Linear Power Supply Teardown and Autopsy With Appearance by Dave Jones

Vocademy - Free Vocational Education

Solid-state Devices and Analog Circuits - Day 9 Supplemental

While working on my laser communicator, my power supply failed, and not failing to a safe condition, it put 23 volts across my laser pointer. Let's see what happened.

[BoxingDayAC]

Summoning Dave on command, what a legend. Throws me off seeing a linear regulator without a 78XX or other discrete IC. Makes it an interesting example.

[mikesradiorepair]

I think you may have one error on your schematic. The electrolytic capacitor you have listed as 2.2uF should be 220uF according to the factory schematic I have on file. Almost every power supply like this you ever see will be around 13.8 VDC on the output. Most common thing to fail on these linear power supplies is the pass transistor. Even higher end Astron power supplies. At least the higher end supplies have crowbar circuits to protect the output from over voltage conditions. If the output goes high it fires a SCR which dead shorts the output to ground and blows the fuse. Don't have enough fingers and toes to count all the pass transistors I have replaced in customers power supplies over the years. Why do they fail, because people put to much load on their poor power supplies. I repair power supplies for amateur radio operators all the time. When I question the owner what they were using the supply for I find they are using them at near full rated output power. They don't bother to read the manual / spec's that shows the supplies can only do full output for intermittent duty cycle. 100% duty cycle is usually only available at 70-75% rated output. Mike KC3OSD

[techNK]

This is nice video ... More video like this sir

[rolfzetterberg]

That resistor is needed.I removed it in a similar power supply and it immediately started to oscillate which means full output voltage!So we just have to live with the fact that it is there for a reason.

[pashireddinuthula]

Yay! dave jones is here eevlog, i love his accent though

[mikejones-vdfg]

Cool, these are helpful learning tools seeing a real circuit and diagnosing its problem. I just watched someone bias a transistor on a breadboard and that cleared up a lot of confusion as well. I went for a cheap meter over the holidays myself and looks like they're offering a lot more for $8 these days, this one has an AC voltage detector, autoranging even a flashlight and was 8.33 CAD shipped to my door. Amazing. Although It measures on the high side compared to the rest of my meters, so i dont know what to think. They all read different. How do we know which one is accurate for sure? the more expensive one?... Anyway thanks for the knowledge. Maybe that resistor is there because our meters are inherintly wrong and theres a hidden error in all circuits? ... I dunno

[EEVblog]

That EEVBlog guy really has an annoying voice.