On August 30, 2009 at 06:08, Ernie Bornn-Gilman said...
Irregardless,
The ones I've seen will be ruined if there are no stop switches. They'll continue to draw current, at heavy load because the motor can't move, and eventually burn out.
Think about it this way... If you stall the motor, depending on the motor technology, it may eventually burn up.
A common low tech fix for this, if you wish to use something like a timer to perform the open and close, is to use what is called a Polymeric PTC. Raychem is one manufacturer of PTC's. These devices have been built into the window motors of cars for many years. (They are the reason you can hold the switch in the up or down position forever, and not burn out the window motor.)
The PTC is so named, because it is a Positive Temperature Coefficient device. When it heats, it's resistance increases. The idea is that you wire it in series with the power supply and motor of the device you intend to protect. Under normal operation, it has a small resistance, and the motor/actuator works normally. If the device becomes stalled (cabinet door blocked, at the end of its throw, etc) the motor draws more current. The PTC is chosen to activate at this higher current, and it then reduces the current draw, until the current decays under a threshold value.
You need to choose the PTC based on several parameters. The "Hold Current" is the maximum current the device can pass, before it trips. The "Trip Current" is the current at which the device is guaranteed to trip. You choose a PTC device where the normal current for the actuator is below at slightly above the "Hold Current". You also choose a device where the stall current of the motor is above the "Trip Current".
The following paper on Raychem's website gives lots of great info:
[Link: circuitprotection.com] You would probably prefer a radial leaded polyswitch device. The following area on their website serves to help select a model and exact part number.
[Link: circuitprotection.com]I'm guessing that something in the RXEF series at the link above would meet your needs. If you pick an actuator, and then can give me numbers on the normal operation current, and the current the device draws when stalled, I can help you pick a device. Or, usually an email to their tech support staff, and they'll recommend a device for your application.
This may be overkill for your application, as it requires a bit of work to choose the right device, but the devices themselves are very inexpensive. You might want to go to DigiKey or Mouser, and see what devices they stock, and choose from those, as it will make sourcing the device really easy. Searches or Polymer PTC should turn up devices at DigiKey or Mouser.
Regarding the current needed for one of these actuators, they will require very little current, as you are going to have very little force the motor must work against. The actuator will probably need to exert about a 1lb force to move the door. As a result, the current needed will be much less than the fully loaded current for the actuator.
Last edited by bcf1963 on September 1, 2009 01:24.
