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'[EE] firing solenoids'
2005\09\22@170603 by Damon Hamm

picon face
I am attempting to fire 8 solenoids from a 16F628 and am inexperienced
at playing with solenoids.

The setup:
One 16F628 connected to an optoisolator (necessary?) then to 8 power
MOSFETs (with necessary resistors in tow).  8 mini 12V push solenoids
(with flyback diodes) on the high side of the MOSFETs.

Upon a trigger, I want to fire the solenoids (sometimes all 8 at once)
and hold for 2 seconds or so.

I am having somewhat success using PWM to control the initial current
then drop it down once the coil is saturated to avoid burnout.
Schematic:
www.damonhamm.com/other/pic_solenoid_schematic1_pwm.gif
(Initial tests do not utilize the #1 RC 'decoupling' circuit.)

But, the MOSFET (1.5A? in a TO-220 case) heats up to be quite hot to
the touch, which I wouldn't consider viable.  Using 5V at the MOSFET
gate (full on or PWM at 100%) doesn't produce enough current to fire
the solenoid, while using 12V with PWM at 20% for 100ms, then at %1 to
hold for 2s, still causes everything to heat up much too much after
several cycles.  I am still surprised that a duty cycle of 1:255 (~1%)
still provides too much current!  Is there something wrong with my
setup or are the MOSFETs just too wimpy?  I have some 6Watt DIP
MOSFETs on the way to test out.

Has anyone had any experience with solenoids that they can offer?  Any
resources, circuit critique, and component suggestions would be
greatly appreciated!

I have looked at DRV101T solenoid drivers (PWM), but would rather save
$100 and use my solution.  My multimeter always reads 0 current
flowing when I attempt to see what the solenoid is using (can one not
read current in series with inducors or is my MM fried? - had a 120V
short with it years ago but everything else works fine).

Thanks!

- Damon

2005\09\22@184003 by Harold Hallikainen

face picon face
I don't think the opto does any good. I use them only where there's
actually some isolation required.

What's the size of the resistor in series with the gate? You'll have
substantial "Miller Effect" gate capacity, so any series resistance will
slow the gate down, causing the FET to not be saturated.

The FET will only get hot when it's dissipating power. The power being
dissipated is Vds*Id. The problem area is generally that the drain is
being pulled out of saturation when the FET is on (giving you both Vds and
Id). Ideally, when the FET is on, Vds is very low and Id is whatever is
required.

I'd watch the drain voltage on a scope to see what's going on. Also, have
a look at the gate voltage. If you run 100% duty cycle, I'd suspect the
gate to rise to some plateau, then stay there for a while as the drain
voltage falls (Miller Effect), then the gate goes to the PIC output
voltage. On the drain, the voltage should fall very quickly, then stay
down as long as the solenoid is on. If you find the drain voltage starting
to rise, the FET is being pulled out of saturation. The solenoid current
(since the solenoid is an inductor) will (ignoring the inductance change
due to slug movement) will linearly ramp up until the core saturates, then
linearly ramp up faster (due to lower permeability) until it is limited by
resistance. If, as you mention, the solenoid current gets too high if held
for a long period of time, you can PWM it, as you're doing. The trick is
to PWM it at a low enough frequency so you don't have tremendous gate
drive requirements (gate current), and high enough so the noise is not
bad.

Finally, look at the datasheet on the FET. It should show Id vs Vgs. What
current SHOULD you get through the FET when the gate is driven with the
voltage you're driving it with (probably 5V out of a PIC)? Different FETs
have different Id vs Vgs curves.

So, like Google, an oscilloscope is your friend. Look at Vg and Vd...

Good luck!

Harold

--
FCC Rules Updated Daily at http://www.hallikainen.com

2005\09\22@184756 by Richard Prosser

picon face
I find your circuit a bit confusing - you have the opto driven MOSFET
in parallel with exverything accros the power supply. No wonder it
gets hot!

If you are having trouble with the high Rds-on of logic level fets you
may need to go to "standard" MOSFETs. However, these will not switch
effectively with only a 5V drive from the PIC so you need to add a
driver circuit. There are examples in the archives. (Russell has
provided a good one). The other thing to watch is that the switching
speed is not too high. MOSFET gates are very capacitive so it takes an
appreciable time period for a PIC pin to turn  them on fully. Again,
if this is the problem, slow down the PWM frequency or use a driver.
The project I am just about completing drives contactors with up to 3A
of coil current and operates at ~30kHz., At this frequency the
risetime, even with a driver, is becoming noticable.
When driving contactors & solenoids note that the inductance changes
as they pull in (the magnetic circuit closes). This can also create
strange effects if you are trying to figure out what's going on.
And if you do use a driver circuit - make sure it is properly decoupled.

