Turn AC on before DC or Turn DC on before AC power switches

[danny davis]

Some circuits want you to turn on the AC power before the DC power or Vice verse, turn on the DC power before the AC power switch

Why is that? what circuits do you guys know that are like this?

 

[Golden Electronics]

Can u explain any application in which such switching require

 

[danny davis]

That's why I'm asking you guys

Can you guys explain to me which circuits and designs are like this?

 

[betwixt]

Most circuits rely on DC supplies so in instances such as motor/machine control, it is often necessary to ensure the control system is powered up and ready for operation before the power it controls is enabled. It really depends upon the individual requirement of the equipment though as in many cases the DC is derived from the AC supply anyway so switching DC first would have no effect.

Brian.

 

[danny davis]

How can it damage the unit or circuit? if you turn on the DC first or AC first before the other?

 

[betwixt]

Nobody can answer that without knowing what the circut does and why there are two supplies in the first place.
Let me give you an example:

A motor turns and makes a conveyor belt move. At the end of the conveyor there is a light beam sensor that turns the motor off when an object breaks the beam. Suppose the sensor is operated by the DC supply (as would be normal) but the motor is controlled by the AC supply. If you switch the DC on first, the system can immediately check if the beam is broken already or if it is safe to turn the motor on. If the AC was turned on first, the motor might move and leave a pile of broken items on the floor as they drop off the end because the sensor wasn't working.

Brian.