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Playing with the Atten PPS3005S DC power supply

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bobski:
I'm a noob here, but what the hey.
I'm starting a thread since doing this in someone else's classifieds ad seemed a little inappropriate. I posted this yesterday and updated it a few mins ago:

--- Quote from: bobski on June 25, 2011, 03:02:06 am ---One of these arrived at my doorstep this afternoon... It seems to have a few issues that need re-engineering.
- The buttons are slow. If you just quick punch a button, you'll get no response no matter how hard you press. You have to slowly and very deliberately press the button to get the desired action. It really gets annoying when you're trying to move the selected digit over.
- The switch to constant-current is slow, or maybe there's too much filter capacitance on the output. If you set the supply to 10V, 10mA and (expecting the current limiter to do it's job) touch the output to an LED, you'll end up with a quick flash and one less LED in your parts collection.
- The fan is constant speed, always on, and it's distractingly loud.
- The zip ties keep things tidy, but for some reason I feel like the AC lines straight off the transformer shouldn't be bundled with the wiring for the DC regulators.

On the up side, the unit is controlled by an ATMEGA88PU running at 8 MHz and 5V. There's a nearby 6-pin header which may or may not be ISP (I'll have to trace the pins)... This is all visible in the pic. I tried swapping in a 16MHz crystal to see if that would help with the control responsiveness, but got no noticeable change in behavior what so ever. I wonder if someone forgot to set the fuse bits and the controller is running on its 1MHz internal clock.
The displays appear to be driven by a bunch of 74HC595D shift registers and a mess of SMD current limiting resistors. Low tech, but effective.
Since the 'mega88 doesn't have on-board USB support, I expect the 4-pin connector consists of the TX and RX lines along with 5V and ground. That's all that would be needed to interface with a USB UART bridge.

[edit]
Yeah, it's ISP. From left to right (left most pin is numbered 1, so 1-6): Ground, VCC, MOSI, MISO, reset, SCK.
Let the hacking begin.
[/edit]
--- End quote ---

So next I'm going to try hooking it up to my AVR Dragon to check my theory on the fuse bits.

bobski:
Bingo.


Unfortunately, the limiter operation still seems flakey.

saturation:
Good work!  If we can get some of these Chinese test gear working properly, its does give you a bang for buck.  Can you characterize the PSU for us in terms of its ripple and output stability?

bobski:

--- Quote from: saturation on June 25, 2011, 09:10:57 pm ---Can you characterize the PSU for us in terms of its ripple and output stability?
--- End quote ---
There's does seem to be some kind of harmonics going on.

The peaks hit about 8 mV at 15 us invervals, while the noise at the base is about 0.8 mV.

How would you characterize stability? Output voltage drift over long periods?


--- Quote from: BoredAtWork on June 25, 2011, 09:41:35 pm ---IMHO there is no need for such restrains.
--- End quote ---
Well, If nothing else, it was in the wrong forum section. ^_^


--- Quote from: BoredAtWork on June 25, 2011, 09:41:35 pm ---Seriously, that design decision might be the reason the fan needs to blow constantly.
--- End quote ---
Yeah, ok... I can see that. They would need temp sensors at board-level as well as the heat sink at the back. But still, variable speed would have been nice. Don't necessarily stop the fan, but merely spin it down so it isn't so noisy. So long as there's airflow, the heat sink temperature should suffice, as all the air flowing through the case has to pass around or through the sink on its way to the fan and out the back.


--- Quote from: BoredAtWork on June 25, 2011, 09:41:35 pm ---Is the ATMega part of the control loop (digital controller), or just providing the settings for analog control loop (digitally assisted controller)?
--- End quote ---
I'm not sure. Since the surge-y behavior of the current limiter doesn't seem to have changed with the clock speed, I suspect it's just digitally controlling an analog loop.

bobski:
Can anyone recommend some good, free (or close to it) diagraming software? I worked with multisim in school, but it's not exactly affordable for the hobbyist.
I've got the basic AC-in to DC-out path of the analog board down on paper:

That covers about 1/3 of the board's area. Another quarter is simple 5V and 12V supplies based on 7805 and 7812 linear regulators. This is starting to seem manageable. Not included on that scribble are various little circuit offshoots from the main traces, as well as whatever controls the big NPN transistors... It looks like a couple of daisy-chained transistor amps, but amplifying what I don't know. Expanding the diagram so it's actually useful is going to get messy, so I would prefer to transfer what I have to software and continue from there. Which leads back to my original question: Does anybody know some good diagraming software for the hobbyist?

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