Cheap Power Supplies


So today I received some 12V power supplies that I ordered a month (or three) ago from AliExpress. I had ordered a 12V @ 2A for AU$4 and 12V @ 4A for AU$7.
On first inspection… man are they lightweight.. My digital weighing scale didn’t even register that a load was put on.

So how good or bad can it be? Let’s find out..

What’s A Power Supply

Let’s start off by understanding what exactly these power supplies I purchased are. The power supplies I purchased were 12VDC switched power supplies, rated at either 2A or 4A.


If you’re curious about how SMPS (switched-mode power supplies) work, take a few minutes to watch these videos

Essentially - to decrease physical space requirements and improve power efficiency, a SMPS converts the rectified supply voltage into a high frequency PWM AC current that can be stepped down with a transformer; where the high frequency current allows the internal capacitor to be smaller (capacitance and frequency have an inverse relationship). Pretty neat, huh?

What make a good power supply

I’m no electrical engineering (jk I am), but to me there are a couple of things that I look out for when buying power supplies.

What if these criteria aren’t met?

Especially because power supplies deal with - you know - power.. there poses an electrical risk as well as potential fire hazard if something were to go wrong.
A damaged case could expose live electricity, or a loose part could short circuit the power supply - both which are obviously hazardous.
Perhaps a badly designed or overloaded circuit could overheat, melt, and cause a plethora of other issues…

Therefore it’s quite important to ensure that your power supplies are of a good quality - especially if they’re going to be left plugged in whilst unattended.


Given how amazingly light these power supplies were, I was dubious as to whether the power supplies could actually output their advertised currents of 2A and 4A.
From my experience, lightweight power supplies mean that corners were likely cut in terms of component quality; something I’d be wary of.

Perhaps technology has evolved and new technological innovations have allowed power supplies to become basically weightless.. But having a look at the power supply circuit myself, and of what it was (i.e. SMPS).. I doubt that…

To test the power supplies, we can utilise Ohm’s Law V = IR and attempt to verify that a certain value of I is achieved during real-world testing.

12V @ 2A 0.5A supply

For a 12V 2A power supply, we can plug these V and I values into the formula

V = IR
12 = 2 * R
R = 6

In other words, for a 12V power supply, if there was a 6 Ohm load, we should expect 2 amps of current to flow through the circuit (I = V/R).

Unfortunately I don’t have a 6Ω resistor… but I do have two 10Ω resistors which in parallel give around 5Ω (because, physics..!)
So for a 12V power supply, with a 5Ω load we should expect I = V/R = 12/5 = 2.4 amps of current.

This 2A power supply shouldn’t be able to provide more than 2 amps of current, so I’d expect the measured current to be around 2A


Hahah… so my 5Ω resistor (measured 5.2Ω) had a measured voltage drop of 2.5V… a measly 480 mA running through the circuit.
It’s nowhere near the supposed rating of 2A…

At the very best, this power supply is really a 12VDC 0.5A power supply

What’s Inside

Looking at the circuit components, we indeed see the layout of a SMPS, some sort of full-bridge rectifier (MB10F IC) into some MOSFET / PWM controller (CR5218), which then gets stepped down through the transformer and then into a rectifier stage (diode + capacitor).

The transformer allows looks very feeble. I don’t think I’d trust it to have 2A induced through its coils..

12V @ 4A 1A supply

So what about the 4A power supply?

Reusing my 5.2Ω resistor, I should also be expecting around 2.3 amps - which is well within rated current capacity of 4 amps.


This time, I measured a 5V voltage drop across my resistors - meaning that only I = V/R = 5/5.2 ~= 1 amps was being provided. Below the expected 2.3A, and way below the rated 4A.

So in effect, this power supply is in fact only a 1A power supply.

What’s Inside

Quality feels a bit better here. Whilst the transformer looks more capable (given it was able to transfer 1A) and there is are some more circuit components; the sheer overall lack of weight of the board (and of the transformer) still makes me feel uneasy about using this power supply.

For the input power stage, the same full-bridge rectifier (MB10F IC) is used.
For the PWM management, some other IC is used (PL3368 CB136W08 - can’t find online)


For only AU$7, the 12V 4A-but-actually-1A power supply isn’t too bad of a deal, but I would much prefer to purchase a 12V 1A power supply which holds true to its labelled rating for QA and trust… But otherwise, if you’re temporarily (please don’t leave these power supplies unattended) needing a 12V 1A power supply, then I bet you that they’re “OK”.

That said, I’ll definitely be asking for a refund as the products don’t match their description, and I’d rather throw these out than for someone in my household to mistakenly overload these power supplies.

EDIT: Buyer Seller Protection

Unfortunately AliExpress isn’t the most helpful with customer support… Fortunately I only wasted around $10 on this ordeal - no biggie.
But it’s a shame to see that customer support doesn’t serve to really support the customer.

Imagine if this was a more expensive product that didn’t serve its purpose and I didn’t get my money back.

To be fair, if the seller was just dropshipping these products … then sure. However at various times this seller replied to other concerned customers and claimed that they were the manufacturer of these power adapters. If they’re the manufacturer… then they’d be more than aware that their product isn’t meeting what they’re advertising. Definitely scamming people then unfortunately!

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