Question about my cheap Chinese bench power supply

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AVA

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Hi All,

As you may or not know, I'm a hobbyist, so I have to put money into the right areas, and save where possible.

When looking for a DC bench power unit, my needs were relatively simple...I just needed reliable and consistent power, up to 24V.

Enter the Longwei magical mystery box from China lol
It's a 30V/10A'er which is well over what I would normally need. So.... perfect!
Plus it was like $30 tops.

It actually seems to filter noise too, which is amazing, since I'm currently stuck living in Taiwan and the power here is always on, but...inconsistent ha. No 3 prongs anywhere either!

Anyway. Works great. Love it.
Zero info online about it. But there are some YouTube reviews that taught me some basics.

This is new to me, and may be obvious to a lot of you, but apparently to set the maximum amperage level, I have to first short the two leads together...which is kinda freaky imo...but again, I'm no expert.

When operating normally, the display for my voltage, on the top, shows whatever the voltage I set it to us (often 9v or 12v), and the bottom generally doesn't show anything. I guess I'm just relying on the fact that it won't go over the 1amp that I set it to initially?

The current display only displays when I short the leads together for the purpose of setting the max current.

Anyway, recently I've noticed that it will show .02A when I hook up this one specific project... (Pictured below)

Question:
Is my project shorting somewhere? It doesn't appear to be. I've triple checked everything.
Or
Is my magical mystery Chinese power box just showing me how much current I'm drawing, and for the first time im drawing enough current for it to actually appear on the meter? As in, all my other projects have fallen below the radar.

Thanks in advance!
Hope you're all well.


Alex
 

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rock soderstrom

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Or
Is my magical mystery Chinese power box just showing me how much current I'm drawing, and for the first time im drawing enough current for it to actually appear on the meter? As in, all my other projects have fallen below the radar
I think that could be reality. I can't see any malfunction from a distance. This can be easily tested by using a circuit that draws a little more current or by taking a tube and supplying the heater with the appropriate voltage. Then you can see if the flowing current is shown correctly on the display. Take a power tube that draws a little more current. In the end, any resistor will do if you follow Ohm's law and the power rating.

I=U/R
 

PermO

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My guess is, it's fine, it's just showing the current draw, when nothing connected there's no current draw, so it shows no reading.

Hook up a halogenbulb set it to 24V and see if you can trigger the current limmiter.
 

AVA

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Ya you guys are right, it looks like it's just telling me what my current draw is...

The only other time I saw a number reveal itself, it was because I had an op amp installed backwards lol and it didn't fare well for the circuit.
So seeing a number pop up again made my brain stem tingle every so slightly.

Thanks for your advice and time folks!!
 

Bo Deadly

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It actually seems to filter noise too, which is amazing

Doubtful. Those supplies are notoriously noisy. They're cheap SMPS that are simply not designed to be low noise. Common mode AC noise will go right through the SMPS transformer and inject massive noise into your ground. There are tricks to make it quieter but you would be better off buying a few MeanWell LED supplies and make a bench supply with some extra parts like really big film caps between the 0V output and earth ground.

I would also look into finding some kind of power strip that has earth ground and then plug everything into that. Even though the earth is not connected to anything at the wall, the power strip will at least provide a common earth ground bus. And you might also find a way to connect the earth to a water pipe or some such. But I know nothing about power infrastructure in asia or outside the US so there's probably a proper way to work around such things.
 

AVA

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Doubtful. Those supplies are notoriously noisy. They're cheap SMPS that are simply not designed to be low noise. Common mode AC noise will go right through the SMPS transformer and inject massive noise into your ground. There are tricks to make it quieter but you would be better off buying a few MeanWell LED supplies and make a bench supply with some extra parts like really big film caps between the 0V output and earth ground.

