Help understanding output protection circuit on lab power supply.  - Page 1
Forum section: electronics / beginners

author: rodcastler
author_rank: Regular Contributor
timestamp: January 14, 2020, 11:37:07 am
content: This happened to a Korad 3005 Power Supply.I managed to attempt a Lipo battery charging -against common sense and probably some manuals- by directly connecting the battery to the output of the PS.To make matters worse, I realized I had the polarity reversed right after hearing the loud pop sound and the magic smoke.Upon inspection before fixing this little unit, I found two things that called my attention:First, the casualty was D1 (below the current sense spring wire). It had blown into pieces. Mi question is, how much protection is there after the diode has blown and the circuit gets open? I suspect that if the diode is in parallel with the output, there's no protection left. The output was working fine with the missing diode.Second, after removing the diode, I realized that the silkscreen shows it in reverse orientation, with the cathode to the left side!!. Am I missing anything? I'm just glad I double checked against pictures on the internet for reference before soldering the replacement diode. Otherwise I would have soldered it wrong.

author: ArthurDent
author_rank: Super Contributor
timestamp: January 14, 2020, 05:51:39 pm
content: I don't believe it is a good idea to use a power supply to directly charge any battery. If there is any problem in the supply causing a short, or the battery is connected backwards, the battery can supply whatever short circuit current it is capable of into the supply. In he case of large batteries this can be up to a hundred amps or so. This can easily blow up a diode (as in your case), or burn runs off the circuit board, or start a fire.If you're trying to charge a battery with a bench supply, always have reverse current protection so the load cannot feed power back into the supply. The minimum is a diode in series with the battery load but there are other more efficient ways as well, one simple way is shown in this video.https://youtu.be/IrB-FPcv1Dc

author: bob91343
author_rank: Super Contributor
timestamp: January 14, 2020, 06:19:05 pm
content: The proper polarity of the shunt diode is with the cathode connected to the positive side of the supply. That makes the diode conduct when reverse voltage appears but it blocks for normal voltage.I recently acquired a power supply for very little money. It had a blown output diode like yours. I replaced it and the supply works great.

author: rodcastler
author_rank: Regular Contributor
timestamp: January 15, 2020, 11:08:20 pm
content: But then, if the diode is fried, the protection is gone. right?

author: xavier60
author_rank: Super Contributor
timestamp: January 15, 2020, 11:36:42 pm
content: Quote from: rodcastler on January 15, 2020, 11:08:20 pmEven if the diode fused open circuit, the reverse polarity applied to the rest of the regulator circuitry need not necessarily have caused extra damage. Current sourcing topologies generally tolerate briefly applied reverse polarity. My 2 self made PSUs have no diodes fitted for the very reason that damaged your PSU.The big unknown involves the damaged spiral wire resistor. Is it really the CS resistor and where is it electrically positioned?I assume there is no schematic available for the model?

author: ogden
author_rank: Super Contributor
timestamp: January 15, 2020, 11:58:32 pm
content: Lesson: get proper battery charger. Something like IMAX B6. Even clones do the job well. Ebay/Ali is full of such, even hobbyking now sellclones.

author: gcewing
author_rank: Regular Contributor
timestamp: January 16, 2020, 12:40:17 am
content: When charging lithium batteries, it's especially important to use a circuit designed for the purpose, since abuse can lead to fiery explosions.

author: KeepItSimpleStupid
author_rank: Regular Contributor
timestamp: January 17, 2020, 06:36:03 pm
content: Battery charging with a lab supply REQUIRES a series diode.

author: xavier60
author_rank: Super Contributor
timestamp: January 17, 2020, 10:55:59 pm
content: A series diode will be effective only if permanently attached to the battery's charging port.It wont be effective as part of the power supply.