# Circuit design help

Discussion in 'Electronic Design' started by Mechdesign2k4, Mar 22, 2007.

1. ### Mechdesign2k4Guest

Having not much knowledge in the field of electronics, I neeed
assistance in designing a circuit. Basically I want to control a
solenoid air valve. I would like to be able to push a button and have
the valve pulse on and off and be able to independently vary the on
and off times. I was thinking on basing this circuit around a 555
timer. QAny help would be greatly appreciated.

2. ### D from BCGuest

If I understand the prob correctly..
When the button is pushed an oscillator with independent on/off
control is turned on?

I'm guessing at the function in words..
A Ton timer triggers on a rising edge.
Ton elapses.
Ton's falling edge then trips the Toff timer.
Toff elapses.
Toff's rising edge is fedback to the Ton timer.

Yup...sounds very 555ish to me.
D from BC

3. ### Mechdesign2k4Guest

right now the solenoids are 120volt, NC. I'm guessing a relay would
between the "circuit" and the solenoid. Within a 3-5 second cycle, I
would like to have the solenoid fire about 6 times. In the energized
state, the solenoid open, it would remain that way for only 20-30
milliseconds. Again I would like to be able to adjust these times
manually, potentiometer maybe? I have a 5-15v power supply, 13.4 amp,
as well as a 24v 2.5 amp supply that I could use for this.

4. ### Rich GriseGuest

Even though this sounds suspiciously like homework, I'll try to help
withoug doing the whole thing for you. A 555 should do the trick,
although you'll most likely need to add a transistor to control the
solenoid. You could use a pot to adjust the "on" time, but I'd think the
"off" time would be determined by "until you press the button again". You
might want to differentiate the button-push to the trigger - I'm not sure
what happens to the output if the trigger stays active through the timing
interval, but I think it just stays high; ergo, you want a very narrow
trigger pulse.

Good Luck!
Rich

5. ### HawkerGuest

How big a solenoid's?
You will probably need a second 555 (or small MPU) to act as a PWM
modulator. for all but very small solenoids you need to give a high duty
cycle pules to close it then back off the duty cycle by a large amount
to hold it close or you will burn up the coil.
It may just be easier to pick a small 8 bit processor to handle all the
timing from the push button and PWM. But you are correct a 555 should be
able to do it for you. I would need more info to give you a drawing.
Look for "1 shot" drawings in National Semi's web site of LM555 for some
ideas for your timer part of it.

Hawker

6. ### Noway2Guest

The first thing you need to do, is determine what the product
requirements are. Spell them out, write a specification, add drawings,
etc.

An example of what you might include: you say independently vary the on
and off times, but how do you want them set? buttons, dials, or other?
Do you want a digital display, 7 segment LEDs, or none?. These types
of decisions will at least impact, if not determine many aspects of the
system design.

7. ### Mechdesign2k4Guest

LOL thanks Rich,
I can assure you this isnt homework. We do have an inhouse electronics
engineer, but lets just say if you want it done in a hurry, you might
get something in a couple of months from him...

8. ### MooseFETGuest

The 20-30mS time worries me. Are these things run from 60Hz AC or
120V DC?

Doing a time that small on an AC powered thing adds a layer of
trouble.

9. ### John FieldsGuest

---
From what you've written, I think you have a solenoid that uses
120VDC to go from what you call the "normally closed" state to the
energized state, which is what you call "open".

Also, you have a "cycle" which can last from 3 to 5 seconds, and
during that time you'd like the solenoid to go from the normally
closed (de-energized) state to the energized "open" state, to stay
open for 20 to 30 milliseconds and then revert to the closed state
six times during that 3 to 5 second period.

Also, you want to be able to adjust the cycle time, the number of
times the solenoid fires during that cycle and the "open" times when
the solenoid is fired.

Am I right?

Oh... and how much current does the solenoid need to operate and how
long does it take to go from closed to open and from open to closed?

10. ### Guest

Yep. He's going to ask the same sorts of questions as John Fields has
asked, and you haven't answered, so he'd have to go and find out the
answers himself, which does use up more of his time. For some odd
reason, people who have a more detailed idea of what they need done
get faster service from in-house engineers.

11. ### Guest

Don't think: Instead BUY a couple of DIN rail timers. They will work directly
with the solenoid too. Once you have done the thinking, the power supply, the
relay driver, the PCB and the EMC stuff then the 555 is very expensive indeed!

12. ### Noway2Guest

Your comment about "all but very small solenoids" caught my attention.
How small is very small?

I have an application I a 12Vdc 20W solenoid I am planning on driving
with a ~24V PWM. I was planning on adjusting the PWM duty cycle to
provide an RMS equivalent of 12Vdc with a frequency of about 100Hz.

I hacked together a test circuit and verified that it worked and let it
run for a while. I monitored the RMS voltage and current and the
readings look fine. The coil did not appear to get hot or suffer any
side effects either.

It sounds like you have some experience with doing this sort of thing.
Based on your experience, does this approach sound reasonable or would
you recommend an initially large duty cycle and then back it off as you
say? The hold current is far less than the pull in current, so backing
off on the duty cycle makes sense.

13. ### HawkerGuest

You will have to measure the temperature of the coil.
Your approach should work as you have it, but as I think you know, you
will need a much higher pull in current than holding current. For this
reason you can shift your PWM after about 200mS to the holding current
and save power. If heat is fine (because the coil was built over sized
enough) then the only advantage is power saving. If you don't care then....

What often happens, to save cost is manufactures make the coil smaller
than can sustain the pull in current indefinitely. In this case if you
don't back of the current after pull in the coil will eventually burn
up. Not everyone builds them that way. The very large stuff has to be
built that way, but you are in the size that could go both ways. 20W
seems like alot of juice to have burning up if you only need 2W to hold
it though.

BTW for whatever reason I don't know the PWM frequency is usually in the
1-5Khz range. Much higher the effects of the inductor take over.

I have designed a few PWM controllers for very large 3-phase
contractors. For them I think we had something like 90% duty cycle pull
in with 10% duty cycle holding or some such.

HAwker

14. ### Noway2Guest

Thank you for the reply.

You have given me some things to think about and experiment with. You
are right, 20W does seem like an awful lot of power just to hold in a
solenoid. In this case, it is on a little valve on a 1/2" plumbing
line. I have honestly been wondering why such a large coil was used in
this application. The problem is that I don't have any specifications
telling me what the requirements are and there is no history from the
person who chose the original to explain his rational. I do have
confidence in that person's abilities based on what I know of them, but
there is no way to ask them anymore.