Host resistance vs driver efficiency.

[kengps]

Can someone educate me (or direct me) on the relationship between host resistance and regulation of the voltage to the LED by the driver? Specifically does voltage drop due to host resistance aid the driver or hinder it? Another way to say it....are you better off heating up the contact springs or having very low resistance and allowing the driver to shed the voltage instead? Which is more efficient? does it matter?

 

[LukeA]

Unless you plan for the resistance and design your own driver, you want the host to have as little resistance as possible. Otherwise, any resistance in the host has negative effects on the driver's efficiency.

 

[kengps]

How do drivers shed voltage? Are they not variable resisters? And if so what would be the difference between a host shedding 50% and the driver 50%, versus a low resistance host shedding 10% and the driver 90% ? Are the drivers regulating by some other more efficient method?

 

[AnAppleSnail]

How do drivers shed voltage? Are they not variable resisters? And if so what would be the difference between a host shedding 50% and the driver 50%, versus a low resistance host shedding 10% and the driver 90% ? Are the drivers regulating by some other more efficient method?

Drivers are not variable resistors, but I don't know exactly how they work. Something about capacitors, inductors, and transistors, it's magic to me. Much more efficient than burning power in resistors though.

 

[kingofwylietx]

I would think the preferred method would be for the driver to shed the voltage. The driver, if designed correctly, will be able to shed excess heat through the head of the flashlight.

If you use the spring to shed the voltage, heat will not be dissipated as well. It can then build up inside the flashlight and create issues.

 

[HarryN]

Hi, there are a couple of ways that an LED driver can work. The most common:

a) No driver also called "direct drive"
- just rely on the battery voltage and system resistance to be about right for the current needed.
- Obviously, this means that things have to be operating in a target range
- You have to be tolerant of imperfect regulation
- Resistance in the light turns into heat

b) Resistored light
- This is similar to the direct drive, but a fixed resistance is placed in the circuit
- If the battery voltage is within 1 - 2 volts of the LED forward voltage, this is a viable approach, although imperfect
- Same limits as the "no driver" approach
- Resistance in the light turns into heat

c) AMC type / Variable Resistor (sometimes called "linear drivers")
- More or less, this is a variable resistor that helps to maintain a constant current by changing the resistance of the circuit
- Each chip can hande a fixed amount of current, I think around 350ma
- Works well again for 1 -2 volt maximum voltage drop from the battery to the LED Vf.
- Commonly 60% efficient, the rest turns into heat.

d) Inductor based designs
- Take in power (battery voltage x battery current) from the batteries, and output this power to the LEDs.
- The voltage is adjusted automatically by the driver to provide the desired current
- If the battery voltage is too low, you use a "boost" driver.
- If the battery voltage is too high, you use a "buck" driver
- There are also combination versions of these that can do both, but they are less common.

Efficiency for well designed models can hit 80-90%. Efficiency for lower quality versions is about 50 - 60%.

Coming back to the original question, if you use an inductor based driver, then you want the minimum resistance possible in your light for maximum efficiency. If you use the other methods, then it does not matter that much.

 

[kengps]

Thats kinda what I thought.....heat is heat whether from the resistance in the driver or the springs. The sums will be equal in the end. I think a lot of people are running Direct Drive set-ups and have no idea how hot (maybe even glowing) their springs are inside. I was surprised that a 3 amp current would heat up the spring in a McKlicky. It got my attention real fast while I was holding it in my hand. So it sounds like a DD set-up would waste a lot of energy in heat as opposed to a high current capable inductor driver with heavy springs and switch.

 

[LukeA]

Another connon tactic to reduce a spring's resistance is to take a stranded copper wire, such as a piece of desoldering braid, and attach it to each end of the spring, either by just by compression or by soldering it in.

 

[kengps]

That sounds good. I am planning on building something this summer with my new H6CC 6.7A driver. I have some heavy motor brushes laying around with braided copper wires. I wonder if a graphite brush properly shaped and attached would be a good conducter?