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This is so embarassed!

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Hi fellow simmers, This is embarassing here, I have forgotten how to use OHMS law. I have been into electronics for 6 years, and still can't figure out what kind of value of resistor that need to be used. I guess I have been into digital, and forgotten everything about OHMS, Faraday, and other laws except for digital's Maybe I could use your help to find the equation, or calculating the resistor value that need to be used. I am working on 7 segment digit display for MCP and radio. And I need to know the value of resistor along with power supply that is 13.8 VDC, 12 surge amp, 10 constant amp, and 250 watts. The 7 segment digital display can only allow 2.8vdc and how can I find the value of resistor for 13.8vdc power supply? On other issue, for those who have output module and digit display module, would watts be matter for the circuit? because on the other instruction, they said that 3 watts of power supply would be recommend. If so, is there any way I could reduce the watts from the power supply? let me know I'd be appreciated.

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Guest MikePowell

Arthur,When driving LEDs, the current is generally the important variable to control. For many panel LEDs, a constant current of about 10 to 20 milliamps works well. If you have a supply voltage of 13 volts, you select a resistor to limit the current to, in this case let's say, 20 milliamps. A LED drops about 2.1 volts at this current so the voltage across the resistor will be 13 - 2.1 = 10.9 volts. Ohm's law is used to find the resistance by dividing the voltage by the desired current: 10.9 volts / 20 milliamps = 545 Ohms. In this case I would use a close standard value: 560 Ohms.Okay, that's the simple example. Your question implies a slightly more complex situation.Seven segment displays are a collection of LEDs, seven for the segments plus zero, one or two for decimal points depending upon model. Each LED needs a resistor. Further complexity comes from the fact that seven segment displays are often driven from a multiplexed bus and do not see a constant current. Each display is pulsed on, in sequence, for just a short length of time. This allows many displays to share the wiring and drivers. In this case the current is higher than the simple example above. Just how much higher depends upon the duty cycle of the pulsed current driving the LED.You don't need to worry about the wattage, given that you have a more than adequately sized one. You just want to make sure the voltage is not too high for the output board to manage.Do you have any other information on the LEDs and the multiplex duty cycle percentage?Mikewww.mikesflightdeck.comInfo for simpit builders

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Matt O., Thank you for the website... It refreshed my memory of OHMS law into my brain. Mike, Thank you for these wonderful details on the performace of these LED display. I need one of those teacher like you. Anyway, that is what I thought, I was right at first. But I was not sure since my mind is a bit of rusty with ohms law. I wanted to have second confirmation from someone who knows the OHMs law. The 7 Seg LED display, I order from Jameco and manufactrued by LITEON didn't really have exactly information I was looking for. I was looking at the technical description and I don't see it labeled about multiplex duty cycle percentage, The only thing I see is the power infos on duty cycle. With the information a LED has 2.8vdc, 20mA, 80mA Duty cycle maximum rating, 60mW power dissipation per segment. I have the pdf website address that has all technical specfications in case you wanted to take look at it. click on this website: http://www.jameco.com/Jameco/Products/ProdDS/151976.pdf Last night, I did a few experiment with one of old LED display from radio shack. to emphasize it, that LED display is COMMON CATHODE not Common anode and can only hold up to 2.0 VDC, what I did was attach the display on protoboard, and attach 180 ohms resistor to the anode 'F' pin. I had 5vdc power supply attached to the end of the resistor (I wanted compare and determine which power supply I should use between 5vdc and 12vdc for my panels ) And of course grounded it toward to common cathode's pin" The result are sucessful. I could see that red led lits up without any problem. The next thing I did was connected up with 12vdc, adding the 1K ohms resistor.. the results is same, not a problem too. So I switch the old LED display, into Jameco's LED display. Again to emphasize, that Jameco Display is COMMON ANODE not Common cathode, those display(s) would be one that get to display on my panels. Anyway, I attached the exactly same way I did with radio's shack common cathode display using 180 ohms resistor for 5vdc and 1K ohms resistor for 12vdc. And of course grounded toward to the common anode pin's. Unfortunately, It did not lit up any yellow leds. I can't figure it out why it wouldn't. What may be wrong? I figured out, maybe its because Common Anode pin needs to be inverted into common cathode. I was told that the grounds can only work with cathode not andoe. Correct me if I am mistaken. Let me know soon. Again, Thank you for your reply

