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Hiland Power Supply Kit Voltage Spike
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ledtester:

--- Quote from: Audioguru on May 23, 2018, 12:48:22 am ---I think this cheap Chinese power supply kit replaces a defective copy of a 15 years old Greek power supply kit that I helped fix at Electronics-Lab.com.
Isn't its voltage adjusted with a pot that might be intermittent?

--- End quote ---

I don't think so. This one uses a microcontroller instead of op-amps in the voltage control loop.

Link to this kit:

https://www.banggood.com/0-28V-0_01-2A-Adjustable-DC-Regulated-Power-Supply-DIY-Kit-Short-Circuit-Current-Limiting-Protection-p-1060253.html

The one I think you are referring to:

https://www.banggood.com/0-30V-2mA-3A-Adjustable-DC-Regulated-Power-Supply-DIY-Kit-p-958308.html

The_Boots:

--- Quote from: Audioguru on May 23, 2018, 12:48:22 am ---I think this cheap Chinese power supply kit replaces a defective copy of a 15 years old Greek power supply kit that I helped fix at Electronics-Lab.com.
Isn't its voltage adjusted with a pot that might be intermittent?

--- End quote ---

I think this is a different design. I've seen the one you're talking about (with the TL081s that are being driven too hard, right?). There is a trimmer pot used for initial calibration, but the actual voltage control is entirely through the two front encoders and the Atmega.


--- Quote from: ledtester on May 22, 2018, 10:02:26 pm ---You could probe the R2R digital output pins when the voltage is set at (or as close as you can get it) the "bad" voltage values to see if there is a discernible pattern (like a lot of the low-order bits are 1).

Hopefully your method of probing won't disrupt the R2R ladder output. I'm pretty sure it would be safe to use your oscilloscope for this.

It would also be interesting to know what the R2R control word is when the voltage is set to it's highest value. Perhaps you are not seeing the 0xDF spike because the control word never gets that high.


--- End quote ---

So I probed at the R2R ladder and tried to get the sense for it, but it seems a bit weird. The pins don't seem to be behaving in traditional base-2 patterns with respect to the voltage, so it's a bit hard to track which is which. However, when I got it to a problem spot, not only a single spot went nuts-- they all did together! And it was the pins themselves. The controller is going nuts at these voltages (you were right, btw, there were other problem voltages). However, the common thread was the pin that I would expect to be the MSB (right by the output) always was due to change state right around the problem voltage.
I'm posting two pictures: one showing two pins going high together, and one showing the voltage compared to a pin going high at higher speed. That noise is pretty impressive, but it also read as 33MHz, and my scope is only 25MHz/100MSpS, so I don't know how accurate that really is.
I guess it's also worth noting that Ch1 in that high speed image is going almost to 10V, when it's one of the digital out pins, and in the other picture of the two digital pins, channel two is going to -5V... I didn't think they were supposed to do that. Probes were grounded to the same spot and hooked in right between the resistor and microcontroller. Probes both set to 10X.
ledtester:
Just a quick comment on "channel 2 going to -5V"... note that CH2 is AC coupled, so it's probably going from +5 to 0.
The_Boots:

--- Quote from: ledtester on May 23, 2018, 12:34:54 pm ---Just a quick comment on "channel 2 going to -5V"... note that CH2 is AC coupled, so it's probably going from +5 to 0.


--- End quote ---
Ack! You're right, of course. The scope doesn't remember configurations between uses, and I'd disconnected it in the middle. That explains a lot of the weird behavior I saw at the end. Still too new to remember to check it! I'll redo it tonight making sure it's DC.

You know, after sleeping on it another thing occurred to me: I don't think there CAN be a 1-1 relationship between the R2R ladder and displayed voltage. If I understand it, it's basically an 8-bit DAC, right? That means 2^8 = 256 possible configurations. The display goes from 0-28.0  in 0.1V increments. That's 281 possible settings. Do you think that for the extra settings, the digital output oscillates around the set voltage if it there's no DAC output configuration within the allowed error for that given voltage?
How does voltage detection happen in the first place, since Vout could be (if it were working correctly) a max of 28V? Does the circuit attenuate or divide the voltage somewhere, and if so, is that a potential place to look?
Still wishing I had more channels to monitor more pins at once... It'd be nice to see all 8 at once. Could a cheap logic analyzer do that?
Thanks again to all who are helping me along. I really appreciate the explanations! Tracking this down is actually proving to be a lot of fun.
ledtester:
One of those simple USB based 8CH analyzers you can get for < $10 should work.

If you get one, make sure it is usable with sigrok: https://sigrok.org/

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