cheating the reponse time in transistors  - Page 1
Forum section: electronics / projects

author: unknown_h
author_rank: Contributor
timestamp: December 13, 2023, 02:01:13 pm
content: Hi all,I am currently working a on control that outputs a somewhat tightly timed square wave output to a somewhat dangerous device. I have already implemented an optoisolator with a momentary switch to control the power on the high side. I am looking to use a transistor in between the IC responsible for the square wave and the optoisolator as a second protection that will be controlled by a delay circuit triggered by a momentary button. I have made sure to select an optoisolator with a tight rise and fall and I want to ensure I avoid to much more interference with square wave. Is it possible to use a pnp transistor and instead of having the button and delay circuit provide the power have the square wave provide the power through the emitter and constantly excite the base using using the button and delay circuit. My thinking is that button and delay circuit will allow the transistor to pass the square wave freely instead of the transistor needing to switch on and off if it was controlled by the square wave.Thanks for all the help in advanced

author: moffy
author_rank: Super Contributor
timestamp: December 13, 2023, 02:22:33 pm
content: Can you draw what you mean, even just an approximation? A schematic is worth a thousand words.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 13, 2023, 04:32:07 pm
content: If you need a tightly timed transition, then an opto may be a poor choice. They are slow.Reading between the lines in a darkened room I think you want to find a way to drive an opto with an "or" function.I've no idea of where your button is. I cant see your diagram that shows how the opto is driven by the pulse generator so who knows where to stick your pnp?Make sure "dangerous devices" fail safe, ie if your control circuit stuffs up, will the danger cease. Is your button a kind of dead mans handle?No picture no love.......

author: MrAl
author_rank: Super Contributor
timestamp: December 13, 2023, 04:56:49 pm
content: Quote from: unknown_h on December 13, 2023, 02:01:13 pmHello,I have to second what the two replies before this one had indicated:1. Draw at least a rough drawing of what you are doing.2. Opto couplers are not very good at accurate timing.The internal LED will age with time making it less and less efficient which will add to the time, so you have to keep the forward LED current low, which could be a problem in itself.For really fast response maybe a pulse transformer would be better. You could look some up on the web and decide if one would suit your needs. If you can't find one and you have some small ferrite toroid's around, you may be able to make your own. I did that once for isolation for a triac speed control, although that was simpler because it only had to put out a short pulse.

author: unknown_h
author_rank: Contributor
timestamp: December 14, 2023, 03:10:03 am
content: when in doubt attach a circuit, also the signal is a pulse and not a square wave not sure why I wrote that. The comment on opto-isolator aging is interesting and it's something I am thinking about a better solution to.

author: IanB
author_rank: Super Contributor
timestamp: December 14, 2023, 03:21:36 am
content: Are you trying to say that if the transistor is turned on then the pulse will pass through the transistor, otherwise it will be blocked?If we say the transistor is already on before the pulse arrives, then the shaping of the pulse by the transistor will depend, I think, on the size and internal capacitance of the transistor? A small, high frequency transistor would presumably work better?

author: moffy
author_rank: Super Contributor
timestamp: December 14, 2023, 04:49:50 am
content: If you debounce the switch with a capacitor and feed it into a schmitt trigger e.g. 74LVC14 or 74HC14, you can use that to drive the PNP (through low value resistor divider) the trick though is to not saturate the PNP, and for that you have to match the input level and current into the emitter, base and drop across the opto coupler, not the easiest. There are transistors designed for switching e.g. 2N2222a but they have storage times of around 200ns, just how critical is your timing acuuracy requirement?

author: unknown_h
author_rank: Contributor
timestamp: December 14, 2023, 01:25:31 pm
content: Quote from: IanB on December 14, 2023, 03:21:36 amthat's exactly what I am trying to propose

author: PSR B1257
author_rank: Frequent Contributor
timestamp: December 14, 2023, 01:54:22 pm
content: Why is it so difficult to name concrete numbers, when asking others for help?Wesomewhatcan't read your mind.

author: S57UUU
author_rank: Regular Contributor
timestamp: December 14, 2023, 04:59:38 pm
content: In your circuit:If the base is at 5V, it depends on the supply voltage of your emitter current source whether the pulse will pass. If the current to emitter comes from 10V, the pulse will pass to the LED. If your emitter current source is limited to 5V or less, the pulse won't pass. Hint: will current flow from E to B?If your base is at 0V, the pulse would always pass, but there will not be enough voltage on the LED to turn it on.Why not put a NPN "under" the resistor? Then button from 5V through 1k to base?Any modern small transistor will be faster than the optocoupler.

author: unknown_h
author_rank: Contributor
timestamp: December 15, 2023, 12:16:35 pm
content: Sadly the external device requires a 24V dc pulse so the optocoupler servers the dual purpose of allowing the ic to control the higher voltage along side the isolation.Thanks for all the help it has helped me find some interesting alternative options