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Guest MikePowell

Arthur,I should ask what you are planning on using to drive your seven segment displays. It is possible that you do not need any resistors at all. For example the EPIC digit driver module appears to use an ICM7218C 8 digit driver chip. These chips have internal resistors that limit the peak current per segment to about 30 milliamps. You can find a datasheet for this chip on the Maxim/Dallas Semiconductor website www.maxim-ic.com .The specsheet on your LED displays indicate that they have a maximum per segment current rating of 80 milliamps. As the 7218C driver limits at 30 milliamps, your displays look like a good match to that driver.Of course, if you are not interfacing to that sort of driver then some resistors may in fact be necessary. Mikewww.mikesflightdeck.comInfo for simpit builders

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Mike, Yes, I have EPIC seven segment digit module, and are you sure I don't need to add any resistors for 7 segment digit displays?? I read Ray Sotkiewicz's Bluesidesup website other day, it includes information about connecting up the 7 seg displays to the modules and indicated that would I need to include 8 resistors for each andoes pins which would prevent over current through the led displays. Eventhough if the EPIC digit display moludes appears to have ICM7218C, would that limit the voltages? This time, I might as well need resistor to test those leds, to see if that can lit up let me know... are you familar with those common anode? speaking of, wonder what may be wrong not having those led display to lit up? (from previous message i indicated)Thanks

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Guest MikePowell

Arthur,I have taken a look at the Bluesideup site and Ray is quite clear that resistors are needed. On the other hand, the product datasheet for the Maxim version of the ICM7218C clearly says they are not necessary, and I see nothing in the EPIC instructions about using resistors when wiring displays to the board. Nonetheless, I am extremely reluctant to second guess Ray. He has much more experience with EPIC than I. Quite possibly, the 7218 is used by EPIC in a non-convensional manner. Not having the schematics in front of me, I cannot say.The 7218 is a CMOS chip with a typical operating voltage of 5 volts. The picture of the card on the front of the documentation shows what appear to be two voltage regulators. I would assume(dangerous word) they are LM7805's or equivalent. Likely one for each pair of 7218C's. If we assume (darn! there I go again) that each LED segment drops a typical 2.1 volts, there will be 2.9 volts across the resistor, again assuming(!) that the 7218 can supply the full 5 volts to the display. If we want to limit the peak current to the segment to no more than 30 milliamps, the resistor should be 2.9 volts / .030 amps or about 100 ohms.Perhaps Ray will see this thread and offer some advice.Returning to the question as to why the common anode display fails to light in your test protoboard...Sounds like the power is inverted. Plus goes to anode, negative to cathode.Mikewww.mikesflightdeck.com

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Mike, You are making sense.... I believe you are right. Those chip 'ICM7218' has resistors in it which would that makes unnecessary to add fixed resistor toward to the 7 segments digit displays. If the chip has a reistors I wouldn't know why Ray would recommend to add fixed resistors. What I can say is I will give a call to Flightlink.. the EPIC technical supporter, or send e-mail to Ralph Robinson and ask them to see if I need to add any fixed resistor to the display modules since that chip has a resistor. Then I will let you know the results. About LED display that wouldnt lit up. I had already solved it out myself, before I get to read your last statement. I figured out by using diode theorem from the technical specification of those displays. As for common cathode, The postive polarity has to be connected to the common cathode. And for the rest of pins, it has to be connected to negative toward to resistors if possible. For common anode display, it's opposite.Again Thanks

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Mike, Resistors is required! Its only if you re wiring directly to module, you would need to add 1K ohms resistors on each pins. If it's for the EPIC's 1+4 display board, you wouldn't need to add any resistors. I spoke with Tech supporter today about those ICM7218C chips has resistor including in it, he said that is used for the resistance on the chip itself, since 12vdc and 3 amps would need to plug in through circuit. If there isn't any resistor in that chip it self, it would burn out with overcurrent of 3amps and would have damaged the board. So, it's not the resistance for the displays. So at last, I have found the results... hope you have something to add, if not, again thank you for all of the replies.Happy Flying

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Guest MikePowell

Arthur,Sounds as though you have found the proper answer. Thank you for keeping me in the loop.Mikewww.mikesflightdeck.comInfo for simpit builders

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