An affordable and convenient DC-DC converter.

In every electronics hobbyist’s life—and not just theirs—there are times when a small, compact, inexpensive step-down voltage converter would come in handy. This review is about just such a device.

I’d had my eye on converters like these for a while, but the amphibian wouldn’t let me pay $4 each at local stores. Later, I saw them on one of the electronics hobbyist forums for about $10 for 10 pieces. During the March 25 sale on AliExpress, I decided to pick some up.
Honestly, I chose based on price—in other words, the cheapest ones. There may be even cheaper options, but at the time this was the best deal.
Note: The seller’s listing is now $7.50 for 5 pieces, whereas when I bought them, it was 10 pieces per lot. Pay attention.

The specifications listed by the seller (though they’re the same for all boards like this).
Input voltage: 3.2–40 V (the chip manufacturer recommends 4.5–40 V)
Output voltage: 1.25–35 V (naturally, the input voltage must be at least 3 V higher than the output; even then, the output capacitor will be right at its limit)).
Output current: up to 3 A (I wouldn’t be so optimistic; given the components installed, 2 A seems more realistic to me).
Efficiency: up to 92% (quite realistic under certain conditions)
Default output voltage: 5 V.

Board dimensions: 41 mm long, 21 mm wide, 15 mm high at the tallest component.

The package arrived a few days ago in a small box. My order was wrapped in a piece of bubble wrap; the main items were the converters, plus a couple of other small things.
I’ll review the other bits later, but I decided to write about the converters now.
So, this is what the package looked like when it arrived—


The seller either has trouble with math or has his own idea of what a discount means, because I received 9 pieces instead of the 10 I ordered. He saved himself 10%. I messaged him about it and am still waiting for a reply.
It reminded me of a story I overheard at a street market.
A trader wanted to buy scarves to resell and haggled with a Chinese seller for a long time over a discount. He got 20% off, but when the order arrived, he was very surprised to find that each scarf was 20% smaller.

Each converter was packed in its own sealed antistatic bag.


The board looks noticeably different from the one shown on the seller’s page.
The main difference is the use of ordinary, cheap aluminum capacitors at the input and output.
For anyone interested, their specs are as follows:

Input: 100 µF, 50 V; ESR: 0.13 Ω; actual capacitance: 102 µF
Output: 220 µF, 35 V; ESR: 0.077 Ω; actual capacitance: 225 µF.
Both are rated for 105°C.
The manufacturer is some no-name company called Chengx (not even the Chinese brand Chang, which many people haven’t heard of).
I’d replace them with something better, since the PWM chip is sensitive to capacitor quality, and add ceramic capacitors in parallel.
LM2596S chip — datasheet — www.ti.com/lit/ds/symlink/lm2596.pdf‎
The circuit is almost exactly as shown in the datasheet (I’ve included the closest matching circuit I could find; the input capacitor value and diode type differ, and the inductor value is listed incorrectly, since the 330 marking means 33 µH).

A single chip like this costs about $2.50 here.
The installed inductor is 33 µH and looks about the right size for an inductor rated for around 3 A (12 × 12 × 7 mm).


The board also has a multiturn trimmer for fairly precise adjustment, and a Schottky diode that looks a bit undersized to me (and to the PWM regulator chip manufacturer as well).
It’s marked SS36, but after comparing its size with the datasheet, I’m inclined to think it’s a relabeled SS26 — www.vishay.com/docs/88748/88748.pdf‎
If you want to improve efficiency, reduce heat, and increase reliability, it’s better to replace it with a more powerful one (the chip datasheet recommends a 1N5825).
The input and output pads are labeled.


The back of the board acts as a heatsink. Directly beneath the chip, there is a grid of plated-through holes to transfer heat to the other side of the board. The board barely warmed up during testing.


Naturally, I wanted to check the efficiency of this converter.
The photo below shows the input current with the output set to 5 V (the most commonly used voltage for various devices), a 5 Ω load (1 A, 5 W), and a stable 12 V input (again, a very common value). The input current is 0.5 A, which means 1 W of losses and an efficiency of about 83%.


In the second example, the output is set to 10 V, with a 10 Ω load, giving 1 A and 10 W. The input is a regulated 15 V. The input current is 0.74 A, for losses of 1.1 W and an efficiency of about 90%.


Obviously, the measurements have some margin of error, but they are very close to the actual values.
I'm not sure I'd buy these converters from this seller a second time, even though the price was good. The seller's carelessness (9 units instead of 10) and the rather poor input and output capacitors, which are best replaced, offset the advantage of the low price.
You can desolder the required diode from a faulty computer power supply, and it doesn't cost much on its own. Capacitors are also usually easy to find in any electronics hobbyist's parts stash, but it's best to check with the seller about which ones are fitted (though that can be tricky with Chinese sellers).

On the other hand, if you buy all this locally, the price is excellent even with these shortcomings. Still, it's better to buy a lot of 10, since smaller quantities will cost almost as much as they do in stores.

To sum up, I highly recommend buying the boards themselves. Whether to buy these particular ones from this seller is up to you; I've described the drawbacks as best I could.

P.S. With a relatively simple modification (adding a few resistors, a transistor, and a capacitor), you can add constant-current regulation at the output and turn it into an LED driver, though the efficiency will drop.
Below is the circuit for modifying this board to regulate the output current.
The load is the LED from this review


Sorry about the photo quality; they came out a bit dark.
If I missed anything, let me know and I'll try to add it.

Update: The seller refunded me for the missing board.

UPDATE: The seller's lot is now $7.50 for 5 units, whereas when I bought it, the lot was 10. Be careful.