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PWM Current buffer to drive Proportional Solenoid from PLC

Oecist

New Member
Hello

(First time poster with rusty familiarity with vaccuum tube audio amplifiers)

I am working with a Siemens 1212C PLC and looking to power a Burkert proportional solenoid via the PLC HSC outputs. Smaller solenoids have been successfully powered this way, but I need a bigger orifice size which requires a bigger solenoid. The maximum rated output of the PLC is 20V @ 500mA (ON) and the Burkert 2875 requires 24V @ 750mA to fully open. (In the smaller valves the 20V maximum is very nearly fully open and an acceptable compromise). Rather than add expensive AD/DA modules, is there a basic current gain MOSFET / transistor amp that I could employ? I'm not that familiar with transistor design, but have added a simple design that may work (?). I have a 24V supply to work with and don't need a big gain ~x2 current, and ideally a small voltage gain (which this design certainly won't provide). It's a 900Hz signal, so I imagine it can be a very simple circuit - added C2 to tune the output to a square wave and C1 to add some local capacitance. No doubt missing a diode.

Can anyone point me in the right direction? I have some IRF540N in the drawer, so if that works for easy prototyping - great!

Siemens 1212C
Burkert 2875

Thanks for any help,

Ben
 

Attachments

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eTech

Well-Known Member
Here's an option.
I think the solenoid inductance is more like 50mh (just a guess).
Below shows solenoid current at 10% duty cycle.
Solenoid is normally closed with no current flow. The circuit is not a "failsafe" in terms of a short circuit because a shorted transistor could cause the valve to remain open, but neither are the others.

1662649984593.png
 

danadak

Active Member
Will the solenoid drop out at 50 mA ?

Alternative (i used this time 10 mH for relay coil L) -

1662657865354.png



Relay is getting shut off for sure.

Divider values at input of IRF540 to insure its Vgs never exceeds 20V, spec limit.

Note diode pk current, layout of this circuit should NOT daisy chain grounds, rather
PLC ground and MOSFET ground should join at power source ground as close as possible
to that point.

Also notice, this is 10% duty cycle. So fine control of valve position would need better
control than this due to long decay of large L in solenoid. Only real way here to fix that
is drop PWM freq to say 100 Hz..


Regards, Dana.
 
Last edited:

danadak

Active Member
So here is 100 Hz sim, assuming 10 mH solenoid (confirm what this is with vendor).

Very little Pdiss in MOSFET (as desired), 10% duty cycle current in solenoid allowed
to decay fully.

You will have to experiment with this circuit. Possibly raise the ratio input divider to
MOSFET to get closer to say 17-18V (to not exceed its limits) that would make turn
on faster.

1662659404651.png



Regards, Dana.
 

Oecist

New Member
Looking at L (relay) current not fully off because PLC cant generate 24 Vdc
to fully shut off Q1 Vbe. Either drop 24V to 20 or use NPN/ NMOSFET
as you redrew in post #9. With series Gate R dropped to 50 or more ohms.

Or drop 24 to 20 with a 4 V 5W Zener, on a heat sink. I would fuse the 24 V line....
just a thought.

View attachment 138480

Here is sim with 24 Vdc changed to 20 Vdc

View attachment 138481

Relay shuts off.

Regards, Dana.

Dana, thanks so very much for this - I'm building both variants and will report back when I have tested them!
 

Oecist

New Member
Remember the 20V was when sourcing 500mA; when sinking the maximum should be higher.

Assuming the specific output in use can sink current, I'd not though of that - some can and some cannot.

If not, then adding a zener around 5 - 6V as you mention, connect the 270 Ohm to ground and connect the PLC output to the zener <> resistor junction via a diode, to bypass the base current when the output is high.

The part is being sent back to me now - I'll build these buffers and report back once I have tested them. Thanks again for your insight!
 

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