Using LEDs as "clamps" and warning indicators - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: June 15, 2025, 11:49:11 am
content: Consider the scenario.You have a 5V IC on a prototype board and you accidentally connect a 0-6V clock signal to it. There is real chance the IC takes power from the clock pin. This might even end up on it's VCC pin as a pulse 5V-6V square wave.Having some form of "clamp" diode which diverts anything greater than "supply side" 5V to ground... via an LED.The LED can be labeled "STOP" or "DUMB!".Is this possible? Would it be required on all ICs and all consumers that could back feed the circuit?The first hurdle I see if the forward voltage of the LED. A mosfet ... not going to get a low enough differential either, not soon enough. A BJT however ... using it where it acts like a diode when A>B but as soon as B tries to flow current towards A the transistor will "switch" and allow whatever current I need to flow through the LED to warn me.

author: MariuszD
author_rank: Regular Contributor
timestamp: June 15, 2025, 02:15:42 pm
content: It can be done. If you need to protect circuit from this, you'll probably also make other mistakes that aren't covered by any safeguards. You need to learn how not to make mistakes.Schematic below is Liion cell balancer but it does what you want. It can be trimmed to 5V. It is accurate to several millivolts (after trimming) and can sink several amperes.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 15, 2025, 10:36:16 pm
content: Most LED's can only handle a small current, and that is a severe limitation in this sort of circuit. But you surely can add a LED as an indicator, and even make it part of the active circuit. 30-odd years ago I designed a stepper motor controller which used linear current sources and it used LED's as reference voltage generators to set the current sources. (Current source = LED + BJT + Resistor). It worked, and you could see nice patterns on the leds, but overall efficiency was extremely bad. It needed big heat sinks and a sizalble (and loud) fan.

author: Benta
author_rank: Super Contributor
timestamp: June 15, 2025, 11:19:56 pm
content: Wouldn't it be simpler to double-check your setup first instead of concocting additional (potentially disturbing) circuits?That would be my approach, but then I'm a KISS guy.

author: Buriedcode
author_rank: Super Contributor
timestamp: June 16, 2025, 03:36:00 am
content: Its why I use low value series resistors on prototypes - given the massive input impedance of logic, and with speeds under ~20MHz, a 100R works just fine. But ultimately - why add a lot of extra circuitry tomultipleIO just to prevent a mistake that should rarely happen. Just double check schematics and pinouts before wiring something up.I'll admit though, breadboarding, stuffing wires, does come with hazards and this is one of them. I still occassionally use solderless breadboards, but its becoming rarer because they have a number of pitfalls, and if you're working with a tried/proven circuit, then theres just no need to breadboard it first. I usually go for soldered prototypes (with stripboard, tripad, or other prototyping board) straight off so I don'thave to worry about dodgy jumper wires, accidental shorts etc..