Electronics Questions

[crackerkorean]

I am not sure where this would go, since I am building a triple seoul bike light I thought I would put it here.

I am using a bFlex to drive the LEDs.

I was wondering while I am waiting for my 14.8v Li Io battery to come in if I could use a 15v 1000mA DC walwart to test it out.

On that same note
could I use a 12v 35Ah battery also? I read some where that I could use a lower voltage battery, the light would just pull more than 1 amp to keep the power level up where it should be.

If so what is the equation so that I know how long any given 12v or any battery for that matter would power my light.

 

[georges80]

15V 1A wallwart will do just fine for testing.

12V would be marginal for 3 LEDs at 1A. Their Vf would be close to 11.1V and you would need something like 1.5V above that to have bFlex in full regulation.

To work out runtime, just measure the input current to bFlex. If you know your battery amp.hour, divide it by input current and you'll have your runtime.

Obviously if you dim bFlex, the input current will drop and runtime will increase...

Oh, and bFlex is a buck converter, it will never draw more than the output current from the input. It will go out of regulation before that happens.

cheers,
george.

 

[crackerkorean]

I can do runtime with a battery that is sufficent.

Maybe I misread but reading what I did I was under the impression that if I had (for example) a 12v 35Ah battery.

But needed 15v 1A

Basically what I read stated that running at a lower voltage the current driver would just pull more current since its a lower voltage to make up for the lack of voltage.

So the 1.2A @ 12v (completly made up numbers) to meet the required 15v 1A

Is this the case?

 

[PhxCycler]

There's a discussion going on right now on the mtbr forum that may help you with your question. There are lots of example calculations for buck and boost options. You can find it here:
http://forums.mtbr.com/showthread.php?t=344591

Good luck!
-Mike

 

[georges80]

I can do runtime with a battery that is sufficent.

Maybe I misread but reading what I did I was under the impression that if I had (for example) a 12v 35Ah battery.

But needed 15v 1A

Basically what I read stated that running at a lower voltage the current driver would just pull more current since its a lower voltage to make up for the lack of voltage.

So the 1.2A @ 12v (completly made up numbers) to meet the required 15v 1A

Is this the case?

You misread... Remember bFlex is a BUCK converter - it steps voltage down and then current regulates.

If your starting voltage is too low the most it can do is not step down and feed the input voltage (less a couple of diode drops) to the load. Buck converters do not step UP. Only a BOOST converter (or a buck/boost) can step the voltage up and then it will draw more current from the input to provide current at a higher voltage to the output. Basically:

input_power = output_power + conversion_losses

Think of a buck converter like a step down transformer and a boost converter like a step up transformer (simplistic comparison).

Again, bFlex can draw no more from the battery than what the load is drawing. So, if you have set bFlex to 1A output, the most it can ever draw from the battery is 1A (in reality a bit less, since it will fall out of regulation before the 1:1 ratio occurs). The rest of the time bFlex will always draw less than 1A.

Also, the amp.hour rating of your 12V battery has no effect on what the current draw will be.

cheers,
george.