I'm planning to use the circuit for driving three Luxeon V's in series at 700mA
since each has a voltage drop of around 6.8V, three will need almost 21V to run. and I will use only a supply of 10-12V.
the circuit is a basic boost converter (constant voltage) using TL494 (since these are readily available) and I simply added a transistor Q4 that turns on when the voltage across a 1 ohm resistor R10 reaches 0.7V (0.7V/1R = 0.7A) this the increases the input voltage at pin 16 of TL494 reducing the PWM which reduces the output voltage thereby limiting the current across the LED.
and if the output is open circuited, the ratio of R6 and R7 will keep the voltage constant so the circuit runs on constant voltage with load currents of less than 0.7A.
atleast that's what I'm hoping.
the values for CT and RT and the inductor is not listed since I haven't decided on any yet. any recommendations?
the diode shown is 1n5400 but I will be using a schottky on that. probably 1N5822.
helpful comments before I start wasting parts on this??😀
since each has a voltage drop of around 6.8V, three will need almost 21V to run. and I will use only a supply of 10-12V.
the circuit is a basic boost converter (constant voltage) using TL494 (since these are readily available) and I simply added a transistor Q4 that turns on when the voltage across a 1 ohm resistor R10 reaches 0.7V (0.7V/1R = 0.7A) this the increases the input voltage at pin 16 of TL494 reducing the PWM which reduces the output voltage thereby limiting the current across the LED.
and if the output is open circuited, the ratio of R6 and R7 will keep the voltage constant so the circuit runs on constant voltage with load currents of less than 0.7A.
atleast that's what I'm hoping.
the values for CT and RT and the inductor is not listed since I haven't decided on any yet. any recommendations?
the diode shown is 1n5400 but I will be using a schottky on that. probably 1N5822.
helpful comments before I start wasting parts on this??😀
Attachments
Is it required to connect the LEDs to the GND of the circuit? Of not, then you can place the current sense resistor between the GND and the LEDs, and you don't have to use that transistor stuff.
no, it's not required.
well, I've thought of that before and it used both comparators of the TL494. one limits output current and another limits output voltage.
the resulting circuit is a little bit more complex such that I would need two feedback paths going back to the TL494 so I thought of this one where there is only one ending up with a little bit smaller PCB layout.....
any recommendations on inductor value and oscillator freq?
well, I've thought of that before and it used both comparators of the TL494. one limits output current and another limits output voltage.
the resulting circuit is a little bit more complex such that I would need two feedback paths going back to the TL494 so I thought of this one where there is only one ending up with a little bit smaller PCB layout.....
any recommendations on inductor value and oscillator freq?
I decided to build this thing last night but with some component changes and it works wonderfully right after installing the components.
the current limit works but not with very good regulation but acceptable.
I also added two resistors and a cap that will make the circuit shut down when the batt voltage drops to below 9V.
it is quite capable of powering three luxeon V's (max) but I'm still waiting on my LED's to test if it can power only two.
it can also work as a constant voltage output powersupply with current limit if yu do not draw enough current to trigger the sensing transistor.
if anyone is interested in the new schem, just post here.......
the current limit works but not with very good regulation but acceptable.
I also added two resistors and a cap that will make the circuit shut down when the batt voltage drops to below 9V.
it is quite capable of powering three luxeon V's (max) but I'm still waiting on my LED's to test if it can power only two.
it can also work as a constant voltage output powersupply with current limit if yu do not draw enough current to trigger the sensing transistor.
if anyone is interested in the new schem, just post here.......
- Status
- Not open for further replies.