DC DC Step Down Power Module 5-16V To 1.25V/1.5V/1.8V/2.5V/3.3V/5V Universal Adjustable Buck Voltage Converter Board 3A For LCD (WHL #77)
Just a brief service tweet announcement for today, no in-depth review of the thing – these super helpful, small and cheap “CA-1235” DC-DC buck converters, commonly available for 50 cents a pop (plus postage), do use a Monolithic Power MP1495 TSOT-23-8 control chip labelled “IACSE” – or a clone thereof. While that connection is not immediately obvious and not mentioned by MPS (it says chip labelling “ACS”), teh Googles knows about this and leads to a suitable datasheet. Some variants of the chip itself are not recommended for new designs, but of course there’s still a lot of stock and probably clones available out there, even Mouser lists a couple of original parts with hundreds of units available.
The module itself is 3cm x 2cm (ha, my centimeter grid AND the 3:2 camera sensor really work well here!) and has a couple of SMD jumpers that can be bridged for setting the desired voltage. If left open, default voltage is 1.25V. Max current is 3 amps, which is actually taken from the datasheet and no wild overstatement from our Chinese friends. Input range is 4.5V to 16V, output range (pure buck operation) is 0.8V to Vin minus 5 to 10% due to the workings of the overcurrent protection.
Single-sided load, very little routing on the back. Vin- and Vout- are directly connected, this is not an isolated supply.
The long track from the Vout+ pin also gives away the output voltage setting circuit layout – the core part of this post. You see, the individual setting resistors seem wildly unrelated, but this is due to this architecture (taken from the datasheet):
05C -> 11k -> sets 5V 30C -> 20k -> sets 3V3 333 -> 33k -> sets 2V5 85C -> 75k -> sets 1V8 22D -> 165k -> sets 1V5 513 -> 51K -> R1 913 -> 91k -> R2
913/R2 is a fixed feedback resistor that is always connected to ground. The other resistors are chosen so that when bridging the SMD pad, their combined resistance (with R2) is such that the labelled voltage is achieved. That’s clever design, yet difficult to tweak if one doesn’t know the formula and has to reverse-engineer the circuit including the inner workings of the buck regulator.
Thankfully that is pretty easy (and in accordance with my own work…duh) with the datasheet, original formula is put above. 0.807V is an inner reference voltage that limits the minimum output voltage. Rearranging the equation yields
Vout = 0.807V * (R1/R2 + 1)
which makes it obvious that even without R2, so R2->inf, Vout will be set to these 0.807V.
Now, with that in mind, and please check your board for identical resistors (!), we got these easy options to vary output voltage:
11k bridged -> Rtot = 9k8 -> 5.00V 20k bridged -> Rtot = 16k4 -> 3.32V 33k bridged -> Rtot = 24k2 -> 2.51V 75k bridged -> Rtot = 41k1 -> 1.81V 165k bridged -> Rtot = 58k7 -> 1.51V none bridged -> Rtot = 91k -> 1.26V
These match very well actually, and real output does follow suit. But of course since that is just an array of parallel resistors that can be added and removed, we are not limited by just bridging one of them. Their combination no longer matches the silkscreen on the board, so be careful and label yourself once you do such things, but these are the combinations with two resistors…:
11k+20k bridged -> Rtot = 6k6 -> 7.06V 11k+33k bridged -> Rtot = 7k6 -> 6.25V 11k+75k bridged -> Rtot = 8k7 -> 5.55V 11k+165k bridged -> Rtot = 9k3 -> 5.25V (great for USB!) 20k+33k bridged -> Rtot = 11k0 -> 4.56V 20k+75k bridged -> Rtot = 13k5 -> 3.87V 20k+165k bridged -> Rtot = 14k9 -> 3.57V 33k+75k bridged -> Rtot = 18k3 -> 3.06V 33k+165k bridged -> Rtot = 21k1 -> 2.76V 75k+165k bridged -> Rtot = 32k9 -> 2.06V
And so on and so forth with three resistor bridged in parallel to the fixed R2:
11k+20k+33k bridged -> Rtot = 5k5 -> 8.31V 11k+20k+75k bridged -> Rtot = 6k1 -> 7.61V 11k+20k+165k bridged -> Rtot = 6k3 -> 7.31V 11k+33k+75k bridged -> Rtot = 6k9 -> 6.80V 11k+33k+165k bridged -> Rtot = 7k2 -> 6.50V 11k+75k+165k bridged -> Rtot = 8k2 -> 5.80V 20k+33k+75k bridged -> Rtot = 9k6 -> 5.11V (also great for USB) 20k+33k+165k bridged -> Rtot = 10k3 -> 4.81V 20k+75k+165k bridged -> Rtot = 12k4 -> 4.12V (not perfect for lithium cells, but you do you) 33k+75k+165k bridged -> Rtot = 16k5 -> 3.30V (actually closer to 3V3 than the designated 20k!)
