Auto off switch underload?

[supak111]

Hey again guys, I am trying to make a switch that turns off automatically under too much load.

I'm running an 12v geared DC motor, only about 90 degrees or 2 seconds. A momentary DPDT switch will operated the motor backwards and forwards. What I need is some type of protection so if someone holds the DPDT switch ON too long so that the motor does not overheat and burn out.

I was thinking maybe a thermal switch/circuit protector, like the device used on turn signals in cars? Would this protect the motor even if someone was crazy enough to keep the switch ON for like 1 hour?

Is there anything easier or more suitable for this?

Thanks in advanced everyone. BTW any ides or comments appreciated, I'm a newbie but willing to research anything that could work. PS trying to keep everything simple and with as little wiring as possible.

 

[CocaCola]

Limit switches, that kill the appropriate power on each side...

 

[(*steve*)]

fuse? Circuit breaker?

 

[supak111]

Yea but if I use a fuse or circuit breaker I will have to replace it everytime I accidentally hold the switch ON too long. I need something that will continue to work

 

[CocaCola]

Yea but if I use a fuse or circuit breaker I will have to replace it everytime I accidentally hold the switch ON too long. I need something that will continue to work

Use limit switches just like they do on DIY CNC for overrun failures... When the motor is at full left it trips a switch that won't allow any more 'left' current to flow... When it's at full right it trips another switch that won't allow anymore 'right' current to flow... Dead simple and foolproof, unless something fails...

 

[(*steve*)]

limit switches are the best idea.

Fuses or circuit breakers can help if the limit switches don't work (or the motor jams)

Circuit breakers can be reset. Fuses are cheaper than motors.

 

[supak111]

Can anyone answer this:

I was thinking maybe a thermal switch/circuit protector, like the device used on turn signals in cars? Would this protect the motor even if someone was crazy enough to keep the switch ON for like 1 hour?

 

[(*steve*)]

fuse? Circuit breaker?

edit or ptc device -- possibly the simplest practical answer.

I have no idea what turn indicators do for protection. Is there any problem with them being left on for hours at a time?

 

[BobK]

Can anyone answer this:

I was thinking maybe a thermal switch/circuit protector, like the device used on turn signals in cars? Would this protect the motor even if someone was crazy enough to keep the switch ON for like 1 hour?

What you are describing is a circuit breaker, which has already been suggested. Tell us why neither that nor limit switches is good enough for you.

Bob

 

[supak111]

I like limit switches but makes my device too complicated and too much extra wiring.

Circuit breaker would work great if I can find one that:

auto resets back ON after the current overload is gone
works for 12v and breaks at probably only 0.5 amp
and is cheap, can't be more then a few $5 so my device stays cheap


PS again best easiest cheapest thing I could find is using a thermal flasher, circuit breaker. Its the device used to make your cars turn signals blink.

 

[Rleo6965]

Motor still heats up even protected by fuse or circuit breakers. Motor still be damages in the long run. Limit switch was still the best option. Most industrial design was to use limit switch.

 

[supak111]

Is there such a thing as a heat variable resistor? where the resistance goes up with temperature?

EDIT: Never mind, I found it. positive thermistors

 

[supak111]

Now could anyone help me figure out what size PCT Thermistors I need? I'm working with 12-14v and my device uses about .5amps at the most. Anything past that and the PCT needs to heat up and increase resistance.

 

[(*steve*)]

Is there such a thing as a heat variable resistor? where the resistance goes up with temperature?

EDIT: Never mind, I found it. positive thermistors

Yes, and the current through them heats them up. They are also sometimes called re-settable fuses.

I mentioned them a few posts back. This link may help you decide if they're what you want.

 

[supak111]

Thanks for the link. Yea it looks like exactly what I need and its cheap.

I still don't get how to pick the right size PCT. If I want the PTC to start heating after .5 amps which size do I need?

 

[supak111]

I can't find anywhere if the volt rating on PTC matters. Say Im working with 12v and I need a .5amp PTC. Is it ok to use a 70v .5amp thermistor PTC?

I know higher voltage rating is fine on regular fuses, but now use with these since they are kinda of a resistor.

 

[(*steve*)]

Only the current and resistance matters. Just don't exceed the voltage rating. In some respects it's like a fuse.

Ensure that the Ihold rating is sufficient to allow the motor to run.

Ensure that the Itrip value will be reached if the motor overloads.

Ensure that the voltage rating exceeds 12V

Then try it out and see how it works.

 

[supak111]

Thanks, I just ordered some and I'll see how it goes. Im really wondering how long they take to heat up and open. I got some that trip at 300mA, my motor pulls 80-200mA at normal operation and stall draws 510mA so it should be perfect.

 

[(*steve*)]

Let us know how they go.