Badboy vs Downboy

[Billson]

I'd like to know what differences they have when used in 2x123 lights. Both seem to be able to handle the voltage so how do you decide which one to use?

Thanks.

 

[kakster]

It depends on what Luxeon you plan to use. 1 watt Luxeons and the Lux3 require a voltage of 3.5-4 volts, so you would need a Downboy converter to 'step-down' the voltage of 2x123, which is around 6 volts. The Luxeon V 5 watter needs around 7 volts+, depending on the bin, so you need a Badboy converter in this case to boost the voltage from 6v.

 

[mut]

I believe it is the Vf of the emitter/star.
I'm sure more experienced will elaborate on this, but I think this is the main consideration.

mut

 

[balrog]

mut is right. Here's some elaboration...

The general rule is that if the forward voltage (Vf) across the Luxeon is greater than the convertor input voltage (ie, the batteries) then use a Badboy (termed a boost convertor). If Vf across the Luxeon is less than the convertor input voltage then use a Downboy (termed a buck convertor).

The typical Vf across a Luxeon 1W or III is 3.7V at 700mA or 3.9V at 1000mA. The typical Vf across a Luxeon V is around 6.84V at 700mA. You should now be able to work out whether to use a Badboy or a Downboy. There are some other factors to consider when the input voltage is equal to or just above Vf, but for CR123s, you don't need to worry about that.

BTW, you might want to consider the Wizard which is a buck/boost convertor so it gives you versatility in the battery combinations that you can use at the expense of efficiency and lower runtimes.

I just remembered this thread: which convertor diagram. /ubbthreads/images/graemlins/ohgeez.gif

 

[Billson]

There's something I'm missing here. The description on the shoppe indicates only input voltage range but does not specify output voltage. How do the converters determine what the output voltage should be considering leds have different vf.

 

[dat2zip]

Billson,

For the Badboy, the converter will try to generate a higher voltage than the input and it will do so until it gets to the current as defined by the sense resistors installed. When the converter senses the current to the load has been achieved it throttles back on the "gas" pedal so to say. Otherwise, it has a lead foot and stomps on the gas.

The downboy does exactly the same thing, with the exception it is trying to put out less voltage on the output. The downboy steps or converts the input voltage to a lower output voltage. This is called a step down (buck) regulator. It too will try the put out power until the output current is achieved. Since the Downboy is wired differently, it does not have run away condition like the Badboy does.

The reason both could be confusing is they both will work on the 1W or the 5W and like others have explained above the output LED sets the output voltage (Vf of the LED). The Vf output voltage is the mid point. If you battery voltage is above Vf, you need to step down (Downboy) and if your battery voltage is below Vf then you will need to step up.

For the 1W/3W the midpoint is appoximately 3V-4V depending on the Vf bin code of the LED.

For the 5W, the midpoint is twice that or around 6.2V-7.8V.

So, a 2X123 voltage is 6V which is higher than the 1W/3W Luxeon yet lower than the 5W Luxeon.

The two key factors are Battery voltage (fresh and near dead) and the Vf of the LED.

With these two you should now be able to determine if you need to step up the battery voltage to the output Vf voltage or step down the battery voltage to the Vf of the LED.

 

[Billson]

Wayne,

I think I understand now.

Thanks.