Hi,
I need to design a NPN output circuit (as the output from traditional proximity switch) with a TTL input. The current required is 150 ma.
Does anyone can help me on that in terms of components (may be optocoupler?)
and schematics?
Many thanks Eugenio
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So, what you're saying is that you need a bipolar transistor output that operates at bipolar transistor logic levels?
Do you mean open collector output?
Why do you feel you need an optocoupler? Do you have a need to isolate two circuit stages, or does the word sound useful?
If it weren't for the optocoupler, I'd suggest you start by looking for a driver chip that'll do what you want all in one package.
But you haven't given enough information. Do you mean a real TTL input that doesn't load its driver any more than a single TTL gate? Or an input that's compatible with TTL output, but may need more current? Or something yet again? What speeds are you going to be operating this at? Temperature? Etc.
A 2N4401 with an appropriate base resistor will turn on adequately at
2mA base drive, and really hard at 10mA. So a circuit like this one would satisfy a set of requirements that includes what you've specified, but may still fail miserably at your task:.-----o | | 470R | Vin ___ |/ o------|___|----o----| e.g. 2N4401 | |>
| | .-. | | | | 4K7| | | '-' | | | | | === === GND GND (created by AACircuit v1.28.6 beta 04/19/05
Stupident. Go away.
It sounds like you are after an open collector output. An NPN Darlington would be my preferred approach - the Darlington has two benefits over a regular NPN in that the higher gain means you need less current, and its switching voltage is naturally in the middle of the TTL undefined range. A current limiting resistor is still advisable but its value is not critical.
A BC517 comes to mind with a gain of at least 30,000, meaning even fairly miniscule currents will saturate the Darlington for 150mA. Connect the emitter to ground and the load to the positive supply and the collector. Connect the TTL output to the base via a series limiting resistor. A 22K resistor will limit the current to around
100uA but anything between perhaps 1K and 430K should be fine, however you are pushing things at the extremes of that range.
A Darlington-connected pair, however, only pulls the output voltage down to one volt or so, not the 0.2 volts that a single NPN can do (or that TTL needs). A solution that does this, if you've got the room, is an emitter follower with collector to a power rail, followed by a pass transistor.
Since the OP has not bothered to respond to requests for more information, it's impossible to know what he really needs.
: L
its
ing
SNIP:
Huh! I had an impression for 'open collector' with the symbol of NPN.
I think 2Nx will do the job as pointed by you.
//ali
Take a seat on the bench over by the door. Your homework tutor will be with you shortly.
And go over the the shop class. I am sure someone over there would be happy to get the grade you're not willing to get your self.
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