modified: Sunday 23 April 2017
author: not set
Making a bench PSU out of a cheap buck/boost module off the web
For the past few years I have thrown together all sorts of random things when I've needed a power supply:
From left to right:
- An old laptop brick (20V). Cut apart to compare it's guts with a cheap PSU, glued and taped back together.
- Grand nailer, the ATX power supply from Halvalla.
- A 5V phone charger that lost its significant other.
The ATX power supply is an old group-regulated design that expects a 5V load. Without this load the 12V rail sits around 10 to 11V, especially under load, which tends to be a bit lower than I want. Its 5V rail is also akin to a spot welder, with little resembling over-current protection. The sharp nailbed and fake LiPo sticker makes up for some of these failings.
The little phone charger and the computer PSU are both fixed voltage, which means I rarely use them. But they do at least have functioning over-current protection.
Overall: something with an adjustable current limit would be nicer. And as of today, I need not dream anymore.
The synchronous current recycler from hell
I had been reading a few things about the LTC3780 boards that you can buy off eBay:
- Constant voltage and constant current controls
- Ridiculously high power output and efficiency. 8A anyone?
- Boost and buck (transparently, causing you no issues), so you can use any other PSU you have lying around to power it.
... in other words, almost a complete bench power supply for a fraction of the price of a normal one.
Alas, we'll also need some nice front-panel controls:
Modifications for the pots
After receiving and testing my 3780 board I immediately went onto modifying it to use the new pots.
This turned out to be harder than expected. My new pots were 5Kohm max and the old ones were many hundreds of Kohm max. I presumed that the circuit would use these pots as voltage dividers, so this would not make much difference, but that didn't turn out to be true.
The pots were connected via three wires to the PCB, as shown by the three circles at the bottom of this diagram:
On the right we have the voltage coming out of the power supply and on the left we have it going back to the main LTC chip. You can see that two of the pins for the pot are commoned together, effectively turning it into a two-terminal variable resistor.
When I switched out the original multi-hundred Kohm pot for my measly 5Kohm one, the usable voltage range I had out of the board was greatly reduced. This was because of the fixed 14K resistor shown in the diagram above. After some quick calculations I decided I needed to replace it with a 120 ohm resistor instead.
Going from 14Kohm to 120ohm is a massive jump, and you should always be wary when you do this. Luckily however this 120 ohms would (almost) always be guaranteed to only have 1.25 volts across it (due to how the feedback system worked), and this would always be coming from the main power supply output, not the poor LTC chip directly. All good.
As per usual: I didn't have any replacement SMD resistors at hand, so I had to bodge a dinosaur-size 1/4W resistor in instead. It looked quite neat and pretty. Sadly I don't have any pictures of this.
Instead I have photos of my horrific failed attempts to rescue the situation after accidentally snapping this resistor off the board:
"Ooh, that resistor does not look straight. I'll just bend it a litt--" SNAP!. There goes the pad. Cue many hours of reverse engineering this board and trying to bodge replacement parts in.
And please don't mention that little purple wire. Trying to solder it to a specific pin on the LTC, without making solder bridges, was a nightmare. Those pins are spaced every 0.65mm.
Unfortunately my repairs did not work. It seems my constant attempts to re-solder a wire to the LTC killed it, and it refused to spit out much more than a volt after that without over-heating.
Let's buy another one!
I was nowhere as enthusiastic as this title would suggest. This is money I could have spent on several packets of chocolate biscuits.
Let's compare new and old:
Note that I've removed the heatsinks from the old (right) board, and attempted several other repairs (look for blobbly solder bits).
Whilst these two boards use the same layout, there are a few differences. Sadly the new one used narrow holes for the pot pins, so I wasn't able to solder 2.54mm headers in their spots like I did in the old one. And the screw-holes for mounting were slightly smaller/different.
I'm not sure why they went for a different number of turns on the main inductors. Different core material? Different parts cheaply available? It's likely a wide range of inductances will work well with this chip.
Primary power supply
This LTC3780 board can't accept mains power directly. I needed a PSU between it and mains.
Luckily this PSU can be almost anything I want. I have a stack of old laptop chargers that would fit the bill, but I decided that I wanted to try and use one of the toroidal transformers I had lying around instead. A bonus of this is that my power supply would become completely isolated, which is handy when using an oscilloscope.
Unfortunately none of the toroids I have scavenged over the years were suitable. Let's take Bertha, for instance:
She outputs a nice 12VAC, but weighs a few kilos and is absolutely massive. One day I might make a spot welder out of her several jiggaamps.
I did have a few other toroids, but they all outputted voltages that were a bit low for my liking (eg 7VAC) or too high for the board (30VAC).
Rewinding a toroid
This wasn't anywhere as bad as what I had expected. The outside layer of wires was the secondary, so I just cut it off and installed a new one.
I wound two wires together at the same time. I didn't know how much wire this would take, so I guestimated a length and cut it. This didn't end up being enough so I had to do this twice and put a couple of joints halfway around my toroid, but this didn't bother me.
It worked remarkably well:
Note the custom-made primary board with a fuse holder (bridged by my multimeter) and the secondary board with a bridge rectifier.
I looked through my collection and found a nice aluminium box, previously used by the power supply for a CCTV camera. This old analogue camera was fun whilst it lasted, but then something in it disintegrated into cat piss.
I was however slightly annoyed that this box already had an input fuse and switch:
Bastard. I threw out my earlier boards (including the rectifrier one because it was too big).
This is what the final insides looked like:
Mains power starts on the right, goes through the toroid in and ends up on the left. Also shown in this picture is the lid where I mounted my pots and banana jacks.
And here is the final product:
Problems still to be addressed:
- Poor current output range. I'll need to look at the current pot's circuit. It should be able to do many amps, and hopefully even less than 70mA.
- Cooling. I might throw a small fan inside to stir the air
Other things not mentioned:
- The clear guard over the rear wiring appears to be hot-glued in place. It's also siliconed, the hot glue was just to hold it whilst this set.
- The voltmeter on the outside is a small standalone one off eBay. I have no integrated way of measuring current just yet.
- Feet were made out of offcuts of wooden dowel.
- Yes those aubergine capacitors are from the February Element14 haul from last year. No I have not been able to convince anyone to take any off me. Please, tell me you want some!