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joshlisa
11-06-2009, 12:59 AM
Can someone please help me out with the best way to drop 24 volt rectified AC to 10 Volt. I have large 24 Volt power supplys running my show but have a few 10 volt LED strings. All my other LEDs are 24 Volt and I have setup my system for the majority of the Sets.

Could this be as simple as a resistor in line?
If so how would I work out the draw of the 10 volt strings?
Thanks in advanced all you learned people.
Josh

dirknerkle
11-06-2009, 01:09 AM
Rectified AC? Do you mean DC instead?

If it's 24 AC, I'd put in a bridge rectifier to convert the A/C into DC, get a big capacitor to smooth out some of the ripple and feed that into a 12v voltage regulator (many are rated up to 30 vdc input). To that you could add a couple diodes to bring it down to 10v.

If it's 24v DC, you don't need the bridge rectifier but the fastest way to drop it would be with the voltage regulator and diode combination. Some voltage regulators are adjustable (via additional components) and you could dial it right down to 10v automatically and skip the diodes.

The kind of regulator/amperage you'd need depends on how many lights you're going to light and how much 24v current you have to to begin with.

Sky-Lights
11-06-2009, 11:47 AM
Do you mean REGULATED , instead of RECTIFIED ? ( rectified is the conversion of alternating current (AC) to direct current (DC) )

regulated voltage = capable of maintaining the output voltage of a specific unit


read the following wiki .... lots of info there .

http://en.wikipedia.org/wiki/Voltage_regulator

tfischer
11-06-2009, 02:05 PM
Since I'm trying to understand this stuff -- is the 12V regulator + diode solution better than just using a 10V regulator (e.g. 7810)?

dirknerkle
11-06-2009, 02:07 PM
No, the 10v solution would certainly be tidier -- as long as it can accommodate the 24v input! Good idea!!

RPM
11-06-2009, 11:41 PM
Since it's just LED's you don't need a regulated supply... just more unnecessary cost and complexity in my opinion.

I would use a full wave bridge rectifier and a current limiting resistor, and maybe a small filter capacitor if the line frequency flashing bothers you.

Just my .02 worth, YMMV.

joshlisa
11-07-2009, 04:28 AM
Just to clear things up.

I have two large 24volt AC transformers. 500 Watt each.

These are ran to Bridge Rectifiers for each string of lights or LED's.

This is (corrent me please) Rectified AC and not true DC ???

No I dont mind the LED flicker.

I have this present in my other LED's with no magic smoke or children having fits on the front lawn.........

Just looking for the easiest way to do this at minimul cost.

Thanks for the thoughts so far.

If a current limiting resistor is the way to go, how would I work out the correct value????

Josh

budude
11-07-2009, 04:54 AM
I would call it unregulated DC (since it is definitely not AC anymore) if there is no capacitor or voltage regulator in-line but probably more symantics. I would add a capacitor myself just to smooth some of that ripple out...

To calculate the resistor you need to know the current draw represented by "I". You know the voltage drop - which is 24-10 = 14V. So the resistor value would be (using Ohm's Law): R=V/I. Let's say you need 200mA as an example so, R = 14V / .200A = 70 ohms. To determine the size of the resistor, you calculate the power: P = VI or P = 14V * .200 = 2.8W or at least a 3W resistor - more like 5W (or more) to keep it a bit cooler.

So - for you the key thing will be to measure the current required for that string of LEDs at 10V - typically LEDs draw 18-20mA max but can vary a lot depending on the type.

chilloutdocdoc
11-15-2009, 09:18 AM
idea... run the 10V in series, less heat the resistors will be throwing off and you can use smaller resistors...

10V + 10V = 20V

4V is a lot less power on a resistor than 14V....

Just a suggestion, sorry to bring this up from the dead!

D4rknessxkilla
11-15-2009, 01:36 PM
bridge rectifier and a current limiting resistor perhaps?.. :confused:

P. Short
11-15-2009, 04:48 PM
Just to clear things up.

I have two large 24volt AC transformers. 500 Watt each.

These are ran to Bridge Rectifiers for each string of lights or LED's.

This is (corrent me please) Rectified AC and not true DC ???


This is the best description that I can think of. To me these words mean that you do not have an output filter cap (or else you would say 'unregulated DC'), and certainly no regulator.



No I dont mind the LED flicker.

I have this present in my other LED's with no magic smoke or children having fits on the front lawn.........

Just looking for the easiest way to do this at minimul cost.

Thanks for the thoughts so far.

If a current limiting resistor is the way to go, how would I work out the correct value????

Josh

That depends on whether you want to use a computer control the lights, or just leave them on all of the time (at least when the power switch is turned on). If you want to control them with a controller, that would be a more complicated situation, since most of the controllers here are designed to work with either pure AC or with filtered DC.

If you just want to leave them on all of the time, I'd just use a current limit resistor technique described by budude in a previous post on this thread. The only thing that I would do different is start with 33.6V for the calculations (rather than 24) because the peak voltage out of the bridge rectifier will be 24V * sqrt(1), or 33.6V. One other thing that I would do is be prepared for a little experimenting with smaller value resistors in case the 10V lights are too dim. Lastly, if you have so many lights that the current ends up being more than 1A, some other solution may be required because the current limit resistors will be dissipating a lot of power, and thus running quite hot.

P. Short
11-15-2009, 05:00 PM
Also, if the lights are all static, you might do what someone else suggested and place two of the 10V strings in series. This leaves a much smaller voltage drop for the series resistor (if you even bother with it under these circumstances). The benefit of a smaller voltage drop across the resistor is that it will run cooler, and you're wasting a lot less power.