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| PNP transistor switching voltage |
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| David Hess:
Another way to do it which avoids any possible ground loop issues is to use a Darlington optocoupler driven directly from the microcontroller's output pin. Or add a single transistor amplifier on the input side if you want to direct output current away from the microcontroller if 4 to 5 milliamps is too deemed too high. A 20 milliamp output could require a drive current of less than 4 milliamps making this very feasible. For instance, Vishay single/dual/quad K815P/K825P/K845P Darlington optocouplers would be suitable. |
| danners430:
--- Quote from: David Hess on August 13, 2018, 07:58:12 pm ---Another way to do it which avoids any possible ground loop issues is to use a Darlington optocoupler driven directly from the microcontroller's output pin. Or add a single transistor amplifier on the input side if you want to direct output current away from the microcontroller if 4 to 5 milliamps is too deemed too high. A 20 milliamp output could require a drive current of less than 4 milliamps making this very feasible. For instance, Vishay single/dual/quad K815P/K825P/K845P Darlington optocouplers would be suitable. --- End quote --- Hmm, I'll definitely investigate that - cheers David! :-) Sent from my ONEPLUS A3003 using Tapatalk |
| rx8pilot:
It may help to see a visual representation of the current flowing in a transistor. https://youtu.be/Bine_PbyFSQ PS: The important detail is to see that a BJT transistor is controlled by the flow of electrons (current). A MOSFET transistor is controlled by the buildup of electrons (voltage). |
| bson:
--- Quote from: rx8pilot on August 13, 2018, 08:17:18 pm ---It may help to see a visual representation of the current flowing in a transistor. https://youtu.be/Bine_PbyFSQ PS: The important detail is to see that a BJT transistor is controlled by the flow of electrons (current). A MOSFET transistor is controlled by the buildup of electrons (voltage). --- End quote --- Wow, that channel has some excellent conceptual teaching aids! Often youtubers talk a million miles an hour and don't consider how long it takes a viewer to get something; these animations go on for long enough for a viewer to observe and think. It's obviously not complete as it's not quantitative, but absolutely perfect for a conceptual introduction on which a quantitative understanding can be founded. There are op amps, SMPS, and all sorts of stuff explained there! Beautiful! Kind of like 3Blue1Brown physics: (although with a bit more math needed). |
| Zero999:
--- Quote from: rx8pilot on August 13, 2018, 08:17:18 pm ---It may help to see a visual representation of the current flowing in a transistor. PS: The important detail is to see that a BJT transistor is controlled by the flow of electrons (current). A MOSFET transistor is controlled by the buildup of electrons (voltage). --- End quote --- For the sake of simplicity, in a saturated switch yes, but the reality is much more complex. To illustrate this, try building the circuit I posted, using a 470R resistor for the load. Apply a 20kHz signal to the base of Q1 and look at the voltage across the load with an oscilloscope. Now try removing R2, since it's theoretically not needed and merely interrupting the base current should stop the collector current, but notice how much longer it now takes for Q2 to turn off? The base-emitter junction holds a considerable charge, which is responsible for keeping the transistor on. --- Quote from: danners430 on August 13, 2018, 07:16:23 pm ---Hey guys, sorry to have neglected this - very, very busy afternoon all of a sudden!! I've already had a look in terms of transistor specs, but here are the requirements since you asked so nicely [emoji13] The system is to switch a model railway signal, which can only be done on the high side, as they use a common negative. Each transistor will be switching a maximum of 2 LEDs, usually just 1, from a 12V supply, and switched using a microcontroller. VCC = 12V Vmcu = 5V Il = 20mA Rl = 600r Ib = 5mA Hope this helps :-) --- End quote --- Then in the circuit I posted, R1 can be 3k3. |
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