Electronics > Beginners
Hiland Power Supply Kit Voltage Spike
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The_Boots:
So I built this power supply kit from Banggood: https://www.banggood.com/0-28V-0_01-2A-Adjustable-DC-Regulated-Power-Supply-DIY-Kit-Short-Circuit-Current-Limiting-Protection-p-1060253.html
Now, I wasn't expecting it to be amazing or anything-- I mostly got it because I wanted a kit to start with and I needed a power supply anyways. The only real issue I had with the kit  was that when I first powered it on, one of the diodes let out the smoke and tripped the breaker on the transformer (and before you ask, yes it was in the correct orientation), but when I replaced it with another of the same kind, it worked with no other obvious problems.
Everything seemed all well and good until I tried to power something with it (it was a cheap DMM) and... RIP. The meter died without any real warning. No smoke or anything.

Cut to a few days later and I had the bright idea to start troubleshooting with my USB Scope, and I found something I can't really explain. At three specific voltage settings, the output is suddenly spiking to 32-38V.

At 3.9-4V the voltage spikes and exponentially decays back to the correct voltage in about 2.5ms.
At 12.4-12.5V it also spikes and returns in a similar curve, taking  about 840 us.
Finally, at 20.9V the spike returns to normal in about 410 us.

The spikes can almost always be triggered by moving from a higher voltage down to the troubled one, with the spike happening right as I hit it, however sometimes the voltage will spike repeatedly if I leave it at that set level. The 12.5V level seems particularly susceptible to this kind of behavior.
These spikes ONLY occur at these three voltages, and the time I gave to return to normal is actually pretty consistent. There are some hints of a pattern in the timing (3x voltage normalizes in 1/3 the time). This suggested to me that whatever is happening is not a random glitch, but then-- why these specific voltages? I feel like there's a pattern I'm missing.

Note that I am NOT really asking for help fixing it or making it somehow not cheap junk. I've already bought a prebuilt power supply (the Korad KA3005 that people seem to think isn't too awful). I'm far more interested in learning what components and designs might be the culprit.

Does anyone have any hints or suggestions about what causes spikes like these? I'm still very much a beginner at this, so I'd appreciate any advice!
ledtester:
I would check the resistors in the R2R ladder to make sure they are all good and have the right values.

My understanding is that this supply uses a microcontroller in the voltage control loop. The micro emits digital signals into an R2R ladder to essentially control the pass transistor.

The R2R ladder are those 10K and 20K resistors next to the Atmega8.

That said, I wouldn't expect a lot from this kit. Because it uses a micro in the control loop it's going to respond very slowly to changes in the load. And it looks like it only has an 8-bit R2R ladder which only gives you 256 control steps across the entire 28V range (and they may not even be uniformly distributed).

But if you want to try to fix it I would first make sure the R2R is working. There might even be a way you can troubleshoot the microcontroller board by itself - perhaps just using a pot connected to one of the ADC pins to simulate the voltage sense. Then again, it might not really be worth your time.
ledtester:
btw, here are some articles which I believe (although I may be wrong) describe the circuit or at least an earlier version of it:

- http://tuxgraphics.org/electronics/201005/bench-power-supply-v3.shtml

- http://linuxfocus.org/English/June2005/article379.shtml
The_Boots:
Since it's controlled digitally, do you think it could be a pure software thing? Where the code in charge of reading feedback is freaking out at these specific levels and takes a while to bring the voltage back down(is that slew rate? Sorry, I'm still learning)?
I'll look at the R2R tonight, though. I'm guessing they all need to be desoldered to check, so that sounds like it'll take a while! I'll also look at the articles you linked to see if I can figure anything out there, as well.
Like I said, I care less about having a functional power supply at the end than understanding what about this circuit causes the problem. I primarily bought the kit to learn, not to have an amazingly reliable and useful power supply! In some ways (other than cooking the DMM) it's almost better that it's had a glitch like this, since you don't learn as much if everything goes smoothly! It gave me a perfect use case for why oscilloscopes are so darn useful and forced me to learn a lot of the basics about how to use them.
Thanks for the tips!
ledtester:
I could be a software issue, but since this is a popular kit I would imagine the kind of problem you are seeing would have been spotted already.

Because the problem happens at specific voltages I suspect a problem with the R2R ladder or with one of the digital IO pins controlling it.

Here is a possible scenario... Suppose the IO pin associated with bit 4 of the R2R ladder is defective and can't output a high value, and suppose you have set a voltage where the microcontroller has set the R2R ladder outputs to 0x0F. If you slightly increase the desired output voltage, the microcontroller will try to set the R2R ladder output to 0x10, but because bit 4 is defective the actual output will be 0x00 causing the output voltage to drop precipitously. Sensing this the software will overcorrect and set the R2R ladder output to some high value (perhaps even 0xFF) causing a spike in the output voltage. Over time the voltage will settle down to the correct value as you have observed.

Since you see this at three specific voltages, perhaps it is actually bit 5 which is defective. That is, you'll see the problem happening when the R2R ladder output is 0x1F, 0x5F and at 0x9F - the bit patterns being 0001.1111, 0101.1111, and 1001.1111.

The recovery time will be longest for the 0x1F point (because the software has to bring the R2R ladder output from 0xFF down to around 0x1F) and shortest for the 0x9F point. In fact, there might actually be fourth voltage where the spike happens - at the 0xDF point - but the recovery time is so quick that your don't see it.

Of course - this is just wild speculation - but it's the first thing that came to mind.

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