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| 74LS14 testing circuit pls |
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| StillTrying:
--- Quote from: rstofer on April 19, 2018, 01:52:35 pm ---The transistor in the opto device has to sink the collector resistor current PLUS the IIL of the 74LS14 and this total current should be about 0.5 mA. The LS14 has a max IIL of 0.4 mA so that only leaves 0.1 mA for the resistor current. --- End quote --- After much DS reading, I think the 0.5mA is just a 'test condition', it's not saying that it comes out of saturation if it's a bit higher. With 20mA through the TX, I make the RX sinking up to about 4mA before it starts to come out of saturation. http://pdf.datasheetcatalog.com/datasheets/toshiba/TLP832(F).pdf |
| rstofer:
That's correct, the 0.5 mA number is used for the rise/fall time test. The datasheet also gets involved with the current transfer ration which relates the LED current to the transistor collector current. I didn't see where they gave any advice on VCESat at any particular LED or collector current. It is true that the resistor could be much smaller but it doesn't need to be. The LS14 input high current is quite low and the required sink current is well within the test circuit range. I would try it and see. One disadvantage of high value resistors is possible noise. In this schematic, we see 22k resistors used plus a 0.1 ufd capacitor between the collector and ground: http://www.junun.org/MarkIII/datasheets/Controller_SCH.pdf These optos are not slot sensors and the data may not apply directly but it does show one way of dealing with any noise that may occur on the collector. |
| Chriss:
Ok, this is going a bit over my head, this calculations... :D Long time ago when I was in school and do the math's, but I should practice again... So, it the Input LOW Current or the Input HIGH Current what is in the DS of the LS14 actually the current what is needed to feed into the input of the gate so the gate can hold the output LOW or HIGH? Maybe tonight if I don't drop to sleep will try this circuit with a 52K resistor between the VCC and collector. |
| Chriss:
I tested already the circuit like rstofer mentioned in post no #64. It is working great. Thank's all of you with such of help and info sharing. |
| rstofer:
It's good that it works but you should really measure some voltages to prove that it works. The LS14 input is a good candidate. What is the voltage when the input is '0' and again when it is '1'. How does that line up with the threshold voltages in the datasheet? You could try to reduce the LED current with increasingly large resistors until the logic '0' voltage starts to rise. Just to see what the Current Transfer Ration (CTR) actually is. This is not really important and may be difficult to test. |
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