What would be your preference for protecting an intermittent-duty solenoid from burnout?

Thread Starter

Benjamin3000

Joined Mar 28, 2021
8
Obviously everyone would say "don't leave it on for too long" but suppose the machine in question doesn't necessarily have somebody watching it at all times that's aware of the "intermittent duty" nature of the solenoid.

The CPU could impose a time limit, be connected to a temperature sensor, or "pick and hold" via PWM. However, this all would depend on the software working properly, and even a CPU running rock-solid software could still freeze up if the power supply gets noisy or unstable. If it froze while the solenoid was energized, then you'd better hope that someone sees it that knows to shut down the machine when a solenoid is stuck, otherwise you can say goodbye to the solenoid.

I have a couple ideas for how one might protect against solenoid burnout due to a CPU malfunction. A watchdog timer that, if it expires, drops out power to all the solenoids and motors, ideally via a relay or MOSFET that's independent of what the CPU uses to control them normally. That way, this will work even if the MOSFET you're controlling directly from the CPU fails shorted for some weird reason. And, if you design it properly so that watchdog doesn't also reset the CPU, then the CPU could intentionally let that watchdog expire as a backup way to stop everything if the CPU detects, for example, current flowing on the power bus when there shouldn't be any (i.e. current flow that doesn't "add up" to how many devices are active, current flowing when all devices are off, etc.)

Another way, at least for intermittent-duty-only solenoids, would be to have a circuit with a couple 555 timers, where the first one times full power to the solenoid, and then when the first one expires, the second one takes over, doing PWM at slightly above the lowest duty cycle necessary to keep the arm extended/retracted if that's necessary in the application. If it's not, then you would just have one timer that does a "pick and drop" function instead, not letting the solenoid be on for more than a second at a time no matter what the CPU says.

Finally, in keeping with the "CPU independent protection" theme, if you had a temperature sensor on each solenoid going back to a simple comparator circuit, the comparator could prohibit that solenoid from operating if the temperature gets too high, along with sending an alarm signal back to the CPU. This would probably be the ideal solution, because it could catch unforeseen issues such as a malfunctioning sensor causing the solenoid to "machine gun" or an odd software glitch that did not cause a total freeze-up of the CPU, therefore not triggering a watchdog timer.

Note that this isn't a safety-critical system I'm dealing with, only looking out for expensive "domino" failures, particularly with software freezes which may cost literally nothing to fix if it's not a recurring problem: just cycle power, end of story. Even if the CPU's power supply has an actual failure, solenoids are relatively expensive compared to power supply capacitors or a voltage regulator.
 

shortbus

Joined Sep 30, 2009
8,803
Can you give a reference to this type of solenoid that is only for intermittent duty? It's a new one on me. You may be worrying about a problem that doesn't exist, as far as reliability of the solenoid goes. Now that doesn't mean that a program or machine can't use a solenoid that is only on for a certain amount of time, they can and do that. But that isn't the same thing you seem to be talking about.
 

crutschow

Joined Mar 14, 2008
27,710
How many solenoids do you need to protect?

A PWM circuit would only be required if the solenoid needs to be pulled in longer than its duty-cycle rating.

If the solenoids operate at different times (as I suspect they do), then a 555 timer on each solenoid may be a good approach.
The 555, configured as a non-retriggerable one-shot, would be triggered by the computer signal, with its output ANDed with the computer signal going to the solenoid driver.
That way the the solenoid signal can be shorter than the one-shot time (as determined by the computer) but not longer.
 

Papabravo

Joined Feb 24, 2006
16,776
I'm just curious to know if solenoid "burnout" is an actual problem that you have observed on solenoids operated within their specified limits.
 

Ian0

Joined Aug 7, 2020
3,220
Use and edge-triggered monostable (such as a 74HC123). Trigger the monostable with the MCU, the output drives the solenoid. No matter what happens to the MCU, the solenoid only operates for a fixed amount of time.
As the 74HC123 is retriggerable, if it is required to operate longer than the RC time constant, this can be achieved by triggering it several times from the MCU, but each requires an low-to-high transition from the MCU, which will not occur if the MCU crashes.
 

MaxHeadRoom

Joined Jul 18, 2013
23,612
The only solenoid 'Burn-out' I have experienced with is in the use of AC versions, which has never occurred with DC versions in my many years of installing them.
Max.
 

crutschow

Joined Mar 14, 2008
27,710
Here's a Digikey Article that discusses three type of solenoids, so the TS machine in question certainly could have "Intermittent" type solenoids.
And I don't see why a DC solenoid, whatever the type, would be more reliable than an AC type if they are both properly designed.

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Last edited:

GetDeviceInfo

Joined Jun 7, 2009
1,876
I’m having a tuff time conceiving a situation where I’d deploy a solenoid that thought for itself. In my experience, an armature that can’t fully travel due to contamination, physical deformation, etc, will heat. Poor maintenance on contactors is pretty common, as is corrosion in valving.
I’d rather have it burnout than doing something on it’s own. An alert to overtemp, or under travel might have better legs. Measuring and responding to drive current could do that. But hey, as it often happens, an application may have specific needs.
 
I wanted one, a shutter, to operate SLOWLY, because it was next to a $1000.00 1mm thick lens.
Use was once every 2.5 minutes on/ 2.5 off for a normal work day.

I also had to do the "kick" thing for a shutter made from a rotary solenoid.

And the "pulse" for a rotary actuator.
 

