Adrian’s electronics blog

I built a bench power supply for a relative which had to be low cost and I decided to use Bangood as my source of most of the parts to reduce cost. I purchased the case, relay module, DC-DC converter board, panel meter and the output banana jacks from Bangood and the rest came from my spares box. I had a number of issues with these cheap parts as they didn’t work as intended or simply just didn’t work. But more on that later.

The XL4016 based switch mode DC-DC converter board only allowed you to adjust the voltage output and it did not have a current limiter on it. The module is rated at 8 amps output but the tiny heatsinks suggest that it’s probably half that and the transformer feeding it is only rated at 2 amps max. I did a review of the XL4016E1 DC-DC converter module separately; see this link for the first impressions and the flaws I found with it.

So I decided to make an auto cutout circuit that works by monitoring the current via an ACS712-5A sensor. The ATtiny85 switches the PSU load on and off via a push button toggle. Useful as the module I’ve used does not have remote off / on facility and it’s handy to turn the output off whilst connecting up your projects rather than having it permanently on. When the output is on the sensor is read and if the current goes above 1.75 amps the output is disconnected and a red LED lights. A green LED indicates that the output is on and all is OK. I used a dual colour LED that simply turns red or green depending on condition. Originally I was going to measure temperature of the XL4016 module too using an LM35 but there were not enough pins on the ATtiny85 and the switching noise from the module produced erratic analog readings that I could not fully filter out. This didn’t present a problem with current sensing however.

Continue reading →

Surprisingly even now you can still buy power tools that come with NiCad batteries (not even NiMh) which are a poor choice of battery for power tools due to their memory effect and high self discharge rate. NiMh is a better choice of battery to use but I presume NiMh is cheaper hence why some tools still come with them. I have a bosch drill and hedge trimmer that both use the same type of 14.4V battery which have failed, even the spare one and a cheap Chinese knock off that was bought as a spare. This is simply because they don’t get used often and the batteries self discharge when left in the garage. Upon opening the batteries and checking the cells some of them measured zero volts and were short circuit both in the genuine batteries and the knock off clone battery. Many had leaked too.

The internal cells are around the same size of a C size battery but replacing them with NiMH C cells was out of the question as they would not physically fit into the battery casing. As I had a universal RC charger I opted for lipo batteries and charge them using the balance charger but went without the protection board for the low voltage cutoff. This could potentially be a bad idea but I fitted a small bar graph indicator into the battery pack for monitoring the battery voltage. A push button switch connects the indicator across the battery so it does not drain the battery when in storage.

To convert cordless power tools to lithium the correct way would be to use 18650 cells and a proper BMS board as many others have done. This was my original plan but RC lipo battery packs are better suited to high current discharge rates (many 18650 cells are not, the exception being the ones designed for e-cigarettes) and can handle sudden high current spikes. They can also be charged rapidly too. Using cells from old laptop batteries is not a good idea as some will be in better condition than others leading to cell voltages being out of balance. Also they are not designed for high current discharge rates; larger tools may take 20 amps when the chuck is stalled. It’s always best to use brand new cells and a good BMS board. It’s also worth mentioning that if a BMS board is used you must make sure there is back emf protection inside the drill otherwise this will destroy the BMS board when the motor switches off. If the tool does not have such protection across the motor then a small non polarised capacitor of around 10uf should be fitted across the motor. A high power diode in reverse across the battery (after the switch / trigger) should be fitted too. If this is placed across the motor it would be forward biased when the drill is in reverse. This is assuming the drill simply reverses the motor polarity to change rotation direction. I’ve seen several bad reviews for BMS protection boards purchased on ebay / Amazon saying they failed when used for power tool conversion and the lack of the diode and capacitor is almost certainly why they failed.

Here’s some photos of what I did. It’s nothing too fancy as there’s no BMS board or spot welding to do here:-

Continue reading →

This is another unfinished project; I started making a scrolling message display using the LED modules I used for my dual colour Pong Clock around 3 years ago. I managed to get it working to the point where a message can be sent via the serial interface (or a serially connected bluetooth module) and have several display options where you can have big or small text, have it in red, yellow or bold orange. I don’t really have a use for it and the Pong Clock does not get much interest from friends and family as in they don’t want one making so I’m putting the whole lot on eBay over the next few weeks. I’ll keep a couple of modules to play around with or in case someone changes their mind and does want a pong clock making but for now they are done with.

As for the scrolling message display I used modified Adafruit RGB matrix libraries that supports daisy chained modules; I used three for a better and wider display. I don’t know what the maximum number of daisy chained modules you can have but I guess more than three is pushing the Arduino to the limit. The library uses around 1.6kB of SRAM so an Arduino Mega is required even though the code size will fit into the Uno. I’ll release my code for this project for anyone to take and modify themselves as they see fit. Code is being offered without any support, but if you do use the code please credit me. Thanks.

Code for this project Message_Display_16x96

My Trust UPS battery failed on me last year and I was meaning to replace it but the cost of a replacement battery is almost equal to the cost of a cheap brand new UPS from dubious brands. I’ve always used APC SMART ups’s as they are very reliable and never seem to go wrong but man they are expensive.

APC do some cheaper models in plastic casing but they are no better than the cheap Trust / Belkin / Mustek types that often advertise with a massive VA rating but are yet much smaller than their APC equivalents. They often only have a 7 to 9Ah battery in them even in 2200VA models. I doubt they would last long at full load and I don’t just mean the battery. They only have 4 MOSFETS at the most which will likely explode the moment it switches to battery at full load.

Anyway enough intro. I decided my UPS was just crap and not worth repairing; it was rated at 800VA and had a single 12V 7.2Ah battery in it with an advertised full load runtime of 5 mins. I pulled out the mainboard and noticed that it obviously uses the same board for different models with the configuration set by resistors. They seem to contradict themselves but from what I gather my UPS is set for 230V fixed with no AVR (buck / boost) and 600VA rating. No wonder it had a shit fit when I tried to put more than 600VA load onto it. Definitely said 800VA on the front and the box. Going by the transformer it is a offline / standby type although the board supports some kind of auto voltage regulation though it isn’t line interactive. There are two pins on the transformer connector connected to relays and other electronics but are not used. Perhaps by changing the transformer and configuration resistors you can make it a 220V model or one with AVR that switches taps on the transformer to maintain a stable output voltage without switching to battery.

Continue reading →

Well time certainly seems to fly and it’s the end of another year. It’s been a year since I finished my LED animated Pong clock which I initially started about 3 years ago and I’ve been working on an improved version using bi colour LED matrix modules I obtained from a scrap advertising LED board. It’s a slight improvement over last year’s version but it has some issues that still need ironing out. I’ll post a further update and release the source code later when I feel it’s ready. Happy new year everyone!

I mentioned before I was going to make a 12 volt lead acid battery tester at one point. I’ve repurposed the single AA tester I had made back in 2017 and converted it to test higher voltage batteries. Without repeating myself from earlier posts I have just decided to keep this short and just put the 12V tester version on the original project page rather than creating a new one. It works just the same as the AA version in that it displays total runtime and approximate Ah capacity. It is a cut down version of my 24V lithium battery tester which I made for work. The original project page is here. You will want to scroll down to the latest updates section where you will find the schematic and code.