Time Delay on Loss of Power

[NE12345]

I am currently working on a project were I have a primary and secondary brake and the secondary brake needs to have a 1-3 second time delay after the 1st break sets. For this application we need to use two electromechanical brakes and the brakes selected use a solenoid. My thought was to place a capacity parallel to the brake solenoid sized based on the drop out voltage and resistance of the solenoid similar to the attached drawing. My questions are am I missing anything and would this also work with an actuator brake. Thanks.

 

[ScottyUK]

Your capacitors look wrong for a DC application.

Do the solenoids hold the brakes off, or apply them?

 

[NE12345]

The solenoids hold the brake off, springs apply the brake pads to stop the motor.

 

[NE12345]

For the capacitors I thought having 2 polarized capacitors with the negative leads together makes it non polarized and cuts the capacitance in half.

 

[ScottyUK]

You're kind of right about the caps but they are on the DC side of the rectifier which makes it pointless. A standard electrolytic would work fine on the DC side of the rectifier.

A capacitor is probably the only electrical method of holding off the brakes for that length of time. How much current do the solenoids draw?

 

[NE12345]

They have 2 coils with a mechanical switch once the brake is energized initial draw is 4.6-5.9 amps but then switches to .09-.07 amps once repositioned which will be the state I am dealing with on loss of power.

 

[OperaHouse]

One concern with this circuit is when power is applied to the brake. A very large capacitor will look like a dead short and draw excessive current till it charges up. That could be a problem with contacts. To avoid this, a low ohm resistor is placed in series with the capacitor limiting charge current. A diode in parallel with that resistor allows the capacitor to discharge through the brake with no loss when power is removed. A better solution may be to use a smaller capacitor and delay a relay instead, appling delayed power to the brake.

 

[ScottyUK]

Like the cap + diode solution. The brake coil still needs a source of power for the latter solution even if you use a pneumatic timing relay, and if this a safety system then that source may not be available other than from an energy store such as a capacitor or battery.