Help with voltage conversion
jwbernin
Posts: 1
I'm trying to develop a lighting module for a home garden idea, and I need some advice on power conversion. Each module is a set of 5 parallel LED arrays that drops 48-50V. I'm using a constant-current circuit around each LED array to try to avoid massive burnout. Ultimately, each array draws between 0.1 and 0.2 A. I'm starting with 120VAC mains, I need to convert that to 48 VDC or 50 VDC. I can handle the rectification to 120 VDC, it's the voltage conversion I'm having problems with. My research has suggested I need either a zener diode with a power resistor or a switching regulator; I'm tempted to go with the zener/resistor pair, but I'd like some input from the community on which is better. I'm guessing it would be a 48V zener, can anyone give me an example part number from parallax.com or mouser.com that I could use to do some more research?
Comments
But there is a far more profound reason to use a lower voltage DC, and that is safety. With 5 LEDs in series, not parallel, you really need only about 12 VDC and that would be much safer to deal with in an outdoor setting. Also, a simple step-down transformer and then rectification would provide you with isolation from the AC mains to prevent a dangerous shock hazard.
From there, some would argue that the DC doesn't even require ripple removal as it is all for lighting and the ripple will not be noticible.
I do admit that I am thinking rather conventional LEDs and not bright arrays of some sort, but the design principles are still the same if you were to select something like a 24VAC transformer output due to different LED requirements.
The main ideas are Isolation and lowest required voltage. Even two 24VAC transformer can be tied together to output an effective 48VAC which can then be converted to DC and smoothed. This is all very old school, but rather easy to do.
Peter
Furthermore, the isolation transformer is still an important safety consideration in outdoor wiring as shocks to ground are quite easy with moisture and such.
Finally, regulated voltage is not as necessary as smoothed voltage. Raw DC is going to have peak voltages of 1.414 times the rated transformer output. Smoothing with a capacitor filter will bring that down to near 1.0 times and remove the ripple that you see in the lights.
I am sure you can think of lots of reasons and ways to use sophisticated solid state supplies, but do you really have to? And might not one just buy a 48 volt 3amp switching supply if one really wants tight voltage control? A packaged switcher would have isolation build in to it - nice and safe.
4 diodes
for a PSU
MPJA has a few nice ones so does jameco ..
Powering this or any outdoor circuit directly from the AC line can be hazardous. A transformer to reduce the voltage to the lowest practical voltage for the circuit and to isolate the leds from the power line is a good idea.
Using a zener and resistor for this is not very practical due to the power dissipation involved. Also your post leaves some question as to how the leds are actually connected. Can you post a diagram of how they will be connected and a part number for the leds.
Yes, full wave rectification as a minimum. A bridge rectifier. Then the flicker is well above what might bug me.
And yes, use a transformer to get down to safe voltages before the diode bridge. Out doors in the rain I don't want to see 220 or 110 volts.
Don't do this for safety sake.
Duane J