Sorry, I wasn't providing it as advice. Even if powering for short duration low frequency a software mistake can destroy the device. I was just noting that the resistor isn't required for the MOSFET example logic level mosfet switch to work switching an LED. And there are situations where a resitor would not be an option like my example.
Besides, if the OP is using high power LEDS, a simple switch is not all they need as you mentioned. They need a constant current supply for each LED.
I was just noting that the resistor isn't required for the MOSFET example logic level mosfet switch to work switching an LED.
Not sure I understand that either. While one doesn't need a gate resistor using a mosfet (although many recommend a small one) there is still a need for a series resistor in the led/drain circuit to limit led current when the mosfet is full on.
retrolefty:
I was just noting that the resistor isn't required for the MOSFET example logic level mosfet switch to work switching an LED.
Not sure I understand that either. While one doesn't need a gate resistor using a mosfet (although many recommend a small one) there is still a need for a series resistor in the led/drain circuit to limit led current when the mosfet is full on.
A current limiting resistor is required when the LED is always on, or on with a high duty cycle. If the LED is on, say 50%, of the time you can get away with a current a little less than twice the maximum rated on the data sheet. If, as in my example, the LED is on for 10uS out of every 15-20 seconds, then no no current limiting resistor is needed, and the LED can receive more than 5+ amps and survive.
In the eighties, Forest Mims, had an article about using an original 20mA IR LED with low duty cycle pulses of 1+ amps to significantly increase the transmission distance.
In the eighties, Forest Mims, had an article about using an original 20mA IR LED with low duty cycle pulses of 1+ amps to significantly increase the transmission distance.
It works cause it doesn't have enough time to build up any heat. However I would assume that would dramatically shorten its expected life span.
Possibly, but as I mentioned I ran a 300ma constant current Luxeon LED with well over 5 amp pulses through 10,000 cycles without a failure. And Mims device worked for at least several years at his last report with 1+ amp pulses on a standard 20ma LED.
It is up to the user/designer to decide what tradeoffs they are willing to make.
The only real downside is a destroyed low value component in this case. With certain situations, and certain devices it is worthwhile to always stick to those rules of thumb that hobbyists pick up; however, an LED switched by a MOSFET is not likely to fall into that case where there is any real danger beyond destroying a couple of LEDs.
Fet's are good for certain jobs, but I myself would use a power transistor for this job, fets for more special applications, tip31 for example..
Thanks for all the info and side notes.
I'm powering LED spotlight bulbs - so there is a little transformer / buck inside the bulb. No need to limit the current myself.
After comparing several datasheets, I've decided to go with IRL2203N for the FET.
It's cheap enough, has low voltage at the gate 1v - 2v, low resistance and 116A rating.
It's actually the only one I saw at a low price that will use less power than a SSR. - SSR's look pretty efficient, and seem to outperform the other fets at 2A +
Thanks guys
wanderson:
......... I ran a 300ma constant current Luxeon LED with well over 5 amp pulses through 10,000 cycles without a failure. ......
This is a very simple minded approach to circuit design. The fact that you did not destroy your LED does not mean that you did not severely damage it. The damage will have occurred and will show itself in two ways:-
A shortening of the life of the component, in other words it will fail sooner than it otherwise would. This is due to the mechanical stress on the chip's bonding wires causing eventual fracturing and depletion of charged carriers in the semiconductor substrate. There is also a problem of localized heating when the thermal conduction will not allow enough heat to escape to prevent over temperature of the junction.
A reduction in the maximum brightness of the LED. I know you will say it did not reduce in brightness but if your only measure is your perception of how bright it was then that is hardly very objective. It is well known that people can't remember the brightness of a light over a few seconds let alone several months. To properly test this you would have to make carefully calibrated measurements with specialist equipment.
No one is saying you have to have a resistor but you have to have some form of current limiting even on a very narrow pulse. It is better if that limiting is under control rather than the vagaries of a particular component.
Grumpy:
It wasn't intended to be a commercial (or even a long term) circuit. I was determining if I could get enough light to replace spark gap bulbs for photographing short term events. Using edgerton style air gap sparks on pyrex tubes I can get a few dozen shots from each 'bulb'. If I could get two or three times that from LEDS it would be worth it. That I got 10,000 'exposures' would have been fantastic if I could have gotten enough light for my purposes. Unfortunately measurements indicated I would need more LED's than would have made it cost effective.
That the LED's were being damaged is a certainty, but you seem to be missing my points.
Grumpy_Mike:
- A shortening of the life of the component, in other words it will fail sooner than it otherwise would. This is due to the mechanical stress on the chip's bonding wires causing eventual fracturing and depletion of charged carriers in the semiconductor substrate. There is also a problem of localized heating when the thermal conduction will not allow enough heat to escape to prevent over temperature of the junction.
Yep, this is one of the expected outcomes. As I mentioned above even a lifespan of 30-40 flashes would have been fine with me for my purposes.
Grumpy_Mike:
2) A reduction in the maximum brightness of the LED. I know you will say it did not reduce in brightness but if your only measure is your perception of how bright it was then that is hardly very objective. It is well known that people can't remember the brightness of a light over a few seconds let alone several months. To properly test this you would have to make carefully calibrated measurements with specialist equipment.No one is saying you have to have a resistor but you have to have some form of current limiting even on a very narrow pulse. It is better if that limiting is under control rather than the vagaries of a particular component.
I didn't mention it, but in my experiment I was measuring the light output of the pulses, and yes there was about a 20% reduction after 10,000 flashes. As I mentioned, if the lights were bright enough for a small enough number of them then I would have been ecstatic with that outcome. I fully expect that as the bulbs get brighter and even cheaper I will eventually replace my existing air spark gap on quartz glass tubes (home made). Indeed as they become bright enough, I will certainly be able to use a better design that can limit current and lengthen the lifespan. But as I said, the current state of the devices isn't there yet.
For a bright flash, might it be worth charging a capacitor for the flash?
Yes, I was using a high capacitance low esr cap to supply the voltage/current to the LED through a MOSFET. For the short duration pulses I was using, none of my available power supplies would have been up to the task.
Apparently the LED's may be a bit closer than they were when I did my experiments several years ago.