Bench Power Supply - Part 1

Home Laboratory Bench Power Supply Project 

Intermediate Skill (Mains Power 120V/240v wiring)

So what is a Bench Power Supply?

Let's say that you have an SBC, a Single Board Computer like the BBC Microbit, Arduino or Raspberry PI or one of their equivalents. You write some software to turn a motor on and off or change it's position based on some input like a position sensor, or light or temperature sensor. You get it all working and test it using an LED driven through a resistor from the output pin which is 3.3v or 5v. The LED only takes 5-20mA to light it and confirm that the program works. But to make real use of the board and program you might need to drive a servo motor, solenoid or some machinery. You can get motors drive boards (HATS) for many purposes but motors that can do meaningful work will need more power, usually 12/24V at a couple of Amps at least. This will hamper your testing, sure you could hook up some rechargeable batteries but if you have a fault  or accidentally short circuit the wires you could be drawing 20-50 Amps and your wiring and possibly your SBC will get fried, maybe even catch fire.

Drone on fire

This was a drone. So it is a good idea to test any electronics in a controlled environment. You can buy cheap SMPS (Switched Mode Power Supply) modules on Ebay or Amazon for around 10 GBP, 12 USD. They will supply 5, 12, 24 , 36 and 48 volts at 5 Amps. Trouble is that you would need one of these for each voltage that you require, and the current limiter is a safety feature that is set to 5 Amps only, not good enough protection when you are working with delicate computer boards.

LED lighting power supply

But, that is still a good building block. So let's say we buy one of these to deliver 24 volts 5 amps, the input voltage can be switched from 240V to 120V to suit your location and mains supply. Now we really want to be able to vary the voltage from 0-24 or more volts, and ideally limit the current. In fact it would be ideal to have a constant current mode rather than just cut the output off.

Mains voltage setting

After a bit of searching I found the perfect solution. They are sold from China on Ebay or Amazon, simple search for "LTC3780 Automatic Step Up Step Down Buck Boost Power Supply Module". They are generally available from 10 GBP or 12 USD. You will need to be patient for delivery as it can be 2-4 weeks delivery time from China.

The connections, Live, Neutral, Earth, -V x2, +V x2

Helpful note – The Chinese government subsidise postage costs to other countries down to a few pence or even free, so get the cheapest price including p&p. It’s worth waiting a few days longer to keep the cost down as many UK outlets sell the same stuff at almost twice the price. But do watch the estimated delivery dates as some are over a month, try other suppliers and don’t forget to clear the redundant ones from the shopping cart before you commit.

So what is a LTC3780 Automatic Step Up Step Down Buck Boost Power Supply Module?

The LTC3780 buck convertor outputs 1-30 volts

OK so the word automatic is redundant, it can take an input voltage from 5v to 32v and convert it to 1v to 30v, wow that is like magic! Now these can pass 5 amps so try and buy one that has heatsinks over the MOSFets or it will get very hot. How does it do that? Simply put it switches the incoming voltage 50,000-60,000 times a second and dumps it into an inductor, the big copper toroidal (doughtnut shaped) coil. Now you can release the stored energy from that coil in a controlled way and by changing the on-off times of the 4 MOSFets it is possible to produce a range of voltages with great efficiency. So much so that at lower voltages it is possible to supply more than the 5amps going into the circuit! I have seen 11 amps at the output!

So we could feed our 24v PSU into the Buck convertor and get any voltage we could practically need. Now along one edge of the PCB you will see three blue rectangular ten-turn potentiometers (variable resistors), this allows very fine control of the Voltage (V-OUT set) , Current (CC-set) and Under-Voltage (UV-set). Wouldn't it be better if we had front panel mounted controls (like volume controls) to set the Voltage and Current settings.

Three 10 turn potentiometers 50k, 20k 50k, leave the left hand one alone!

So I measured the 10 turn pot's. The first one is 500KOhms (marked 504) and the second one 20k (marked 203). So we can buy 1x 500k Linear potentiometer for Voltage and 1x 20K linear potentiometer for Current limiting. Now professional power supplies have one extra control 'fine voltage' control. So if we buy one more potentiometer a 1x 20KOhm Linear one, for a few pence extra we can wire that in series with the main voltage control and have coarse and fine control. Fine control gives about 2.5 volts difference over it's full travel and helps to prevent mistakes. Leave the UV-set one alone, we only need to tweak than once and not touch it again.


So how much voltage and current will I be making?

We need to know what the output is at any time, rather than tie up your favourite multimeter why don't we add a panel voltmeter and current meter, better still let's add a combined voltage and current display. I found these from 2.50 GBP or 3 USD upwards, we need the ones that are 100 Volts and 10 Amps, this will make the readings suitable for our PSU and do not need a current shunt.

Voltmeter and Ammeter, all in one!

Here is a test rig using a 12v 1a power brick and driving a small computer fan

Testing the Buck convertor on a 12v 1Amp power brick, it is running a computer fan.

I made a decision at this point to make a dual power supply, that is two channels independently adjustable for voltage. Now later I realised that as the current meters use the negative rail to measure the current that there may be some inaccuracy because the negative rail is common all the way through this power supply. Normally one channel would be driving an SBC and the other the motors or loads. So long as you wire the negative leads to their respective terminals there should be no problem. But if the negative connections are connected anywhere else the current readings will have to be totalled up. You have been warned. This will not affect single power supplies or any use as a single channel PSU. You cannot make +ve and -ve balanced supplies from this unit!
The shopping list
Universal Switch Power Supply AC110-220V to DC 24V 5A LED Transformer (other voltages available) £10.12 + free p&p

DC 5V-32V to 1V-30V 10A LTC3780 Auto Step Up Down Regulator Charging Module A1Y2 £8.40 free p&p

218x237x93MM Large Project Box PCB Enclosure Case with Ventilation Slots KE17W £11.99 + £3.70 p&p

DC 100V 10A Voltmeter Ammeter Blue&Red LED Amp Dual Digital Volt Meter Gauge UK £2.99 + free p&p
https://www.ebay.co.uk/itm/1-5-10-20-50-100-500-K-M-ohm-Linear-Logarithmic-Mono-Stereo-Pot-Potentiometer (you want 1x 500k linear and 2x 20k linear) and some knobs £0.99 each
2 x switches DPDT ON-ON Mini Toggle Switch Double Pole Double Throw Lever Flick Car Dash SW20 £2.98 + £1.70 p&p

6 Amp mains cable – do not use more than 1 metre as it is a trip hazard.
3 Core Round Black Flex cable 0.75mm Flexible PCV Extension Wiring 1 metre £2.95 + free p&p

Mains Cable retainer

Dual Female Banana Plug Socket Terminal Binding Post for Speaker Amplifier 4MM G £1.98 for two + free p&p
4MM plug leads- 1M Long Alligator C to Banana Plug Test Cable Pair for Multimeter K4T5 £2.27 + free p&p
Mains plug with 3 A fuse MAINS POWER UK 3 PIN FUSED PLUG £1 buy locally, or whatever your countries plug is.
You will also need flexible cable capable of carrying 5 Amps DC and 240 Volts AC in various colours, say brown and blue for the mains switch and fuse, and red and black for the DC side.

You will also need some finer wires to connect to the front panel controls, different colours will make it easier to work on. I used -

16/0.2mm Single core hookup wire 22m Pack £5.90

You could also try Amazon.com, Banggood.com or Aliexpress.com You will also need -
Mounting screws,spacers, cable ties.
Tools - power drill and bits, soldering iron, solder, desoldering braid, multimeter, screwdrivers, wire cutters, wire strippers, pliers.


In Part 2 - assembly and testing