Circuit design help

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

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

On 3/22/2007 12:15 PM, The digits of Mechdesign2k4's hands composed the following:

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.

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.

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.

--- 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?

-- JF

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!

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.

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

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.