You are better off measuring coil current than trying to measure coil
volatge due to the waveforms involved. I get very widley varying
voltage readings even with a "true RMS" meter but the coil current is
very stable. If you can't get a reading I suspect your meter is
damaged.

RP

On 23/09/05, Damon Hamm <spam_OUTdamonhammTakeThisOuTspamgmail.com> wrote:
{Quote hidden}

> -

2005\09\22@191051 by Mark Rages

face picon face
On 9/22/05, Damon Hamm <.....damonhammKILLspamspam@spam@gmail.com> wrote:
{Quote hidden}

Damon,

I think the MOSFET shouldn't be getting that hot.  What MOSFET is it?

You can calculate the power dissipation of the MOSFET. Look up the on
resistance in the datasheet and multiply it by the current squared:
(I^2)R  Then multiply that by the duty cyctle of the PWM to get the
average dissipation.  If this figure is too high (more than half a
watt, say, if you're not using a heatsink), you can choose a MOSFET
with lower R.   This will mean choosing one with a higher current
rating than you would think you need.

The opto is not necessary.  The R/C is a neat idea, but I think the
voltage on MOSFET gate will be swinging between -2.5 and +2.5 V.
That's probably not going to be enough to turn it "on" all the way.
(I'm assuming the PIC is running on +5V).   I'd just put about a 10K
resistor between the PIC and the gate.

Actually, 5V may not be enough to turn some MOSFETs "on" all the way.
You have to check the curves on the datasheet or just choose a MOSFET
with "logic-level" inputs.

Regards,
Mark
markrages@gmail
--
You think that it is a secret, but it never has been one.
 - fortune cookie

2005\09\22@200048 by David Van Horn

picon face
> I think the MOSFET shouldn't be getting that hot.  What MOSFET is it?

It really shouldn't be getting hot in this sort of service.

RdsON too high for this application, or not enough gate voltage to turn
it on would be my first suspicion.  Second, I'd look at the snubber on
the solenoid coil, and make sure that it's keeping the fet from
avalanching.  If the voltages you see there are anywhere near the VDS
rating, then you are probably in avalanche.
Also watch the VDG rating.

> Actually, 5V may not be enough to turn some MOSFETs "on" all the way.
> You have to check the curves on the datasheet or just choose a MOSFET
> with "logic-level" inputs.

Many don't turn fully on till around 10V.
Simple check, what's the voltage between drain and gate when "on"?
Work that against the solenoid resistance and applied voltage, and see
what your RdsON figure is. If it's not real close to the rated number,
then it's not really ON.




2005\09\22@200258 by David Van Horn

picon face
> I find your circuit a bit confusing - you have the opto driven MOSFET
> in parallel with exverything accros the power supply. No wonder it
> gets hot!

That's how I read it as well!  Yikes.

Is this something like an impact printer?
BTDT.




2005\09\23@043133 by Alan B. Pearce

face picon face
> I am having somewhat success using PWM to control the initial current
> then drop it down once the coil is saturated to avoid burnout.
> Schematic:
> www.damonhamm.com/other/pic_solenoid_schematic1_pwm.gif
> (Initial tests do not utilize the #1 RC 'decoupling' circuit.)
>
> But, the MOSFET (1.5A? in a TO-220 case) heats up to be quite hot to
> the touch, which I wouldn't consider viable.  Using 5V at the MOSFET
> gate (full on or PWM at 100%) doesn't produce enough current to fire
> the solenoid, while using 12V with PWM at 20% for 100ms, then at %1 to
> hold for 2s, still causes everything to heat up much too much after
> several cycles.  I am still surprised that a duty cycle of 1:255 (~1%)
> still provides too much current!  Is there something wrong with my
> setup or are the MOSFETs just too wimpy?  I have some 6Watt DIP
> MOSFETs on the way to test out.

I have serious doubts this is switching the way you think.

1. I doubt the opto switches the FET fast enough.

2. I doubt the current in the solenoid changes fast enough to get the FET
out of "linear" mode - i.e. the current rise and fall times in the FET are
significant resulting in the FET being in the linear region between full off
and full on. I would doubt that a 1% duty cycle would do anything for you

3. You first need to sort out why the 100% duty cycle does not produce
enough current through the FET to pull in the solenoid. It sounds like the
FET is not a logic level one, in which case it probably needs 10 to 15V at
the gate to turn on. Then look at the other problems.

2005\09\23@153808 by Richard J. Pytelewski

picon face
Mark:

What is the current requirement for the solenoids?  Amps or mA's?  I have
used the ULM2003/4 IC's for driving a bank of relays/coils and each of the
drivers handles ~500 ma. There is a max package dissipation and at 2 sec,
and at a few hundred ma a piece that might be OK.  You may want to check out
the chip info....

Rich P.

{Original Message removed}

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