Limit switches

[ronv]

I'm working with someone on an H Bridge but using limit switches to break the motor current at the end of travel. I'm thinking this is not a good appication for a FET bridge due to the arcing of the switch at the end of travel. Anyone have experience with such a plan?
The load is a motor.

 

[KG4MXV]

How about a IR sensor detector pair instead of a mechanical switch?

 

[4pyros]

I have done it with relays and limit switchs. Can you use the limit switchs on the control side? Is it a critical application to switch the motor power with the limit switchs?

 

[KG4MXV]

I agree I wouldn't switch the output side of the drive circuit.

 

[KeepItSimpleStupid]

One comment, I'd like to make is that in many cases a limit switch is used as a safety switch. e.g. It's not activated on a regular basis.

A hoist: It has both an upper and lower limit, but most of the time, they are never activated. An operator with a brain generally keeps the hoist within it's limits.

A scientific instrument (stepper based): It has a low, high limit and an ORG switch. Org is the most important. So, the unit turns on and heads toward low, passes org and then advances to org.

A model gantry crane: I put them into the motor circuit and I had another feedback sensor. if the programming got out of whack, I went for overcurrent motor protection. it required a power recycle to clear the fault.

Shutter mechanism (synchronous motor): The switches were used to sense the ends, but the motor drove against a slip clutch. At least that's how I thought I designed it.

 

[ronv]

Yep, I think relays may be the way to go. I think the limit switches are built into the actuator. Everything I read says you just can't pull the plug on the motor and expect the FETs to survive.

Thanks

 

[johansen]

Yep, I think relays may be the way to go. I think the limit switches are built into the actuator. Everything I read says you just can't pull the plug on the motor and expect the FETs to survive.

Thanks

please enlighten me


provided the h bridge has sufficient capacitance across the bus, and the parasitic inductance of the 4 switches are adaquately snubbered, the h bridge cannot be destroyed except by thermal failure caused by a short circuit.

 

[ronv]

Here is a nice write up. I know the theory is good but......

https://www.4qd.co.uk/serv/MOSFETfail.html

 

[johansen]

dv/dt limits fall under adequately snubbered.

unfortunately, you have to consider worst case scenario when building the snubbers; yet most of the time there aren't any!

 

[ccurtis]

Consider integrating the limit switches with the gate control of the FETs instead of using them to interrupt the motor current directly.

 

[4pyros]

On some equpment I have seen two sets of switches. One for control signals and just after that another safty switch that would kill all power to the motors.