Simple inverter circuit acting weird. - Page 1
Forum section: electronics / beginners

author: PhilippeASP
author_rank: Contributor
timestamp: August 17, 2017, 07:47:46 pm
content: I have a strange problem involving an inverter. I want it to do what (I assumed) inverters are meant to do.When my system powers on, I want a green light to turn on, but when a laser is activated, I want the green one to turn off and a red one to turn on instead. So what I did is I put the system power into the Vcc of my inverter, and the laser’s power voltage goes into the input. When the system turns on, I assumed that the inverter’s output would emit 5v. And only when the laser emits it’s 5v would we see the output turn off (and the red one turns on).Instead I’m seeing something weird. The input spits out it’s own 1.6 volts, even when nothing is plugged into it. While the output stays around 0.04 volts. I tried to use a 10K pulldown resistor to force the input to zero volts, but it stays at 1.6v. If I pull the input to ground with a bare wire, the output will go as high as 3.3 (which is something, but still weird considering I’m powering the chip with 5.3v). But if I leave this super-pulldown resistor the chip starts getting very very hot, like I’ve created a short somewhere.This is the internal diagram of the inverter chip I’m using.I don’t see why shorting the input straight to ground would cause it to heat up. At worst, he’s got that 1.4K resistor that’s limiting the current. I'm proving the power from the digital output of my arduino (I plan on using transistors to power the lights), and I remember reading that arduino is good in supplying up to 20 mA from it's ports. Could it be that the chip is trying to pull more power than the arduino can supply? But if that was the case I imagine it would be the arduino that would heat up, not the inverter.Anyone who might have a clue as to what's happening and be willing to explain it to me? I'd very much appreciate it.

author: Zero999
author_rank: Super Contributor
timestamp: August 17, 2017, 07:59:44 pm
content: Quote from: PhilippeASP on August 17, 2017, 07:47:46 pmYes, that's to be expected. TTL needs a fairly large current to make the output go low.The input stage really looks something like this. The triangle with a circle on the end of it is just an abstraction.https://en.wikiversity.org/wiki/SPICE/NOT_TTLI trust you have a pull-up resistor on the output. The 7406 has an open collector output stage. Q3 and the associated components from the above schematic are missing from the 7406.

author: jmelson
author_rank: Super Contributor
timestamp: August 17, 2017, 08:03:15 pm
content: Quote from: PhilippeASP on August 17, 2017, 07:47:46 pmYes, this is classic behavior for a TTL gate. It defaults to a weak high input condition, and requires a pull-down to ground to indicate a zero. This will all be in the data sheet.QuoteThe chip you show is an open-collector output, so it will not pull high at all, only toward ground. You need a lot more current through the pull-down at the input to assure a zero input state. 1 K Ohm would be marginal, maybe 250 Ohms would be good to guarantee a zero input.You need to read the data sheet for the chip to understand it.Jon

author: PhilippeASP
author_rank: Contributor
timestamp: August 17, 2017, 08:57:08 pm
content: @Hero999The pull-up resistor on the out definitely helped. I'm able to get 0,09v to 5.23v at the output now.But I'm beginning to suspect my whole thought process for this may be flawed. I should probably just implement the inverter logic using the arduin's own digital ins and outs. All the same, thanks for the prompt answers and the help.

author: Zero999
author_rank: Super Contributor
timestamp: August 17, 2017, 09:34:07 pm
content: Quote from: PhilippeASP on August 17, 2017, 08:57:08 pmYes, you should implement the inverter logic using software. The only reason for doing this would be to provide a higher output voltage, than what the MCU can provide. The SN7406 is capable of holding of up to 30V on its output stage, so could be used to interface with automotive and industrial logic, which work at higher voltages than the usual 5V.http://www.ti.com/lit/ds/symlink/sn7416.pdf