank: Contributor
timestamp: February 18, 2024, 11:20:36 pm
content: Thanks to all for these replies. The diodes are no longer frying as I have found the solder bridge. My bad.1.What would be more suitable labels for the outputs of the transformer? I guess these labels make it sound like DC when it is still AC?2.Thanks for info on fuse and switch. Do you know off-hand a diode that will take 1.5 amps?I have another issue where the voltage drop across the outputs are equal to -20.24 : 0 : 15.05V (wrt centre tap). Do you suspect a problem with 7915 regulator?edit - diode amperage

author: srb1954
author_rank: Super Contributor
timestamp: February 18, 2024, 11:54:57 pm
content: Quote from: r00n on February 18, 2024, 11:20:36 pmIf you have a scope check the O/P of the 7905 to see if it is oscillating.The 7905 is more critical than the 7805 on the correct selection of its input and output bypass capacitors for stable operation. Check and follow the recommendations in the datasheet.