Lab Power Supply Project

Often I've ended up doing odd things because I only have a single lab power supply. For example when I was experimenting with op amps I first had to figure out how to create a differential supply from a single supply before I could actually do anything. Also when working on the current load I was using the load to sink current from the lab supply so I ended up running my circuit off a 9V battery until I got a better supply worked out.

The other issue is that the supply I have now is quite primitive. It has a fine/coarse voltage adjustment knobs instead of a multi-turn knob, it does display the output voltage and current digitally but the voltage reading is 200mV off. It has a current limit but you can't see what it is set to unless you short the output. The power output is quite good though (0-14V @ 3.2A and 14-29 @ 2.7A). The load regulation is pretty woeful - I'll get to that later. Occasionally it does some odd things in current limiting mode.

The PSU I have is one of these Jaycar ones that Dave Jones reviewed. It's not too bad really overall and it wasn't expensive. We've used it for all sorts of stuff including as PSU for my son's LIPO charger.

The thing that makes this easier is that all of the work I did with MOSFETs, DACs, voltage references etc for the dummy load is directly applicable here. I'm not starting from complete scratch.

The plan

What I would really like is one of these Rigol DP832A. Of course I could buy one but given I an am using this supply to learn about electronics it seemed a perfect opportunity to build one! Not to mention that the DP832A is close to $1000 here in Australia - that gives me a fair budget for the build!

So In general what I'd like is to have two or three output channels and the ability to run two in dual tracking mode. I'd like it to be at least as good as the Jaycar model but more accurate, more programmable and to have a better UI. So I was thinking it would have

• Two or ideally three output channels (as I said). Probably 0-30V and at least 0-3A. If possible higher current at lower voltages.
• Current limiting.
• Digitally settable output voltage - ideally both via keyboard and rotary knob.
• Display current and power output.
• Ability to individually turn each channel on or off (ideally with a momentary switch on the front panel)
• Pretty high precision - 1mV/1mA resolution, fast response, minimal overshoots, very low noise.
• Over voltage protection, reverse polarity protection (say if you connect a battery) fuses (both mains and DC). Safe grounded chassis, ground terminal on front.

What would be really nice is to:

• To be able to graph the current/power output like the Keysight bench meters do. Even show cumulative amount of energy consumed by the driven circuit.
• To have an Ethernet interface. Maybe a web interface or for it to be LXI compatible or both.
• Some output programmability (LXI). Doesn't have to be quick.
• Be able to save/load output configuration settings.
• Maybe experiment with a 3D printed front panel (for buttons and knobs etc)

Architecture

Looking at teardowns and repairs of other lab supplies such as the Rigol and Agilent units,  the usual thing to do is to use a big, custom wound transformer to generate all of the voltages required. These days the only transformers economically available are (a) Torroidal (which is good) and (b) usually have one or two output windings.

The limited taps means either I use one winding per channel or I use multiple transformers. Using one winding per channel doesn't give me any options for saving energy at lower voltage outputs and no options to boost the current at lower voltages.

Also transformers larger than 200VA are quite expensive.

Another factor driving the overall design is that PCB fab houses are much cheaper for smaller boards. A smaller board is easier to debug too.

These facts led me to decide that what I want is three independent channels that have their own circuitry, control and transformer. This way I can get the design of one right and build another two.

My plan is to use a Raspberry PI as the front-panel controller and have it control each channel via an isolated I2C interface. This way the power for the Raspberry Pi and each channel is all isolated (and floating) from every other channel. The Raspberry Pi supports Ethernet, USB, I2C and there are many touch sensitive LCD panels available. The only downside is the Raspberry PI will require a permanently active power supply and a soft button to allow it to shutdown properly. Also the Pi's boot time is quite long but this could be adjusted with some software pruning.

Each channel will include an AVR (Arduino) processor, DACs ADCs, voltage references as well as a thermal management system. Each channel implements its own current limiting, voltage control, current measurement etc and is controller via I2C. Two channels will include extra hardware to allow them to be shorted together (to create a higher voltage output or dual tracking output). Each channel will store calibration data in EEPROM and provide code to calibrate itself via the control interface.

To limit dissipated heat, my plan is to have relays to switch between either both windings in series or both in parallel. I also plan to build a simple pre-regulator circuit but I don't want to use a switching pre-regulator as they generate too much noise.

I plan to use MOSFET devices as the pass element but this might change if I have trouble stabilising the circuit.

The case is still a problem. To achieve 3A @ 30V I need 160VA transformers which are roughly 200mm x 50mm. Three of these with 100mm x 100mm control boards means I need a big box. Large project boxes are quite expensive and (for safety) the box needs to be metal so it can be earthed.

One option is to use an old shuttle PC case.and build a custom front panel. They have good thermal properties but they are still pretty big.

Open Source Tools

After spending all that time fixing my old Tektronix 475 scope it occurred to me that closed source lab/bench tools are a very bad idea. Without the extensive service manual of the 475 the unit would have been in a land-fill right now.

Furthermore, if you want to do something outside the box with a bit of lab gear, if the device is open you should be able to hack it to do what you want.

I doubt this project will ever be seen outside my bench (and this blog) but just maybe one day someone might release a line of open source lab tools that rival the gear from Keysight or Tektronix. 

Devices with lasered off component markings are very much the opposite of what I would want on my bench.

Next

Over the next while I will be working on the voltage regulator circuit, the pre-regulator and then current limiter. I plan to simulate and bread-board or proto-type each stage separately as I go and integrate as much as I can along the way. I do plan to get PCBs made for this eventually.

Welcome to the next six months of my evenings!

So watch this space! More later.