Neil Fraser

For the past decade I've been a software engineer at Google in California. In my spare time I've created a mélange of open-source software. I've also built some rather unusual hardware. On rare occasions I've even been known to write something interesting.

Broken QW-MS305D Sunday, 26 April 2020

If you don't have a malfunctioning QW-MS305D power supply, you can stop reading now.

The QW-MS305D is a great little 30V 5A adjustable power supply that can be bought for around $50. But after taxing it too hard, mine suddenly stopped functioning. When powered on, the output voltage shot up to 63V, then dwindled to 0V. The knobs had no effect. Here's a video of this behaviour:

Searching online showed that I'm not the first person to have this issue. Some suggestions were made on forums, but nobody followed up with the results (hence this post).

Inspecting the inside revealed no obvious damage. There were four large (TO-220) heat-sinked semiconductors that were my prime suspects. And sure enough, as soon as I replaced the P75NF75 (Power MOSFET), the power supply started operating normally. Easy fix.

[P75NF75 transistor in QW-MS305D]

Hope this helps some other owners of malfunctioning QW-MS305D power supplies.

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