Last Christmas I bought a Velleman MK170 Christmas Star project for my Wife to build. She often shows interest in my various electronics projects and I thought it would be a good way for her to build something that she could show off to friends and family, whilst at the same time teaching her a thing or two about electronics, components, and soldering.
After the build was completed we connected it to a 9V D.C. supply, as instructed by the documentation, and…
Star before mods
I was quite disappointed! The project was definitely functional, but the display was extremely dim and underwhelming. At first I was convinced we’d done something wrong during the assembly, but nope; everything was as it should be!
The camera flatters the result. To the naked eye the brightness of the LEDs is completely unsatisfactory.
I decided to revisit the project for this year. I downloaded the schematic from the Velleman website, and took a look to see what was going on.
Looking at the schematic (click the image for a larger version) you can see that we basically have three sets of LEDs, each of which is split into sub-groups of series and parallel LEDs.
Now, one thing that jumps out straight away is the value of the series resistors; they’re HUGE!
Take, for example, the series chain consisting of LED1, LED2, LED3, LED4. I measured one of these individual LEDs and they don’t even begin to conduct until ~1.7V. So to work out the current through these resistors we have:
(9V – (1.7V * 4)) / 2200R = 1mA!
I looked up the datasheet for the LEDs and I found that the typical forward current should be around 10mA. Practical tests showed that these LEDs conduct 10mA at around 2V forward voltage.
So, to work out more appropriate resistor values:
Four Series LEDs
For the chains with four series LEDs, the forward voltage drop will be 2 * 4 = 8V.
This means the drop across the current limiting resistor will be 9 – 8 = 1V. In order for 10mA to flow, we’ll need a 100R resistor.
100R!! The 2k2 resistors that Velleman fitted are not even in the ball-park!
Two Series LEDs
The same process can be used to determine the appropriate value for the chains with two series LEDs. The drop across the resistor ends up being 5V, so you need around 500R for the current limiting resistance.
Star after mods.
I really don’t know what Velleman were thinking with this project. Their choice of current limiting resistor values are way off. I can only imagine that their intension was to reduce the current down to the absolute minimum, so that the project could be powered by a standard PP3 battery for relatively long periods of time.
That’s all very well, but the result is a project with totally unsatisfactory LED brightness.
I changed all the resistor values on ours, and now we are able to show off the project along with all our other Christmas decorations.
The current consumption on my unit, after my modifications, is 120mA when all LEDs are lit. Not all the LEDs are lit all of the time, but I guess it would be reasonable to assume >90mA average current consumption.
A standard 9V PP3 battery is going to be wasted very quickly at this current consumption. I would estimate 1-2 hours use before the battery drains flat!
That’s probably why Velleman used such high resistor values. But my take on this is that it was a poor design decision to build the project around a PP3 battery in the first place. These batteries have very low capacity, so they were on to a loser from the beginning.
We are powering ours from a 9V DC adaptor, which is the only realistic way forwards for a project like this.