I would also look into finding some kind of power strip that has earth ground and then plug everything into that. Even though the earth is not connected to anything at the wall, the power strip will at least provide a common earth ground bus. And you might also find a way to connect the earth to a water pipe or some such. But I know nothing about power infrastructure in asia or outside the US so there's probably a proper way to work around such things.
Hmm.. Maybe the electricity going into the unit is clean then? I don’t see any extraneous noise on my oscilloscope…which is also from China haha

That’s exactly what I’ve done, plug everything into a power bar with ground.
Some apartments in Asia have basic thumbscrews that come out of some wall plates, mostly in the kitchen or bathroom.
Appliances like toaster ovens come with long wires that have horseshoe connectors on either end haha. And a matching thumbscrew on the chassis somewhere. This is the band aid solution I usually see.

But this is also becoming rare as the newer apartments are being built American style. And the older apartments aren’t going to get retrofitted.
So they also have adapters that go from two prong with thumbscrew to three prong and viseversa. It’s messy…

Mine is a strange in inbetweener where the odd plug has a proper ground, but nowhere near my work table…of course. But I could use it if I really felt compelled.

Dig this pic I attached haha
This is fairly common in Asia. Not so much in Taipei where I live, they’re working hard to clean it all up. Wireless internet sure helps!
 

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PermO

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Oscilloscope on the PSU, I tried that wih my recently build PSU,.. all I get to see is a line.
It has a max resoluion of 2 mV, but still I see nothing.

What would be considered a 'clean' powersupply for audio, how much ripple can we have ?

Do you use a scope on the PSU or do you take the audio output as reference point ?... as anything noisy will show there.
 

AVA

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Oscilloscope on the PSU, I tried that wih my recently build PSU,.. all I get to see is a line.
It has a max resoluion of 2 mV, but still I see nothing.

What would be considered a 'clean' powersupply for audio, how much ripple can we have ?

Do you use a scope on the PSU or do you take the audio output as reference point ?... as anything noisy will show there.
Sorry I should have explained...

I mean that if I set up a buffer op amp on a breadboard, and put a 1kHz sine wave through it at 500mVpp, I don't see much more noise than when I test the output of my signal generator directly...

There is noise, but it could be any of the various inexpensive things that I'm using, or the fact that I'm surrounded by the electronics of a mega Asian city haha.
And it's negligible for my needs...If I can't hear it in my audio circuits, it's good enough for me until I get back home to Canada. Then I'll be more picky.

*Thats with the PSU basically straight into the op amp
But I hardly ever just run the power straight from the PSU into my breadboards

I made a little power conditioner on a protoboard as one of my earlier projects a long long time ago.

It just takes X voltage through a bunch of shunted electrolytic capacitors, then splits the supply in half and buffers it, making X/2 and runs it through some more shunted caps.
It's also got an LED so I do less stupid things with the power on.
So when I'm actually working on stuff, thats what I run it through.
Clean as a whistle.


Cheers!
 

PermO

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Oh sorry, my questions where more in general, not specificly aimed at you 😀

I've got a big box of wallwarts, that's my bench PSU...

So my guess is testing in circuit is the only way to figure out noise levels, also, you can do a nice very quiet PSU, but if your grounding scheme is not ok you'll have noise anyway...

I'm happy at -80.
 

ccaudle

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I would also look into finding some kind of power strip that has earth ground and then plug everything into that. Even though the earth is not connected to anything at the wall, the power strip will at least provide a common earth ground bus.

Safety ground in a proper 3-wire outlet is to protect from power to chassis shorts (the purpose of safety ground connected to neutral at the power entry), and limit AC power common mode rise during lightning events (the purpose of safety ground connected to ground rod at the power entry).

If you connect all your 3-prong equipment into a power strip, but that power strip has no connection to safety ground, basically a big bench size cheater plug, you have made sure that any power to chassis short on any piece of equipment on your bench has now made every other 3-prong equipment on the same bench power strip also hot on chassis. Why let only one piece of equipment have a potentially lethal fault when you can make sure every piece of equipment becomes potentially lethal?
 

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