Mr_Light
Enlightened
Sorry for this fairly basic question, but do resistors used for dimming extend run time or just dissipate current as heat?
Do resistors used for dimming extend run time?
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Fallingwater
Flashlight Enthusiast
They do both. How much they extend runtime depends on how much current they are dissipating; the more heat they make, the more power they waste.
mudman cj
Flashlight Enthusiast
A resistor impedes the flow of current. When the current flowing through the circuit is reduced, this also reduces the current flowing from the battery. This is why run time is increased. Some energy is wasted as heat by the resistor, but depending upon the current level and the resistor needed for the application, a simple direct drive circuit as they are called with just an LED and a resistor can rival the efficiency of many of the less expensive circuit boards available.
csshih
Flashlight Enthusiast
you mean, more expensive, cj?
Since it's a simple series circuit, the power ratio is just the voltage ratio, so efficiency is Vf/Vbat. That pot is probably not helpful, though; 1M might give you a couple microamps of current at low, so it'll be very sensitive at one end, and practically useless at the other...
However close Vbat is to Vf at maximum current determines the peak efficiency. If things work out for a direct-drive, that'll be 100% efficient, but if you have a little resistance (either added, or simply non-negligible body and switch resistance), it'll be slightly less, same as any DD light. As you ramp the resistance up, driver efficiency (power to LED / power from battery) will drop to ~70% or so, but this will be partially compensated by the increased LED efficiency (light output / power to LED) at low current, so total efficiency (light output / power from battery) might drop only to 80 or 85% of its original value.
If it's not a DD on high, just multiply the fixed voltage ratio for high by the variable ratio (which can be drawn from the LED datasheet). Suppose Vbat is twice Vf
eek; you'd get 50% (high) to 35% (low) driver efficiency.By way of qualitative comparison, a current-controlled dimming driver might maintain reasonably flat driver efficiency across the current range, and thus have an actual gain in total efficiency at low levels. A PWMing dimmer can have very good driver efficiency (although it needs to be cascaded with a fixed current driver for regulation, incurring additional losses), and will be quite flat over the range. But since a PWMing driver at low output is still driving the LED with full current, it doesn't pick up the low-output LED efficiency boost, so the overall efficiency will also be flat.
mudman cj
Flashlight Enthusiast
I meant to say that typical efficiencies for less expensive (cheap) driver circuits are only around 50-70% compared to 80-90% for good ones. In typical flashlight applications where a resistor is used to create a low mode, efficiencies can be in the 60-70% range in my experience. But, as Benson has shown, the actual efficiency depends upon the particular load, drive level, and battery voltage.
I think direct drive circuits have sometimes received an unduly poor reputation. They are far simpler and therefore less prone to failure; and despite the attraction of flat output with regulation, there is something to be said for reduced output with extended run time nearing the end of battery life which serves to warn the user that they need to charge or change the battery soon.
Calina
Enlightened
A 1M Ohm potentiometer might be too high but you could still use it if you connect it in parallel with a smaller resistor.
OK, in my experiments, I am finding that I can hook up higher voltage battery source to the LED through a potentiometer and not burn out the LED.
For example, I have a 12v battery hooked up to a Cree LED through a 1M potentiometer. The LED lights up dimly.
I have also hooked up a 3v battery to a Luxeon 1 LED through a 10K potentiometer and can see it glow (barely) at night with lights off and curtains shut. Pretty creepy, looks more like glow in the dark spec of paint rather than a light source.
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abvidledUK
Flashlight Enthusiast
You should really also have a low value resistor in series with the pot so you can't overload the leds if you turn the pot down to 0Ω
abvidledUK
Flashlight Enthusiast
In my DIY 9v PP3 torches, I use a 100kΩ to 470kΩ resistor in series to provide a night glow, and they run for over a year like this, so they reduce current consumption, some of which may be given off as heat, but at << 0.03ma this would be negligible.
