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Old 05-15-2019, 06:21 PM   #21
stj
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Default Re: DPS5020 dual power supply build

no idea about the mosfet.
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Old 05-15-2019, 10:28 PM   #22
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Default Re: DPS5020 dual power supply build

Oh right, forgot about the power switches. I should place an X cap across them like I've seen in other devices which use a physical switch to switch transformers (see audio amps).

So I should keep the resistor/cap idea after all.
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Old 05-15-2019, 11:21 PM   #23
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Default Re: DPS5020 dual power supply build

Do you 220V guys always use DPDT power switches?
North America is 120VAC and SPST switches. Safety codes do not generally allow disconnecting neutral connection in a product.
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Old 05-16-2019, 12:06 AM   #24
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Thumbs up Re: DPS5020 dual power supply build

Yes, I believe on just about every device I've seen so far around here that uses a mechanical switch for the mains it's going to be a DPDT to "cut" both wires going to the plug. Makes perfect sense, since you want to isolate BOTH wires. The plugs are not polarized so L and N can be flipped around at the luck of the draw - you have a 50% chance of inserting the L into the N when plugging the cord in

Would THIS be a good way to connect the X-cap ? Or should it go across the switch and not the load ? In that case, I imagine I'd need two: one for each set of contacts. I also have no idea what value it should have - I'll probably just trash-pick one from a drill or some SMPS
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Old 05-16-2019, 12:27 AM   #25
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Default Re: DPS5020 dual power supply build

I would have a cap across each switch contact. But I am not knowledgeable on the European usual method. I think 10nF is minimum and I use 33-47nF for larger transformers to 1500VA.
My boss yelled at me - "the {DPST} switch contacts are never identical. One opens first and another closes first". So the power off arc could occur at either contact.

From the DPS5020 manufacturer:
"if you use AC-DC converter or transformer coil, please add rectifier (rectifier bridge) and filter (more than 4700uf filter capacitor). If input power supply is unstable, please add LC, L is over 47uH, C is over 2200 uf."

So I think 10,000uF is perfect. The LC filter is probably if you are supplying noisy power from a SMPS. It's poorly translated.
You can see two power mosfets at the IN(-) on the PCB so not sure why.

Last edited by redwire; 05-16-2019 at 12:33 AM..
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Old 05-16-2019, 02:44 AM   #26
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Default Re: DPS5020 dual power supply build

your boss was almost right.
it would be more accurate to say the tolerances are such that you never know which contact will move first.

the mosfets may be for rectification/reverse protection.
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Old 05-16-2019, 09:53 AM   #27
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Talking Re: DPS5020 dual power supply build

I swiped THIS from "The Key Master" today when he wasn't looking It's the 230v input filter I was talking about.
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Old 05-16-2019, 09:56 AM   #28
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Default Re: DPS5020 dual power supply build

Quote:
Originally Posted by stj View Post
the interface is a serial>usb adapter with a WinChipHead chipset (a damned good chip)
now here is the interesting bit, in the picture it looks like it's already opto-isolated!!!!!
Confirmed:
CH340 serial to USB conversion (standard application configuration)
EL357N phototransistor optocouplers (2 pcs: Rx & Tx)
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Old 05-16-2019, 10:23 AM   #29
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Default Re: DPS5020 dual power supply build

(According to this source)
Not disappointing when pushed to near absolute max output current limit...
Noise stays within acceptable boundaries, and very important no voltage spike.
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Old 05-16-2019, 01:49 PM   #30
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Default Re: DPS5020 dual power supply build

My worry is not damaging the DPS module due to the output on/off mosfets. What is weird about the modules is the low-side ground switch they have.

Stray capacitance on the power transformer means things can float with noise and hum, so I suggest adding a 10nF and 1MEG to give leakage currents a path to earth ground. If things floated say to 50V it would damage the DPS mosfet when the PSU output was switched off. Output switched off -> (-) floats

The DPS5020 has a couple fuses on the PCB. I don't think you need an input fuse - it has F3 on the pcb. F1 I think is for the MCU and display.

This is the circuit I might try.
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File Type: jpg dps5020-adjustable-power-supply-50-v-20-b.jpg (257.6 KB, 4 views)
File Type: jpg dps5020-adjustable-power-supply-50-v-20-a.jpg (187.1 KB, 3 views)
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Old 05-16-2019, 02:26 PM   #31
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Default Re: DPS5020 dual power supply build

Why is the on-board FET a cause for concern here exactly ? The way I understand it is when Q4 in your schematic (which I'll kindly ask you to remind me which software you used to create since I keep asking and keep forgetting ) "opens" (gate goes low) it causes a spike on the input side of the module which can exceed the rated 50v input ?

I see TWO sets of resistor+capacitor: one on the input, one on the output. These act as "quenching" devices so to say ?
Also I believe there's an error with the symbols for the X caps across the switch, as they're shown as electrolytics

Like I said in my original post, I already built a similar unit and I now realise there's a LOT of room for improvement and I'm probably running it improperly there: no arc-suppression on the AC switch, no zener diode across the input terminals of the DPS, no caps on the input and output the way you suggested....I'll probably open it up again and improve it as well, especially since this one is a single transformer setup and don't have to worry about keeping anything separate.

Last edited by Dannyx; 05-16-2019 at 02:28 PM..
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Old 05-17-2019, 10:33 PM   #32
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Default Re: DPS5020 dual power supply build

All you do is measure ACV between each output to earth ground, with the DPS on but output disabled. That will tell you if it's floating at high potential, enough to damage the mosfet.
I take extra care against things floating up or ESD hits to the DPS jacks when I'm connecting things. The mosfet takes the hit. Normally it's not a concern in power supplies because things are always connected both jacks- there is no mosfet switch that can be off. I could be wrong too.
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Old 05-18-2019, 02:10 AM   #33
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Default Re: DPS5020 dual power supply build

In the case of my first supply, with the single DPS module, earth ground is not actually connected to the negative side of the bridge rectifier - it's only connected to the chassis of the transformer and that's it. By "earth-ground" I refer to the third prong of the IEC 13 plug on the back, so I don't know if my meter would actually measure anything between this prong and GND. I now realize I should've probably tied GND and that earth terminal together - seems industry-standard. Even better add those Y-caps+1meg resistors you suggested.

I noticed on some Samsung TVs I worked on that cold GND (GND of the 5/12/etc. power rails of the SMPS) is connected directly to the metal frame which is then tied to the earth prong on the mains lead with no Y cap in-between. I definitely remember seeing blue Y-caps on SMPS boards, somewhere in the corners near the metal mounting lugs but I can't remember for sure where they go. I think they're between HOT GND and earth (chassis).

Last edited by Dannyx; 05-18-2019 at 02:16 AM..
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