RustyShackleford1
Electrical
- Feb 7, 2015
- 5
Hi there, new member ...
I want to build a low voltage system consisting of a 120vac-connected custom power supply that drives multiple series-connected strings of removable LEDs.
A 32vdc power supply would drive multiple current regulators, each consisting of an LM317 and a current-setting resistor, and driving a series-connected string of up to 8 removable LEDs. The LEDs would have a forward voltage drop of about 3v and a nominal forward current of 350ma or so.
Allowing for the 3v drop of the LM317, and some wiring drop etc, the 32vdc supply should allow 8 or 9 LEDs in each string (assuming the variation in LED forward voltages averages out). If a string only has 1 or 2 LEDs, there will be a pretty significant drop across the LM317, so they should have good heat dissipation, perhaps TO-220 packages screwed to a metal chassis.
By putting a potentiometer in series with a fixed resistor on each LM317, dimming should be achievable. The fixed resistor would be sized so that when the pot is at 0 ohms, the LEDs in the string are at maximum desired brightness (current 2-3x nominal, or up to 1 amp). With the pot rotated the other way, the string is effectively "off".
Since the LEDs in each string are removable, they can potentially be connected with reverse biasing (correct polarization of the LEDs cannot be guaranteed). So each LED will be packaged with an "anti-parallel" (anode of each connected to cathode of the other) ordinary silicon diode (such as 1N4007). When the LED/diode pair is installed one way, the LED is forward-biased and the 1N4007 reverse-biased (the diode can handle the possible 32v reverse voltage indefinitely). If installed the other way, the forward-voltage spec of the 1N4007 insures that the reverse voltage on the LED cannot exceed 1.5v or so (and the Cree LEDs I'll likely use tend to allow 5v).
The same problem as old-fashioned Christmas lights exists: if one burns out, the whole series string is killed. But LEDs have lifetimes in the 10,000s or hours, so the MTBF of a string of 8 should still be long. The culprit can quickly be identified by reversing each connection until the string lights up (because the 1N4007 paired with the dead LED will then be forward-biased).
LEDs in each string will be guaranteed to have the same current and similar brightness. Extra-bright assemblies can have 2 or more LEDs in series (protected by a "backwards" silicon diode).
I'd appreciate any comments on the above, thanks !
I want to build a low voltage system consisting of a 120vac-connected custom power supply that drives multiple series-connected strings of removable LEDs.
A 32vdc power supply would drive multiple current regulators, each consisting of an LM317 and a current-setting resistor, and driving a series-connected string of up to 8 removable LEDs. The LEDs would have a forward voltage drop of about 3v and a nominal forward current of 350ma or so.
Allowing for the 3v drop of the LM317, and some wiring drop etc, the 32vdc supply should allow 8 or 9 LEDs in each string (assuming the variation in LED forward voltages averages out). If a string only has 1 or 2 LEDs, there will be a pretty significant drop across the LM317, so they should have good heat dissipation, perhaps TO-220 packages screwed to a metal chassis.
By putting a potentiometer in series with a fixed resistor on each LM317, dimming should be achievable. The fixed resistor would be sized so that when the pot is at 0 ohms, the LEDs in the string are at maximum desired brightness (current 2-3x nominal, or up to 1 amp). With the pot rotated the other way, the string is effectively "off".
Since the LEDs in each string are removable, they can potentially be connected with reverse biasing (correct polarization of the LEDs cannot be guaranteed). So each LED will be packaged with an "anti-parallel" (anode of each connected to cathode of the other) ordinary silicon diode (such as 1N4007). When the LED/diode pair is installed one way, the LED is forward-biased and the 1N4007 reverse-biased (the diode can handle the possible 32v reverse voltage indefinitely). If installed the other way, the forward-voltage spec of the 1N4007 insures that the reverse voltage on the LED cannot exceed 1.5v or so (and the Cree LEDs I'll likely use tend to allow 5v).
The same problem as old-fashioned Christmas lights exists: if one burns out, the whole series string is killed. But LEDs have lifetimes in the 10,000s or hours, so the MTBF of a string of 8 should still be long. The culprit can quickly be identified by reversing each connection until the string lights up (because the 1N4007 paired with the dead LED will then be forward-biased).
LEDs in each string will be guaranteed to have the same current and similar brightness. Extra-bright assemblies can have 2 or more LEDs in series (protected by a "backwards" silicon diode).
I'd appreciate any comments on the above, thanks !
