From Newsgroup: sci.electronics.design

I've dug through the Art of Electronics-3rd Edition (& X Chapters), but haven't spotted a half decent diagram for current limiting MOSFET
solenoid drivers. They only list that subject with respect to Power
Supplies.

Looking to limit the current for six MOSFETs driving solenoids that draw around 2A at 28VDC. These are only momentary solenoids and will burn out
if the CPU locks up and the watchdog fails. I need redundancy in this circuit...

An old way for doing that was to have a capacitor in series with the
driver transistor so you only can get a short pulse. That is pretty
simple to implement, but I'm curious if there is anything more modern
that may be more foolproof? With MOSFETs I suspect one could use a non-electrolytic cap in series and get a 1/4 to 1/2 second pulse...

Yeah, I'm not an electronic engineer, but you folks have known that as
long as I've been rummaging through this group.

Thanks as always!

John :-#)#
--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
#7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
(604)872-5757 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

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From Newsgroup: sci.electronics.design

John Robertson <jrr@flippers.com> writes:

Looking to limit the current for six MOSFETs driving solenoids that
draw around 2A at 28VDC. These are only momentary solenoids and will
burn out if the CPU locks up and the watchdog fails. I need redundancy
in this circuit...

An old way for doing that was to have a capacitor in series with the
driver transistor so you only can get a short pulse. That is pretty
simple to implement, but I'm curious if there is anything more modern
that may be more foolproof? With MOSFETs I suspect one could use a non-electrolytic cap in series and get a 1/4 to 1/2 second pulse...

There are plenty of solenoid drive ICs (Search on ti.com for some
examples), which can do initial high drive + PWM hold. The older analog
way with resistor limit is not used much anymore due to efficiency.

You could also use 74HC123 or 74HC/CD4538 series monostable to limit the
drive pulse. If you need controller pulse, you might need AND/OR with
the output and the control signal and have the monostable to the
limiting only. If you do the pulsing in firmware instead of timer, you
can use retriggerable setup. With timer your firmware might lock up and
timer could keep on working.

-- mikko
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From Newsgroup: sci.electronics.design

John Robertson <jrr@flippers.com> wrote:

I've dug through the Art of Electronics-3rd Edition (& X Chapters), but haven't spotted a half decent diagram for current limiting MOSFET
solenoid drivers. They only list that subject with respect to Power Supplies.

Looking to limit the current for six MOSFETs driving solenoids that draw around 2A at 28VDC. These are only momentary solenoids and will burn out
if the CPU locks up and the watchdog fails. I need redundancy in this circuit...

An old way for doing that was to have a capacitor in series with the
driver transistor so you only can get a short pulse. That is pretty
simple to implement, but I'm curious if there is anything more modern
that may be more foolproof? With MOSFETs I suspect one could use a non-electrolytic cap in series and get a 1/4 to 1/2 second pulse...

Yeah, I'm not an electronic engineer, but you folks have known that as
long as I've been rummaging through this group.

Thanks as always!

John :-#)#

Nothing wrong with a capacitor in the gate drive, simple and effective.
--
piglet
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From Newsgroup: sci.electronics.design

Mikko S <mikko.syrjalahti@nospam.fi> wrote:

John Robertson <jrr@flippers.com> writes:

Looking to limit the current for six MOSFETs driving solenoids that
draw around 2A at 28VDC. These are only momentary solenoids and will
burn out if the CPU locks up and the watchdog fails. I need redundancy
in this circuit...

An old way for doing that was to have a capacitor in series with the
driver transistor so you only can get a short pulse. That is pretty
simple to implement, but I'm curious if there is anything more modern
that may be more foolproof? With MOSFETs I suspect one could use a
non-electrolytic cap in series and get a 1/4 to 1/2 second pulse...

There are plenty of solenoid drive ICs (Search on ti.com for some
examples), which can do initial high drive + PWM hold. The older analog
way with resistor limit is not used much anymore due to efficiency.

You could also use 74HC123 or 74HC/CD4538 series monostable to limit the drive pulse. If you need controller pulse, you might need AND/OR with
the output and the control signal and have the monostable to the
limiting only. If you do the pulsing in firmware instead of timer, you
can use retriggerable setup. With timer your firmware might lock up and
timer could keep on working.

Here's one solution:

Since a fried diode will end the game anyway, I guess it's
alright to cut the power. So you don't need to monitor the
current on all MOSFETs individually.

Since you use a 4-to-16 decoder, at most one solenoid will
be active, so you could just put a current shunt or Hall
sensor in the ground connection, to monitor the current of
whatever solenoid is on.

Then you can use two comparators:

One with a high threshold, which is not normally
reached with "normal" current, to detect if the
MOSFET turns on and the diode in the solenoid is
shorted.

