MAX810 as a crude one-shot timer? - Page 1
Forum section: electronics / projects

author: HwAoRrDk
author_rank: Super Contributor
timestamp: June 12, 2025, 07:52:27 am
content: I have what's possibly a silly idea, but I think it might work.I have a need to drive a solenoid, which actuates a latch mechanism. It only needs to be driven with a brief pulse of power (few hundred ms) to release the latch. The solenoid is controlled by a microcontroller, which will be responsible for driving an N-ch MOSFET with the requisite length of pulse to turn on the solenoid. However, I want to protect against the solenoid being stuck 'on', because it overheats if left on for too long - it's not rated for continuous duty cycle. I could imagine this may happen for several reasons: MCU crash/hang, MOSFET fails short-circuit, etc. So, ideally I would like to try and enforce in hardware some kind of on-time limit for the solenoid.I had the idea to use a MAX810 reset supervisor IC to enforce a time-limited pulse. The '810 has an active-high reset output and typical reset delay of 240 ms, which combined will form the time-limited activation pulse for the MOSFET. Then the VCC pin of the '810 is connected to a GPIO pin of the MCU to control it.When the MCU wants to activate the solenoid:1. MCU outputs a logic-high pulse (ideally longer than the '810's delay period - say 500 ms) on the GPIO.2. As the GPIO rises, it will exceed the '810's min. VCC voltage (1V) but be below the reset voltage threshold, which will cause it to assert the reset pin (logic high), which will turn on the MOSFET.3. GPIO rises further, above the reset voltage threshold, causing the '810's timeout period to begin.4. When the timeout period is over, it will de-assert its reset pin (logic low), turning the MOSFET off.The datasheet of the '810 also obviously says that reset will be asserted whenever VCC falls below the threshold, but in that case it'll be because the GPIO has gone low (i.e. ground) and there won't be any VCC with which it can assert the reset output! Or even if it does manage to get off a brief transient (maybe a few microseconds?), I don't think it should affect anything due to the pull-in time of the solenoid.I think this should protect against the following situations:- MCU's GPIO stuck high: won't matter because the '810 will always de-assert its reset output after the timeout period and turn off the MOSFET.- MCU's GPIO stuck low: the '810 is un-powered, and cannot assert its reset pin, so the MOSFET will be off (due to pull-down on the gate).- MCU's GPIO floating: a pull-down resistor will keep the '810's VCC at ground, un-powered, and MOSFET off.There are a couple of concerns that I have thought of:- Datasheet says max. VCC rate of rise is 100V/us. If I have the math right, a 3.3V GPIO rise time of anything less than 33 ns exceeds this. What would be the deleterious effects of a faster rise rate?- Datasheet says max. output current on the reset pin is 20mA, but that can only be sourced from VCC, which is in turn sourced from the MCU GPIO, so I need to still obey the max. source current rating of the GPIO pin.- The reset timeout period can be as low as 140 ms across the entire operating temp. range (-40°C to +85°C), which may be too short for the solenoid. But they don't give any kind of chart in the datasheet to illustrate the relationship between temp. and period. I assume it's lower temp. = shorter period, but who knows?But otherwise, will this work? Is there anything I've missed?

author: jpanhalt
author_rank: Super Contributor
timestamp: June 12, 2025, 08:48:01 am
content: Quote from: HwAoRrDk on June 12, 2025, 07:52:27 amThat does not address a mosfet shorted on.If the MCU stalls, does it have a watchdog timer? Why have two?How will you know whether the latch actually opens/closes? What effect would failure of that solenoid have?(Presumably, the mosfet will be happy with less than 20 mA and 6 volts.)

author: HwAoRrDk
author_rank: Super Contributor
timestamp: June 12, 2025, 09:02:29 am
content: Quote from: jpanhalt on June 12, 2025, 08:48:01 amTrue, but I'm not too focused on that. I don't know how one could mitigate that failure mode anyway without resorting to something like... I dunno, dual back-to-back MOSFETs?Quote from: jpanhalt on June 12, 2025, 08:48:01 amNot sure. I think it'll be running some kind of RTOS. I'm not involved with that side.Quote from: jpanhalt on June 12, 2025, 08:48:01 amThe latch mechanism has a separate microswitch that is closed (to ground) when it is latched, and open once unlatched. So we can tell if the solenoid has failed to actuate the latch.Failure of the solenoid would be highly inconvenient and undesirable, but not dangerous. Unless it fails due to overheating and catches fire.Not that I think it would... but then again I haven't experimented to see if it does indeed reach winding-melting temperatures.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 13, 2025, 07:30:04 am
content: Quote from: HwAoRrDk on June 12, 2025, 07:52:27 amEasy to try.You can also get simple timeout using a capacitor to the gate. With short turn on times, the MCU is in control, and the FET switches fast.If the MCU ever locks HI, the cap discharges and turns off the fet.Fall time is a little slower, but this feature is rarely used.You can check the turn off power on Spice.