Computing stuff tied to the physical world

Meet the Current Source Plug

In Hardware on Oct 28, 2010 at 00:01

Here’s another new plug, firmly in the realm of home automation: the Current Source Plug drives two independent channels of Power LEDs with either 350 mA or 700 mA current:

Dsc 2183

As you can see, this plug was a very tight fit. These are based on high-efficiency AP8803 switching power supply chips, which also explains the two hefty inductors on the board. The 2 I/O pins are used to control both channels, and can use PWM to dim the LEDs.

The connector on the bottom is for the LED power supply (sorry for the tight fit!), and can be 8..30V. According to the specs, you can have up to 5 power LEDs in series at 30V, so the highest power this plug can theoretically switch is 5x 3W-LEDs @ 700 mA per channel!

There are some thermal limits though, due to the high amount of power involved in this small board. It is best to keep the voltage differential between what the LED needs and the input voltage fairly low. The two 3.7V LEDs I tried worked fine at 6..12V, but the board started heating up quickly with more than 12V (at full power).

Here’s the test setup I used:

Dsc 2182

The big LED is set to 700 mA, the small one to 350 mA – for a total of 1050 mA output. The nice part is the efficiency of this thing: the board draws less than 1050 mA due to the “buck” switching design. I measured roughly 960 mA @ 6V and 490 mA at 12V. At 18V it heated up a lot, I did not dare take it any higher. The main use I see for this board is two channels, each with 1 or 2 power LEDs in series, driven from a 12V supply.

At full power, the board temperature rose about 40°C above ambient after an hour, i.e. roughly 60..70°C. Hot to the touch, but not scorching hot. When dimmed, temperatures immediately went down. Note that the LED lights themselves were hotter than the board. Power LEDs need cooling!

For PWM dimming up to 500 Hz, just use the I/O pins. I haven’t tried it yet, but this could be done with the same rgbRemote.pde sketch as used for the MOSFET plugs.

The documentation and shop have been updated with this new plug.

More goodies coming, stay tuned…

  1. PWM dimming of LEDs is a very good idea. You get the same apparent brightness for considerably less power than dimming by reducing LED current. (When I first did this, it was controlling the common cathodes of a bunch of LED’s, and used a SCR triggered by a UJT to do it. Cheaper back then than a MC1555. Yes, I’m old.)

  2. Could this be a more efficient way of handling your RGB strips and avoiding the horrendous losses on the Red channel?

    • ‘fraid not…

      The channels on the Current Source Plug don’t have a common pin. What you’d need is a current-controlled switching current sink (perhaps MOSFET + inductance + schottky + a lot of know-how I don’t have), and even then the RGB strips are full of those resistors, which would have to all be manually shorted out.

      In terms of energy efficiency, I think the RGB (and even white) “strips” are so-so (but still considerably better than incandescent). I see them more as background lighting, with nice and cosy power LED spots to deliver the “important” lumens, if there is such a thing.

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