Needed: 5 seconds of AC

[controldude]

Hello all, I hope you can help me with this.

I'm running a motor, and when I lose power I have a mechanical lock with a solenoid design. If I lose power, the solenoid drops and hits a cam on the motor's shaft. Normally this doesn't happen while the motor will be running. But if the motor is running and I lose power, I don't want the plunger to slam into the cam if it was running. So basically, I want just about 5 seconds of 120 AC to the solenoid before it drops out, enough time for the motor drift to a stop. Is there an inexpensive circuit design for this? I'll gladly take pointers in the right direction.

-Dude

 

[controldude]

Thank you, this information is invaluable. I don't think I can use the coil-lock relays, they are around $100. The event that power is lost is an extremely rare occurence for when the motor operates, which is only about 15 seconds to a minute every day. I will attempt to change to a 24V DC solenoid and add the capacitor. I will let you know my progress in a later thread, but that may be a while down the road. Now to get busy...

 

[electricpete]

I am not sure I understand the idea of the dc coil with the cap

When you disconnect power you will have oscillations, which leads to two questions
1 - Barry mentioned a dc cap - won't it cause violent damage to the dc cap when the polarity reverses?
2 - Even if you use an ac cap (my preference), how will the dc relay respond in presence of oscillating ac? I would think you are in danger of dropping out.

To my thinking, if a cap is an option it would be with the existing ac coil. If you are going to dc coil why don't you just use the reverse biased diode?

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[mc5w]

If you use a DC solenoid and capacitor I forgot to mention that you need a diode in shunt across the coil in order to keep the coil from trying to put a reverse voltage across the electrolytic capacitor.

You also need a second diode that will keep the capacitor charge from backfeeding into the control circuit.

 

[jraef]

Complicateder and complicateder... (apologies to Lewis Carrol )

With a $10 cost limit you are going to run into problems with any electrical solution. Have you considered a mechanical change? I am thinking of backspin ratchets on pumps. They work in a similar manner, but what they do is have a sloped side to the end of lock pin and a sawtooth arangement on the tops of the shaft cam lobes. So even though the pin technically drops when the power fails, it "rides" over the tops of the sawtooth cam lobes until the velocity of the shaft is slow enough for it to drop down all the way into the slot (past the slope) and lock the shaft.

"Venditori de oleum-vipera non vigere excordis populi"

 

[PowerfulStuff]

A long long time ago (well, 30 or 40 years) pneumatic timers were not so reliable and it was commmon for time delays to be built using magnetic properties. One that I remember used a copper washer on the end of the solenoid coil with DC energisation. When power was turned off the collapsing magnetic field induced current in the washer and held the plunger in for a long time (up to 30s). This was adjustable by how close the washer was to the core.

I'd say a diode would be almost the same but with more loss may not hold in as long. The cap alternative sounds like it may have problems. The size of the cap to store a reasonable amount of energy would be quite large (and expensive).