To catch a melted MOSFET being on continuously,
or a software bug keeping the solenoids on all
the time, another comparator with a lower
threshold, which detects a normal amount of
current for the solenoid. This should go to a
retriggerable monostable which is reset every
time the current drops to zero. If the current
does not drop to zero for long enough, the
monostable will expire and signal an error.

Then the output of these should go to something like a RS
flop that latches the error and deasserts the output enable
on your decoder, and also disables the power supply, if it
has an enable input. You could also use a high side MOSFET
switch.

If the power supply has a lot of capacitance, and the diode
shorts, a huge current will flow, so you don't have a lot
of time to turn off the MOSFETs by deasserting the decoder's
output enable. The gate resistors will probably have to be
tuned. A fast comparator can be useful here. The comparator
can be a simple BJT if voltage on the shunt in case of a
short circuit exceeds 0.6V.

<https://electronics.stackexchange.com/questions/632538/overcurrent-protection-for-solenoid-driver-circuit>

Danke,

--
73, Don, WD7Q veritas _|_
liberabit | https://www.qsl.net/wd7q vos |

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From Newsgroup: sci.electronics.design

"John Robertson" <jrr@flippers.com> wrote in message news:111vjsi$rmdu$1@dont-email.me...

I've dug through the Art of Electronics-3rd Edition (& X Chapters), but haven't spotted a half decent diagram for current limiting
MOSFET solenoid drivers. They only list that subject with respect to Power Supplies.

Looking to limit the current for six MOSFETs driving solenoids that draw around 2A at 28VDC. These are only momentary solenoids
and will burn out if the CPU locks up and the watchdog fails. I need redundancy in this circuit...

An old way for doing that was to have a capacitor in series with the driver transistor so you only can get a short pulse.

That would be my immediate thought.

Capacitively couple the drive so that the drive from the CPU can
stick high or low forever without causing damage.

The simplest way is usually the best way.

Otherwise sense the current with a low value resistor and
arrange for that to turn off the drive as necessary.
Preferably in hardware only.

That is pretty simple to implement, but I'm curious if there is anything more modern that may be more foolproof? With MOSFETs I
suspect one could use a non-electrolytic cap in series and get a 1/4 to 1/2 second pulse...

Yeah, I'm not an electronic engineer, but you folks have known that as long as I've been rummaging through this group.

Thanks as always!

John :-#)#
--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
#7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
(604)872-5757 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

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From Newsgroup: sci.electronics.design

On 6/29/2026 10:23 PM, John Robertson wrote:

I've dug through the Art of Electronics-3rd Edition (& X Chapters), but haven't
spotted a half decent diagram for current limiting MOSFET solenoid drivers. They only list that subject with respect to Power Supplies.

Looking to limit the current for six MOSFETs driving solenoids that draw around
2A at 28VDC. These are only momentary solenoids and will burn out if the CPU locks up and the watchdog fails. I need redundancy in this circuit...

Are you seeing instances where this has (or is likely to have) happened?

An old way for doing that was to have a capacitor in series with the driver transistor so you only can get a short pulse. That is pretty simple to implement, but I'm curious if there is anything more modern that may be more foolproof? With MOSFETs I suspect one could use a non-electrolytic cap in series and get a 1/4 to 1/2 second pulse...

As you can't predict haow the processor will fail if the watchdog isn't
around to keep it in check (i.e., a double failure), the CPU could just
as easily enter into a loop where it is turning the FET on and off, repeatedly.

Yeah, I'm not an electronic engineer, but you folks have known that as long as
I've been rummaging through this group.

I assume (past posts) you are looking for a drop-in replacement for a BJT
or similar. I.e., a single three-terminal circuit for each device to be replaced?

At one point (in the distant past), SCRs were used and "forcefully" turned
off by bleeding gate current away (now it's nominally a 4-terminal circuit)

Is there a reason you can't replace the existing devices with their
exact replacements? (I assume you are dealing with small numbers/retrofits) --- Synchronet 3.22a-Linux NewsLink 1.2

From Newsgroup: sci.electronics.design

On 6/30/2026 6:08 AM, Don wrote:

Mikko S <mikko.syrjalahti@nospam.fi> wrote:

John Robertson <jrr@flippers.com> writes:

Looking to limit the current for six MOSFETs driving solenoids that
draw around 2A at 28VDC. These are only momentary solenoids and will
burn out if the CPU locks up and the watchdog fails. I need redundancy
in this circuit...

An old way for doing that was to have a capacitor in series with the
driver transistor so you only can get a short pulse. That is pretty
simple to implement, but I'm curious if there is anything more modern
that may be more foolproof? With MOSFETs I suspect one could use a
non-electrolytic cap in series and get a 1/4 to 1/2 second pulse...

There are plenty of solenoid drive ICs (Search on ti.com for some
examples), which can do initial high drive + PWM hold. The older analog
way with resistor limit is not used much anymore due to efficiency.

