Hi all,

I have a project which requires a motor to

• go forwards for ~1 second when the power supply is turned on,
• go backwards for ~1 second when the power supply is turned off, i.e. the system is de-energiesed, and
• remain stationary for the rest of the time.

The power supply is 240 vac. The motor is ~6 vdc, < 400mA.

I can think of a couple of ways of achieving this, all of which are more complex than I think it needs to be. What is the simplest way of achieving this reliably? I'm not bothered if the timing is accurate as long it is repetable and won't drift over time.

Looking forward to your suggestions

 

Hi all,

I have a project which requires a motor to

• go forwards for ~1 second when the power supply is turned on,
• go backwards for ~1 second when the power supply is turned off, i.e. the system is de-energiesed, and
• remain stationary for the rest of the time.

The power supply is 240 vac. The motor is ~6 vdc, < 400mA.

I can think of a couple of ways of achieving this, all of which are more complex than I think it needs to be. What is the simplest way of achieving this reliably? I'm not bothered if the timing is accurate as long it is repetable and won't drift over time.

Looking forward to your suggestions

Can you elaborate a little , what do you mean by forwards and backwards?

I assume it is a rotary motor. Is the load fixed or varying i.e. do you require position feedback or not?
Would a fixed period of activation after power on & power off suffice?

If so I would look at 555 in monostable and a double pole double throw relay or H bridge.
You will need some form of reserve power to reverse the motor after the 240 Supply is removed.

 

Can you elaborate a little , what do you mean by forwards and backwards?

I assume it is a rotary motor. Is the load fixed or varying i.e. do you require position feedback or not?
Would a fixed period of activation after power on & power off suffice?

If so I would look at 555 in monostable and a double pole double throw relay or H bridge.
You will need some form of reserve power to reverse the motor after the 240 Supply is removed.

Or a time delay to turn the power off after motor moves

 

[/B]

Or a time delay to turn the power off after motor moves

That is one possibility, we shall see his answers first and look at the various options.

 

The load is fixed and the motor is rotary. By forwards and backwards I mean that the motor should rotate in one direction and then the opposite direction. There is no particular requirement for position feedback or precise control, a fixed period of rotation would be fine.

Am not close to internet at moment so sorry if my answers are slow.

 

Hi, anyone any suggestions on this? In particular, what to use to provide the backup power supply for when the power goes off. Would like to avoid batteries if possible as don't want the hassle of designing the charging circuit.

Tim

 

Look up "supercapacitor" or "ultracapacitor". If you wanted to supply 400mA for 1 second at 6V, you'd need well under 1 Farad, but the current would start high and taper off as the voltage dropped, so I'm not sure what it would really take. Would one big current pulse do what you need?

But then, if your AC power supply is a transformer, rectifier and filter capacitor, maybe you could detect when the input has been lost, and use the remaining energy in the filter capacitor to revere the motor.

 

Just a few mA drain when power is off. Maybe a little more complex than you wanted, but it's a start.

 

Wow Bernard, that's really impressive that you spent the time to draw that out, thanks. I think I will proceed on those lines but use a h-bridge rather than relay - I already have experience with a suitable h-bridge, library parts drawn up etc. For backup power I think I'll use a supercap, for my application the 6v power supply will be disconnected at the same time as the rest of the circuits, i.e. the power will be cut to the whole thing all at once.

Thanks again,

Tim