BobaMosfet

Joined Jul 1, 2009
1,848
Obviously everyone would say "don't leave it on for too long" but suppose the machine in question doesn't necessarily have somebody watching it at all times that's aware of the "intermittent duty" nature of the solenoid.

The CPU could impose a time limit, be connected to a temperature sensor, or "pick and hold" via PWM. However, this all would depend on the software working properly, and even a CPU running rock-solid software could still freeze up if the power supply gets noisy or unstable. If it froze while the solenoid was energized, then you'd better hope that someone sees it that knows to shut down the machine when a solenoid is stuck, otherwise you can say goodbye to the solenoid.

I have a couple ideas for how one might protect against solenoid burnout due to a CPU malfunction. A watchdog timer that, if it expires, drops out power to all the solenoids and motors, ideally via a relay or MOSFET that's independent of what the CPU uses to control them normally. That way, this will work even if the MOSFET you're controlling directly from the CPU fails shorted for some weird reason. And, if you design it properly so that watchdog doesn't also reset the CPU, then the CPU could intentionally let that watchdog expire as a backup way to stop everything if the CPU detects, for example, current flowing on the power bus when there shouldn't be any (i.e. current flow that doesn't "add up" to how many devices are active, current flowing when all devices are off, etc.)

Another way, at least for intermittent-duty-only solenoids, would be to have a circuit with a couple 555 timers, where the first one times full power to the solenoid, and then when the first one expires, the second one takes over, doing PWM at slightly above the lowest duty cycle necessary to keep the arm extended/retracted if that's necessary in the application. If it's not, then you would just have one timer that does a "pick and drop" function instead, not letting the solenoid be on for more than a second at a time no matter what the CPU says.

Finally, in keeping with the "CPU independent protection" theme, if you had a temperature sensor on each solenoid going back to a simple comparator circuit, the comparator could prohibit that solenoid from operating if the temperature gets too high, along with sending an alarm signal back to the CPU. This would probably be the ideal solution, because it could catch unforeseen issues such as a malfunctioning sensor causing the solenoid to "machine gun" or an odd software glitch that did not cause a total freeze-up of the CPU, therefore not triggering a watchdog timer.

Note that this isn't a safety-critical system I'm dealing with, only looking out for expensive "domino" failures, particularly with software freezes which may cost literally nothing to fix if it's not a recurring problem: just cycle power, end of story. Even if the CPU's power supply has an actual failure, solenoids are relatively expensive compared to power supply capacitors or a voltage regulator.
Use an appropriately rated Thermistor. When _it_ gets too hot, it will disconnect for a period.
 

crutschow

Joined Mar 14, 2008
27,710
Use an appropriately rated Thermistor. When _it_ gets too hot, it will disconnect for a period.
A thermistor resistance gets lower with heat, so you would need to use that with some additional circuitry to shut off the solenoid.
A sensitor resistance increases with temperature.
 

Thread Starter

Benjamin3000

Joined Mar 28, 2021
8
I’m having a tuff time conceiving a situation where I’d deploy a solenoid that thought for itself. In my experience, an armature that can’t fully travel due to contamination, physical deformation, etc, will heat. Poor maintenance on contactors is pretty common, as is corrosion in valving.
I’d rather have it burnout than doing something on it’s own. An alert to overtemp, or under travel might have better legs. Measuring and responding to drive current could do that. But hey, as it often happens, an application may have specific needs.
I used to think solenoid valves were pretty much one piece and there wasn't a way to non-destructively separate the coil and plunger from the valve mechanism.

Pinball machine solenoids are generally intermittent duty. The flippers use an end of stroke switch to switch from full power "kick" to low power "hold".

As far as I know there's not a protection mechanism against a solenoid burning out due to a motherboard freezing up. I have a real "solid state" pinball with a schematic, and I didn't see anything that looked like a watchdog timer. It does, however, have a separate fuse just for the solenoid power supply. So I dare say if a solenoid burned out and the windings shorted instead of opened, the driver circuit and power supply stand a good chance of surviving the incident (but I'm not about to find out)

I'm not going to modify my existing pinball machine. Anything more than a simple watchdog, if that, would be impractical to add to an existing system. Some people have added watchdog timers to machines that didn't already have them, but it's in our house and most of the people that play it, know that a loud constant hum means a solenoid is stuck on, which is bad and it should be shut down ASAP. In an arcade you might not hear it, or might not be able to find the power cord for the right machine in time, etc etc
 

MaxHeadRoom

Joined Jul 18, 2013
23,612
And I don't see why a DC solenoid, whatever the type, would be more reliable than an AC type if they are both properly designed.
The problem is, in an industrial setting anyway, is when a AC solenoid does not completely shift the armature for some reason, or someone pushes the armature over to test, as in a Hydraulic valve etc, resulting in unseating a shifted armature when under power, this usually result in an instant burn out.
The DC variety run the same current when initialized or solenoid shifted.
 

Ian0

Joined Aug 7, 2020
3,220
Yes, there are NTC and PTC Thermistors. Ametherm, Inc. Makes very god ones.
I know - NTC types generally follow the Hart-Steinhart equation. PTC types fall into two categories - those where the resistance is proportional to absolute temperature, and those (generally Barium Titanate) which have an abrupt resistance increase around a certain temperature. But I've never heard the term "sensitor".
 
A thermal overload relay would simultaneously cut power to the coil and send an alarm signal to halt the program.

Or a small thermal switch in good thermal contact with the solenoid body. Could wire a control relay in parallel with it so if the switch is open and power is applied, you will get an alarm signal.
 
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