You could also use 74HC123 or 74HC/CD4538 series monostable to limit the
drive pulse. If you need controller pulse, you might need AND/OR with
the output and the control signal and have the monostable to the
limiting only. If you do the pulsing in firmware instead of timer, you
can use retriggerable setup. With timer your firmware might lock up and
timer could keep on working.

Shit rolls downhill. Your goal is to ensure the actual drive to
the switch (*or*, the power available to it) is constrained, regardless
of how you choose to drive it. Any part can fail so you want to have
the most faith in where the rubber actually meets the road.

Here's one solution:

Since a fried diode will end the game anyway, I guess it's
alright to cut the power. So you don't need to monitor the
current on all MOSFETs individually.

Since you use a 4-to-16 decoder, at most one solenoid will
be active, so you could just put a current shunt or Hall
sensor in the ground connection, to monitor the current of
whatever solenoid is on.

I don't see where John claimed to be driving them from a decoder.
I suspect any combination -- including all and none -- could be
active at any given time and for differing amounts of time,
depending on their application.

Then you can use two comparators:

One with a high threshold, which is not normally
reached with "normal" current, to detect if the
MOSFET turns on and the diode in the solenoid is
shorted.

To catch a melted MOSFET being on continuously,
or a software bug keeping the solenoids on all
the time, another comparator with a lower
threshold, which detects a normal amount of
current for the solenoid. This should go to a
retriggerable monostable which is reset every
time the current drops to zero. If the current
does not drop to zero for long enough, the
monostable will expire and signal an error.

Who's going to *see* the error if the processor has lost its mind?

Any sort of watchdog should bring whatever it is "watching" to
a safe state that can persist indefinitely. E.g., restarting the
CPU just means it can loop between "restart" and "fail", forever.

When I detect a fault, I remove power to the field so nothing
can be actively driven "wrong". If you have designed so these
things "can't happen", then it is easy to rationalize shutting
down when it *does*.

In the applications I suspect John is addressing (based on his past
posts), there's no real guarantee as to what will be available
to address a problem, besides the "FET-replacement-circuit" itself.
I.e., the CPU can be on a completely different board, interconnect
cables can be "open", etc.

[And, I assume he doesn't want to spend much on a solution for
a problem if he's not seeing lots of downside risk to omitting
such fixes]

Then the output of these should go to something like a RS
flop that latches the error and deasserts the output enable
on your decoder, and also disables the power supply, if it
has an enable input. You could also use a high side MOSFET
switch.

If the power supply has a lot of capacitance, and the diode
shorts, a huge current will flow, so you don't have a lot
of time to turn off the MOSFETs by deasserting the decoder's
output enable. The gate resistors will probably have to be
tuned. A fast comparator can be useful here. The comparator
can be a simple BJT if voltage on the shunt in case of a
short circuit exceeds 0.6V.

<https://electronics.stackexchange.com/questions/632538/overcurrent-protection-for-solenoid-driver-circuit>

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From Newsgroup: sci.electronics.design

Don Y <blockedofcourse@foo.invalid> wrote:

Shit rolls downhill.

Scat shat on slight slopes stays grounded.
Lessin' you're a scat kicker or something similar.

Danke,

--
73, Don, WD7Q veritas _|_
liberabit | https://www.qsl.net/wd7q vos |

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From Newsgroup: sci.electronics.design

On Mon, 29 Jun 2026 22:23:28 -0700, John Robertson <jrr@flippers.com>
wrote:

I've dug through the Art of Electronics-3rd Edition (& X Chapters), but >haven't spotted a half decent diagram for current limiting MOSFET
solenoid drivers. They only list that subject with respect to Power >Supplies.

Looking to limit the current for six MOSFETs driving solenoids that draw >around 2A at 28VDC. These are only momentary solenoids and will burn out
if the CPU locks up and the watchdog fails. I need redundancy in this >circuit...

An old way for doing that was to have a capacitor in series with the
driver transistor so you only can get a short pulse. That is pretty
simple to implement, but I'm curious if there is anything more modern
that may be more foolproof? With MOSFETs I suspect one could use a >non-electrolytic cap in series and get a 1/4 to 1/2 second pulse...

Yeah, I'm not an electronic engineer, but you folks have known that as
long as I've been rummaging through this group.

Thanks as always!

John :-#)#

Most solenoids and relays need a lot of current to pull in, and much
less to keep them locked once they are seated. So one can drop the
current after a tenth of a second or something.

A second mosfet and a resistor will work. Or PWM.

Old timey solenoids had two coils and a switch contact to do that.
Like the notorious Lucas overdrive solenoid on MGs.

AC relays and solenoids do the current step down automatically; the
seated magnetic loop has a higher impedance than the open one.


John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
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