And the few options with four and five resistor bridges:
ALL but 165 bridged -> Rtot = 5k1 -> 8.85V ALL but 75k bridged -> Rtot = 5k3 -> 8.56V ALL but 33k bridged -> Rtot = 5k8 -> 7.86V ALL but 20k bridged -> Rtot = 6k6 -> 7.05V ALL but 11k bridged -> Rtot = 9k0 -> 5.36V ALL bridged -> Rtot = 5k0 -> 9.10V
I, for once, needed a tad above 6 volts for a small water pump to not only start reliably but also deal with air bubbles properly (which it did not at 5V), so I went with bridged 11k + 33k for 6.25V. If later on a bit more voltage is needed due to longer wiring, I will add another bridge at 165k (+0.25V) or 75k (another +0.30V).
As for calculating arbitrary voltage-resistor pairs within the realm of the allowed…maybe just use the simplified terms
Vout = 1.25927 + 41157 / R R = 41157 / (Vout - 1.25927) (R in Ω, Vout in V)
which of course only works with the 51k/91k configuration of my unit (YMMV) and R being the additional resistor in parallel.
e.g.: 6.66V requires a 7.62k resistor…which is not actually available in E192 or lower, too bad!
But, when modding the thing to the point that resistors are exchanged, it’s probably easier to remove the 91k unit and solder in whatever you require.
Speaking of the 91k base resistor being removed – that of course also allows for quite a few more voltage pairs with the existing resistors, but I don’t see very many useful/new ones. I think it’s best to keep that one in, as it might save the day when re-using the converter in another project a couple years down the line. After all, all presets are off when that resistor isn’t present – thankfully all voltages go down, so at least it’s not an immediate threat to attached hardware.
NO 91k R2 present on all these combinations! 11k bridged -> Rtot 11k (well, duh!) -> 4.55V 20k bridged -> Rtot 20k -> 2.86V 33k bridged -> Rtot 33k -> 2.05V 75k bridged -> Rtot 75k -> 1.36V 165k bridged -> Rtot 165k -> 1.06V none bridged -> Rtot inf -> 0.81V 11k+20k bridged -> Rtot 7k1 -> 6.61V 11k+33k bridged -> Rtot 8k3 -> 5.80V 11k+75k bridged -> Rtot 9k6 -> 5.10V 11k+165k bridged -> Rtot 10k3 -> 4.80V 20k+33k bridged -> Rtot 12k5 -> 4.11V 20k+75k bridged -> Rtot 15k8 -> 3.41V 20k+165k bridged -> Rtot 17k8 -> 3.11V 33k+75k bridged -> Rtot 22k9 -> 2.60V 33k+165k bridged -> Rtot 27k5 -> 2.30V 75k+165k bridged -> Rtot 51k6 -> 1.61V 11k+20k+33k bridged -> Rtot 5k8 -> 7.85V 11k+20k+75k bridged -> Rtot 6k5 -> 7.16V 11k+20k+165k bridged -> Rtot 6k8 -> 6.86V 11k+33k+75k bridged -> Rtot 7k4 -> 6.34V 11k+33k+165k bridged -> Rtot 7k9 -> 6.05V 11k+75k+165k bridged -> Rtot 9k1 -> 5.35V 20k+33k+75k bridged -> Rtot 10k7 -> 4.66V 20k+33k+165k bridged -> Rtot 11k6 -> 4.36V 20k+75k+165k bridged -> Rtot 14k4 -> 3.66V 33k+75k+165k bridged -> Rtot 20k1 -> 2.85V ALL but 165 bridged -> Rtot 5k4 -> 8.40V ALL but 75k bridged -> Rtot 5k6 -> 8.10V ALL but 33k bridged -> Rtot 6k2 -> 7.40V ALL but 20k bridged -> Rtot 7k1 -> 6.59V ALL but 11k bridged -> Rtot 10k0 -> 4.91V ALL bridged -> Rtot 5k2 -> 8.65V
Well, that’s it – hope it is useful for somebody someday, maybe it’s just forgetful me googling this exact thing in 2026 when using the last unit in stock for arbitrary voltage needs, who knows… 
