Month: February 2022

I bought this Chinese HW-131 breadboard power supply, which seems to be a Chinese clone of another Chinese product, the YwRobot MB U2. There are reports of unreliability with that device run off 12V, the suggestion is to run it of less than 12V if you are drawing notable power from it because heatsinking is marginal, using the small board. And definitely test all the output voltages before wiring this to something valuable.

It uses a AMS1117 3.3 and 5V regulator, and the minimum input voltage is 6.5V, due to the regulator dropout of 1.3V, so I will look around for something more suited to this. A breadboard tends to get shorts easily, and I could see the AMS1117 getting shirty trying to dissipate 12W into a short at 1A current limit 😉 The schematic matches this Addicore one

Absolute maximum junction temp is stated at 125C, and their example calculations say a good board will get thermal resistance of 45C/W, so if we start at 20C the chip can dissipate 2W max. I favour 12V since I can split the 12V off an existing LED lamp power supply, and sockets are a premium around the computer.

It’s hard to ignore some of the dire warnings on the Net about this, it always seems to be the 5V regulator that gives itself to the cause, failing short, which is bad in a series pass device, though typical. So before I put this into service I tested its resilience.

I could reduce the short-circuit voltage with a power resistor or a PTC current limiter. I only had a 12 ohm power resistor, so I used that. Voltage drops to 2.5V into the AMS1117-5 when I short the 5V rail, but it recovers OK, and more importantly the 5V rail recovers to 5V. However, when I replaced the 12 ohm resistor with a PTC 550mA current limiter the series pass device failed short and a wisp of the magic smoke escaped. There’s not much of it. I can see where these get their bad rap from – show me the experimenter who never shorts the power supply rail with the scope probe 😉 As Bob Widlar said in his paper on the design of the LM109 voltage regulator “Actually, the dominant failure mechanism of solid-state regulators is excessive heating of the semiconductors.” And it happens pretty damn quick.

Magic smoke output port. Single use only

So I ordered five of the AMS1117 +5V which set me back £2¹ and set about the spare board with a Dremel to break the + connection from the pin on the socket, to insert the resistor. There’s no need for the PTC 550mA device because it clearly heats up slower than the AMS1117. I shorted the 5V rail, and while you can smell the overheating resistor the +5V rail came back fine, and I diddled about with the short to see if I could catch it out. After about 20 seconds and a wisp of smoke rises from the resistor, although the picture shows it doesn’t discolour too much, and everything is still serviceable. I measure about 10V across it into a shorted 5V rail, across 12 ohms that is about 10W, pushing what is probably a ~2-5W resistor a bit. Resistors tend to fail open, not short, which is good in a series pass device. It was time to introduce this to the old National Semiconductor method of testing voltage regulator short circuit tolerance, dragging the 5V rail over a grounded rough file. As Bob Pease used to say

Don’t just see if it oscillates — see how it RINGS when you tickle it with a pulse of current. In other words, BANG ON IT.

output of 5V dragged along grounded file

Analogue shorted load test equipment

That has this sort of effect on the rail going into the AM1117 5V regulator. Since the DrDAQ I am feeding into the PicoScope only has one channel it is a different instance

12V input to AMS1117-5 with output dragged against shorted file

It’s not subtle, and it’s not clever, but it seems to save the board from swift destruction, at the cost of limiting power drain to 500mA. This one has taken a fair bit of abuse now. I am still tempted to desolder the great big USB socket and bridge a 5.6V 5W Zener across the 5V rail as a better use of the space.

It beats using a big lab power supply on my desk. My lab bench is in a garage, and it gets cold in winter, so for some PIC development I wanted to run a solderless breadboard on my desk. Two reasons – it’s warm in the house, and secondly the lab computer is an old Sony Vaio 32-bit machine. Microchip’s MPLAB X IDE demands 64-bit, because it is shocking bloatware built on Netbeans and Java under the hood. Until recently I’ve used MPLab 8 and assembler, but, well, they ain’t making any more time and assembler is time consuming as complexity goes up.

Tinkering with PIC programming on a breadboard tends to involve a couple of LEDs or perhaps an LCD display, so a few hundred mA is good enough for that sort of thing. If the project needs less than 30mA a decent alternative is to use the PicKit 3 programmer to power the board, but 30mA won’t power many LEDs.

Chinese electronics seems pretty hit and miss. I find buying components like power transistors there is a crapshoot, because these things are terribly easy to relabel², which is probably what did for my SkyTec power amplifier, over and above the rotten thermal design 😉 I’ve had better luck with their modules, which are often the only way to tackle some SMD devices. There seems to be a mix of pick and place and hand soldering – looking at the reverse of the board

It’s a tad on the scruffy side. The jumper wires I got with the kit are OK, I normally use stripped Ethernet cable for jumpers so more colours are welcome. The breadboard is disgusting. The grip on wires is weaker brand new than my 20 year old main lab unit. Life is too short to muck around with janky solderless breadboards, so I will throw it way to save my future self hours of debugging it.