Connecting the input of a 5V logic gate to 12V through a 100k resistor?  - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: June 14, 2025, 02:35:54 pm
content: Suppose we have a single SOT-23-5 inverter gate running from 5V.And we connect its input to 12V though a 100k resistor.Of all of the logic gate families out there with input ESD diodes, will any of them be damaged by this?It doesn't need to operate very fast, and being exposed to 12V shouldn't happen except during faults.

author: langwadt
author_rank: Super Contributor
timestamp: June 14, 2025, 02:44:28 pm
content: the datasheet will usually have a spec on how much current you can safely inject into the inputs

author: TimFox
author_rank: Super Contributor
timestamp: June 14, 2025, 02:45:47 pm
content: I never advise operating a component above the manufacturer’s max rating.Why not add a 68k resistor to ground at the node where the 100k resistor connects to the gate?$0.03 shot to hell.

author: brucehoult
author_rank: Super Contributor
timestamp: June 14, 2025, 02:48:08 pm
content: A simple resistor will limit the current, but not the voltage.You'd normally want to use two resistors as a voltage divider, with values significantly smaller than the input impedance of the logic gate so that most of the current goes through the resister to GND. Something like ... idk ... 10k from the 12V source to the logic gate input, and 3k3 or 4k7 from the logic gate input to GND. Or, yeah, multiply those by 10.

author: langwadt
author_rank: Super Contributor
timestamp: June 14, 2025, 02:52:39 pm
content: Quote from: TimFox on June 14, 2025, 02:45:47 pmbut injecting current is often within the manufacturer rating, e.g. a HC14 datasheet TI "The input and output voltage ratings may be exceeded if the input and output current ratings are observed." and that current spec'ed as +/-20mA per pin, +/-50mA total

author: MariuszD
author_rank: Regular Contributor
timestamp: June 14, 2025, 03:18:22 pm
content: Most of logic gate families have diodes on all inputs and outputs. But not all of them. Example in the attachment. This input can be damaged even with a 100kOhm resistor connected to 12V.

author: TimFox
author_rank: Super Contributor
timestamp: June 14, 2025, 03:37:42 pm
content: Quote from: langwadt on June 14, 2025, 02:52:39 pmIn that particular case, his circuit does not exceed the max rating. $0.03 saved is $0.03 earned.

author: ajb
author_rank: Super Contributor
timestamp: June 14, 2025, 06:50:23 pm
content: If the amount of injected current is a concern, and you can't use a higher value resistor or voltage divide for some reason, you can always fit external diodes, and optionally an additional resistor between the clamping diodes and the logic input. The second resistor will limit the current through the internal diodes even if the external diodes clamp at a slightly higher Vf, but only needs to handle a fraction of a volt rather than the full ~7V, so it can be a lower value.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 14, 2025, 08:01:04 pm
content: Quote from: incf on June 14, 2025, 02:35:54 pmThe gates with clamp diodes to Vcc will be ok, current wise, but ensure Vcc can always sink the 100uA without increasing.The threshold will also be quite low with a single resistor, and also watch for higher icc on cmos gates operating with Vi not close to the rails.A resistor equipped transistor can also do this task.

author: Nominal Animal
author_rank: Super Contributor
timestamp: June 15, 2025, 02:59:01 am
content: I like SN74LVC1T45, dual supply single voltage level translator between 1.2V (NXP) / 1.65V (TI) and 5.5V with direction control in a 6-pin package, but current-limiting resistor on input only protects againstnegativeinputs; inputs must never exceed +6V.74AHC1G17, a single Schmitt CMOS trigger, only draws about max. 2µA on input, and can handle under/overvoltage if the input current is limited to a few milliamps maximum –– so only needs the current-limiting resistor on input. It's a cheap jelly-bean part, and the 5-pin SOT23-5/SC-74A/SOT753/TSOP-5 (NXP 74ACH1G17GV for example) has 0.95mm pitch and is okay to hand-solder, despite being just 3mm by 2mm or so in size. It can draw a few milliamps from the supply when the input is around half the supply voltage, but being a Schmitt trigger, the output is always either low or high (seedatasheet).In my hobbyist experience, it is a good idea for any hobbyist to have a few of these at hand, so that one does not need to resort into ugly bodges even when testing things. Especially so if interfacing stuff to microcontrollers, SBCs, or other complex logic devices with "sensitive" inputs. SOT23-5/SOT23-6 is my favourite, because even though it is small, I can solder it by hand, even directly to wires if I cover the legs and the package in heatshrink afterwards.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 15, 2025, 04:48:15 am
content: Microchip has long had an application note for a light dimmer for one of it's PIC micro's It uses a few series resistors between the mains voltage and an input pin. (The power supply of the uC is referenced to the other mains voltage wire). A few years later, Atmel released a similar application note. Both relied on the ESD protection diodes to limit the DC current to a safe value. This spec is sometimes hard to find. I think it was 4mA for the Microchip part, and 2mA for the Atmel part. In your circuit it's: >>> (12-5)/1e5 = 7e-05 = 70uA which almost certainly is safe, but you still have to check the datasheet.Other options are:* A voltage divider such as brucehoult mentioned.* Schottky diode to the power supply (Assuming the supply current of the whole circuit is always more then the shunted current).* A zener (TVS) diode to GND.Also, for example 3V3 logic (microcontrollers) that is "5V tolerant" usually don't have the ESD diode to the power supply voltage, but they may have some other method of ESD protection. and you'd have to check whether it can tolerate an injection current.And as the AN's above suggest, the input voltage is irrelevant, as long as the current is limited. If the input is 1kV and the series resistor is 1M, then it very closely resembles a 1mA current source, provided of course that the resistor can handle the 1000V safely.