The reduction in applied voltage (& current) is not measurably wasted.
You can't give off "current consumption" as heat; the relevant quantity is power. The loss from the resistor at 0.03 mA (x ~6V = ~0.2 mW) is orders of magnitude smaller than at full drive, but it's hardly negligible, because the power levels through the entire system are scaled similarly, and most of the battery's power is being given off as heat. In fact the efficiency is worst for very low outputs.
I'm not sure what that statement can possibly mean. There's no reduction in current (it's a series circuit), and it's trivial to measure the reduction in voltage; the voltage drop (multiplied by the current through the whole circuit) is wasted. Since this is eminently measurable, how is it "not measurably wasted"?
Your resistor drive is running about 30% efficiency. That's not necessarily a problem; if it's meeting your output and runtime requirements, there's no need to add significant cost and complexity with a buck converter. But while the losses may certainly be acceptable, I can't understand why you claim they are negligible or unmeasurable.
I actually tried this, very quickly, less than a second. My room lit up and I could see smoke coming from somewhere, not necessarily the LED. I think the potentiometer or the wires or the soldering or the alligator clips I am using could be smoking as any one of these may not be rated to handle 12v at max (2A?) current.
The LED is still alive.
Mr Happy
Flashlight Enthusiast
Eek!
A typical potentiometer like the probable 1M carbon device you are using is not designed to handle any significant current at all. They are designed for varying voltages (hence the name: potentiometer = voltage adjuster).
One of the first experiments I tried as a youngster was to try using a pot as a variable resistor to adjust the brightness of a bulb. The sizzling and crackling sound followed by smoke and a damaged pot soon taught me not to do that.
If you want to learn how to make it work right, you might investigate a slightly more complex circuit using a power transistor to handle the current and make use of the pot to control how much current the transistor passes.
Eek!
A typical potentiometer like the probable 1M carbon device you are using is not designed to handle any significant current at all.
OK, you confirmed what I think I saw but could not believe. It was the pot that was smoking!
abvidledUK
Flashlight Enthusiast
Isn't smoking pot illegal !
mudman cj
Flashlight Enthusiast
But it was the pot that was smoking, not Itiu. "Honestly officer, it wasn't me, it was the pot." :laughing:
Depends where you are. I hear that your next door neighbors, the Dutch, are legally allowed.
StandardBattery
Flashlight Enthusiast
DC Power is I^2R, so in the example if the current is .03mA with a 100K resistance were talking about something like .09mW of wasted power (4.7x more if 470K is required for the same current).
I'm not sure howmany regulation circuits with a 9V input would waste less power. Now if your application needs regulation then you have no choice. I like electronics, but the right tool for the right job. And obviously in the 9V PP light, a resistor is the right choice.
If the current is going to be very low then the resister is not a bad choice. On a high-powered light, people usually want several levels and typically want the high level for a reasonable amount of time, even if not the entire time. So in higher powered lights, a resistor is often not the right choice. It's really a design choice.
I'm not sure howmany regulation circuits with a 9V input would waste less power. Now if your application needs regulation then you have no choice. I like electronics, but the right tool for the right job. And obviously in the 9V PP light, a resistor is the right choice.
If the current is going to be very low then the resister is not a bad choice. On a high-powered light, people usually want several levels and typically want the high level for a reasonable amount of time, even if not the entire time. So in higher powered lights, a resistor is often not the right choice. It's really a design choice.
Mr Happy
Flashlight Enthusiast
There is a different way of looking at this, however. If a resistor is placed in series with an LED on a 9 V battery, then 2-3 V is dropped over the LED and the remaining 6 V over the resistor. This means that two thirds or more of the battery power is being lost to the resistor. If this power wasn't lost, it would mean the light could run for three years instead of one year, or it could mean the light being three times as bright for a one year run time.
On the other hand, if the performance of the resistor is acceptable it definitely has the advantage of cheapness and simplicity.
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