Electronics > Projects, Designs, and Technical Stuff

How do you tell what "type" of power a DC power supply is?

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AndyC_772:

--- Quote from: DW1961 on July 24, 2020, 07:40:54 pm ---Good information, but why does that power supply I linked to above have  a CE mark on it and say "LED use only?" Also, how could you test it to make sure it is compliant? I'd love to know that. Does complaint mean ripple and etc are in spec?

--- End quote ---

The standards in question relate to EMC (radio interference) and safety, not functionality. The ripple voltage, for example, could be atrocious, but that wouldn't make it non-compliant from a legal standpoint.

To test it properly, you'd need to take it to an approvals lab where they'd put it in a screened enclosure, connect it to a load (which in this case is an LED lighting strip) and measure the radio emissions from it. It's also tested for its conducted emissions back up the mains cable, immunity to spikes, surges and drop-outs in the mains supply, and immunity to radio interference generated by other nearby equipment.

In terms of safety they'll look at things like electrical isolation between the mains and the low voltage output, the temperature of the case when it's under full load, flammability, current leakage to earth, and whether or not it's correctly marked with voltage and current ratings, type of fuse and so on. A lot of the process is about paperwork.

DW1961:

--- Quote from: james_s on July 25, 2020, 04:57:07 am ---If you have any serious interest in electronics you should get a scope, they've gotten really cheap, relatively speaking. A decent 4 channel scope costs less than most smartphones these days.

--- End quote ---

I've noticed bench power supplies (adjustable) are really cheap on Amazon. Any suggestions under 110.00 bucks? Same for scopes, but they seems like they start around 400+.

DW1961:

--- Quote from: AndyC_772 on July 25, 2020, 06:55:42 am ---
--- Quote from: DW1961 on July 24, 2020, 07:40:54 pm ---Good information, but why does that power supply I linked to above have  a CE mark on it and say "LED use only?" Also, how could you test it to make sure it is compliant? I'd love to know that. Does complaint mean ripple and etc are in spec?

--- End quote ---

The standards in question relate to EMC (radio interference) and safety, not functionality. The ripple voltage, for example, could be atrocious, but that wouldn't make it non-compliant from a legal standpoint.

To test it properly, you'd need to take it to an approvals lab where they'd put it in a screened enclosure, connect it to a load (which in this case is an LED lighting strip) and measure the radio emissions from it. It's also tested for its conducted emissions back up the mains cable, immunity to spikes, surges and drop-outs in the mains supply, and immunity to radio interference generated by other nearby equipment.

In terms of safety they'll look at things like electrical isolation between the mains and the low voltage output, the temperature of the case when it's under full load, flammability, current leakage to earth, and whether or not it's correctly marked with voltage and current ratings, type of fuse and so on. A lot of the process is about paperwork.

--- End quote ---

Got it. I would hope those PSU would at least be safe. A scope could test for voltage and ripple and other undesirable, though, correct? I'm less worried about burning to death than protecting my equipment :)

james_s:

--- Quote from: DW1961 on July 25, 2020, 05:04:35 pm ---I've noticed bench power supplies (adjustable) are really cheap on Amazon. Any suggestions under 110.00 bucks? Same for scopes, but they seems like they start around 400+.

--- End quote ---

The cheapest decent new scope I'm aware of is the Rigol DS1054z which is trivial to hack to 100MHz and costs $349 from Tequipment, that's dirt cheap for a scope.

On a tighter budget there are some 2 channel models from Rigol and Siglent, or if you have space for a "boat anchor" and don't mind a gamble on used gear, I've seen 1990s CRT digital scopes from A-list brands like Tek and HP go for as little as $100. Then there are old analog scopes, you might get lucky and score one of those for free if you're in the right place at the right time, but typically working ones fetch about $1/MHz.

TimNJ:

--- Quote from: DW1961 on July 25, 2020, 05:08:52 pm ---
--- Quote from: AndyC_772 on July 25, 2020, 06:55:42 am ---
--- Quote from: DW1961 on July 24, 2020, 07:40:54 pm ---Good information, but why does that power supply I linked to above have  a CE mark on it and say "LED use only?" Also, how could you test it to make sure it is compliant? I'd love to know that. Does complaint mean ripple and etc are in spec?

--- End quote ---

The standards in question relate to EMC (radio interference) and safety, not functionality. The ripple voltage, for example, could be atrocious, but that wouldn't make it non-compliant from a legal standpoint.

To test it properly, you'd need to take it to an approvals lab where they'd put it in a screened enclosure, connect it to a load (which in this case is an LED lighting strip) and measure the radio emissions from it. It's also tested for its conducted emissions back up the mains cable, immunity to spikes, surges and drop-outs in the mains supply, and immunity to radio interference generated by other nearby equipment.

In terms of safety they'll look at things like electrical isolation between the mains and the low voltage output, the temperature of the case when it's under full load, flammability, current leakage to earth, and whether or not it's correctly marked with voltage and current ratings, type of fuse and so on. A lot of the process is about paperwork.

--- End quote ---

Got it. I would hope those PSU would at least be safe. A scope could test for voltage and ripple and other undesirable, though, correct? I'm less worried about burning to death than protecting my equipment :)

--- End quote ---

Just be aware that there is a lot more sketchy stuff on Amazon than you might think. I don’t know the vetting process for items like power supplies, but there are certainly many which have slipped through the cracks. Some may have questionable safety and/or lack relevant safety documentation. Others might be safe but do not comply with mandated efficiency or EMC requirements. As expected , these items tend to be cheap and thus appealing to consumers, but in reality may be ignoring some regulations intended to help the consumer and protect society as a whole. Not to mention, undercutting legitimate manufacturers who ARE abiding by the laws.

By the way, technically power supplies for LED lighting should be IEC61000-3-2 “Class C” compliant for line frequency harmonic current. This is a measure of current drawn by the power supply at harmonics of the line frequency (50/60Hz). Drawing large currents at the harmonics can cause line voltage distortion, which in turn can mess up other systems on the same power distribution network.

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