Dimming 31V LED Festoons

[Emmien]

Er yes, but they don't use a transformer/rectifier as the power supply. The controller does the polarity reversal.

 

Actually I am not sure. I think they use a SMPSU, but I know that they often get the timings from the AC power frequency.

[wbdreynolds]

Did you every get anywhere with this?

I want to do the same thing - to dim strings of 31V LED fairy lights. Mine have alternating blue and white LEDs in the string.

 

These are the lights I have - http://www.ebay.co.uk/itm/50m-100m-Low-Voltage-LED-Fairy-String-Lights-Wedding-Clear-Decoration-Waterproof-/332163661175?var=541196636849&hash=item4d567f3d77:g:1okAAOSwsW9Y03Wt.

 

I was going to cut off the controller and power through a standard DMX controlled LED driver, but the alternating LED colours seems to imply the groups of LEDs are wired in reverse polarity, so I'd need a PWM controller that goes +ve and -ve rather than 0 to +ve (or at least that's how I assume the PWM in normal LED drivers works)

Of course I could just accept that half of the LEDs wouldn't light up, but I'd rather have them all if possible!

 

Thanks!

 

[bigclive]

Sorry, didn't realise it was an old thread that had been resurrected. I replied to one of the original comments and have deleted it now to avoid making things messy. Edited September 14, 2017 by bigclive

[cedd]

Take a look at reply number 13 - that was the solution I eventually went with. As it happened the thing stopped working recently but I think that may have been my fault rather than the handling of 30V.

If you're looking for a decent quality 30V supply it would appear that lots of HP printers have 30V power supplies.

[DrV]

As it happens I've just made up a dimmer to control 12 independently dimmed sets of a.c. fairy lights for an amDram show. I've chopped the sets up into 12 volt sections. You identify sections by finding the place where there are only two wires between adjacent LEDs rather than three. Each section is 3.1V so 4 sections makes a nominal 12V length. I'm then driving them through L298 stepper motor power modules. One output of an Arduino Mega provides a square wave at about 1kHz and this drives the direction inputs of all the modules and the enable inputs come from the rest of the PWM outputs. The dimming is rubbish at the bottom end, as expected, because the PWM outputs are only 8bit, but otherwise it works fine. The 12V supply has been tweaked up very slightly to compensate for the slight forward drop in the modules. You do need to monitor the current when setting it as there are no series resistors. Each L298 has two H-bridges so can drive two sets and they were only about £1.50 online. The PWM frequency for the enable pins is set higher than the default at 3kHz.

Dave

[JGG]

One output of an Arduino Mega provides a square wave at about 1kHz and this drives the direction inputs of all the modules and the enable inputs come from the rest of the PWM outputs. The dimming is rubbish at the bottom end, as expected, because the PWM outputs are only 8bit, but otherwise it works fine.

 

Hi DrV

 

Thanks for your post. Really useful. Am just looking at building this using an ESP8266 and a few L298Ns. I'm hoping to receive ArtNet over wifi and then drive 10bit PWM to get slightly better dimming at the bottom end.

Can you please clarify for us how the 1kHz square wave is wired into the direction inputs of the H Bridge? Did you use a logic inverter for one of the inputs? My understanding is that if both direction inputs are high or both low no current will flow. Thanks.

[DrV]

One output of an Arduino Mega provides a square wave at about 1kHz and this drives the direction inputs of all the modules and the enable inputs come from the rest of the PWM outputs. The dimming is rubbish at the bottom end, as expected, because the PWM outputs are only 8bit, but otherwise it works fine.

 

Hi DrV

 

Thanks for your post. Really useful. Am just looking at building this using an ESP8266 and a few L298Ns. I'm hoping to receive ArtNet over wifi and then drive 10bit PWM to get slightly better dimming at the bottom end.

Can you please clarify for us how the 1kHz square wave is wired into the direction inputs of the H Bridge? Did you use a logic inverter for one of the inputs? My understanding is that if both direction inputs are high or both low no current will flow. Thanks.

Yes, you're right. In fact I used a 75176 to drive the inputs as there were lots (8) of them and this conveniently provides a non inverted output and an inverted output. One other minor point - some of the L298s were driving 24volts making the on-board 5V regulators a bit hot so I put a 13V zener in series with their inputs to take some of the strain. The zener was crimped into a 2pin Berg which replaced the link between the main supply and the regulator input - quite convenient, and easily reversible!

Dave