LED Formard current
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Posted by GrahamIT on August 8, 2004, 8:20 pm
Please log in for more thread options forward LED current as stated in the data sheet is 50mA, but all the current transfer tests are done at 5mA except the Vce(sat) which is done at 20mA, confusing or what!!! The question is, which LED forward current and voltage would be the best choice, so that the output photo transistor was switched hard on to full saturation but the LED had as long a life as possible? I've been under the impression that a transistors base current necessary to only "just" saturate the transistor should be increased by a factor of 5 to fully saturate. Therefore as 5mA through the LED would only just saturate the phototransistor, an LED current of 25mA would fully saturate it, but their Vce(sat) current was only 20mA If you can make sence of the question, then you're a better man than me :-P | ||||||||||||||||||||||||||||
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Posted by Michael A. Covington on August 8, 2004, 5:13 pm
Please log in for more thread options > transfer tests are done at 5mA except the Vce(sat) which is done at 20mA,
to
> confusing or what!!! > > The question is, which LED forward current and voltage would be the best > choice, so that the output photo transistor was switched hard on to full > saturation but the LED had as long a life as possible? > > I've been under the impression that a transistors base current necessary > only "just" saturate the transistor should be increased by a factor of 5
to
> fully saturate. Therefore as 5mA through the LED would only just saturate
> the phototransistor, an LED current of 25mA would fully saturate it, but > their Vce(sat) current was only 20mA LED current is not proportional to the base current of the phototransistor. 5 mA is probably plenty. If the Vce(sat) is specified at 20 mA LED current, then use 20 mA. | ||||||||||||||||||||||||||||
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Posted by John Popelish on August 8, 2004, 5:36 pm
Please log in for more thread options GrahamIT wrote:
>
> Think of an optocoupler, e.g. ISP847 from http://www.isocom.com the MAX > forward LED current as stated in the data sheet is 50mA, but all the current > transfer tests are done at 5mA except the Vce(sat) which is done at 20mA, > confusing or what!!! > > The question is, which LED forward current and voltage would be the best > choice, so that the output photo transistor was switched hard on to full > saturation but the LED had as long a life as possible? These two requirements need opposite extremes. You need to define how much collector current the output must pass, and what voltage drop is acceptable. Then using the data sheet, see if you can extrapolate what LED current that will require and if it is low enough for acceptable LED stability. This last part is hardest to guess from most data sheets. But it is safe to assume that operating at the absolute maximum LED current is not a good solution and that staying at or below the current where most of the specifications are done will give a reasonable life. > I've been under the impression that a transistors base current necessary to
> only "just" saturate the transistor should be increased by a factor of 5 to > fully saturate. The terms 'just saturate' and 'fully saturate' are not very well defined. What voltage drop can you tolerate? > Therefore as 5mA through the LED would only just saturate
> the phototransistor, an LED current of 25mA would fully saturate it, but > their Vce(sat) current was only 20mA The forward current transfer ration for the ISP847 is more than 50% at 5 mA LED current and 5 volts across the transistor (not saturated, at all). This is 2.5 mA of collector current. The ISP847D guarantees 300% (15 mA collector current) under the same input and collector voltage conditions. At saturation (defined as no more than .2 volts collector to emitter, and a 1 mA collector current, 20 mA LED current may be needed. This represents a forward current transfer ratio of only 5%, a 10 times reduction of the linear case. But it is one point only. If you could tolerate .3 volts drop, a much smaller drive may do. You can get some idea how the drive varies with saturation voltage from the graphical data. Based on what I can only assume is typical characteristics (since it is so much better than the one data point guaranteed), the guaranteed value is very conservative compared to the graph of saturation voltage versus input current and for several collector currents. > If you can make sence of the question, then you're a better man than me :-P
You have to start with what you need. How much collector current must the device pass, and what voltage drop is acceptable? Then you can start the difficult process of extrapolating what LED current may be needed based on the fragmentary info on the data sheet. -- John Popelish | ||||||||||||||||||||||||||||
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Posted by GrahamIT on August 9, 2004, 12:11 am
Please log in for more thread options Sorry for my ignorance, when I said "just starting to saturate" I meant
"just starting to conduct", high voltage drop across transistor low collector current etc. What I actually need is for the output of the optocoupler to be TTL compatible, because it is to feed other TTL logic circuits in a CNC application I'm working on. This part is for the parallel port breakout board so I don't fry my laptop if I get a short from the 36V 9A motor supply. The circuit so far is described as this. Parallel port output > Tri-State buffer / line driver > Opto-Coupler > Drive
Logic with separate and independent Vcc and GND either side of the
Optocoupler. I could use another buffer on this side of the opto I suppose. The tri-state buffer is needed for power up and down stability, so the CNC machine can't move during a reboot, the software I'm using outputs a 12.5kHz signal on a parallel port output pin when ready to send data to the CNC machine and I'm feeding that into a 1mS timed charge pump to the Tri-state enable lines, also because my laptop has 3.3V and 5V output voltages from the parallel port, so I thought I'd standardise at 5V. so the breakout board could be used on desktop PC's as well, without overdriving the opto LED's. 25 years ago I was an Army Radio Telecommunications Technician, it's surprising how much you forget if you don't use it. | ||||||||||||||||||||||||||||
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Posted by John Popelish on August 8, 2004, 8:38 pm
Please log in for more thread options GrahamIT wrote:
>
> Sorry for my ignorance, when I said "just starting to saturate" I meant > "just starting to conduct", high voltage drop across transistor low > collector current etc. > > What I actually need is for the output of the optocoupler to be TTL > compatible, because it is to feed other TTL logic circuits in a CNC > application I'm working on. This part is for the parallel port breakout > board so I don't fry my laptop if I get a short from the 36V 9A motor > supply. Most TTL inputs see anything above about 2 volts as a logic high. See the data sheet for a typical LSTTL gate: http://www.physics.wisc.edu/graduates/courses/623-f03/ds/74LS00.pdf So you can have a tiny bit of leakage and still have a logic high out of the transistor. With a 4.7k pull up resistor (i mA to pull down to ..3 volts) you can have about a half mA passing through it and still have greater than 2 volts out of the coupler. Another good way to make a fast logic output that is leakage and noise immune is to use two couplers, with their outputs stacked to form a push pull totem pole. Drive their inputs alternately. Most TTL inputs will treat an open circuit input (neither coupler on) as a logic high. You can parallel the pull up coupler with a 10k resistor for extra insurance (adds an extra half mA load to the pull down coupler). > The circuit so far is described as this.
> > Parallel port output > Tri-State buffer / line driver > Opto-Coupler > Drive > Logic with separate and independent Vcc and GND either side of the > Optocoupler. I could use another buffer on this side of the opto I suppose. > > The tri-state buffer is needed for power up and down stability, so the CNC > machine can't move during a reboot, the software I'm using outputs a 12.5kHz > signal on a parallel port output pin when ready to send data to the CNC > machine and I'm feeding that into a 1mS timed charge pump to the Tri-state > enable lines, also because my laptop has 3.3V and 5V output voltages from > the parallel port, so I thought I'd standardise at 5V. so the breakout board > could be used on desktop PC's as well, without overdriving the opto LED's. > > 25 years ago I was an Army Radio Telecommunications Technician, it's > surprising how much you forget if you don't use it. -- John Popelish | ||||||||||||||||||||||||||||
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> forward LED current as stated in the data sheet is 50mA, but all the