Rds_on current sensing

Haven't done it myself, but it seems that if you can know the junction temperature of the switch device, a lookup table might get you close enough to knowing what Rds is at the moment, then measure the voltage across & you have current. I suspect you want to just look for a rapid delta in current, like from a stalled motor...so maybe you don't even have to compensate for junction temp to do a decent job in the special case you have.

Den fredag den 16. oktober 2015 kl. 23.21.16 UTC+2 skrev Tim Wescott:

why three resistors, why not single resistor in the supply?

of just stick a intelligent high side switch with over current shut down in the supply wire

-Lasse

How about a device that senses when the plane has crashed?

I have a friend that does radio control flying devices. I do know at one time he used a REED switch with specific turns around it to do something at a specific current event and found a nice spot for it in the plane.

Like I said, I have no idea what he was trying to switch in the plane.

Jamie

Tim, I'm afraid I don't know what Rds_on current sensing is. You're looking at the voltage across the fet pass element?

George H.

In a switching converter that is expected to react within a switching period, Rdson sensing can be complicated by noise and the requirement to ignore normal high voltages developed when the switch is off. Internal and external fet rds on sensing is not uncommon in protective circuitry of low voltage buck converter ICs.

A slower response method sometimes used requires that the switch also discharge a small signal RCD network; if it fails to do so to a specific level, then an overcurrent situation can be detected in a switch cycle or two.

You need to take care with the elaborate methods used to avoid sensing resistor power loss, as a false overcurrent detection, if persistent or latching, could theoreticaly induce the crash.........

Crude limiting to protect the motor windings might be achieved using a PTC - but you wanted to avoid extra (lossy?) parts.

RL

That would never work for most... with the high currents involved.

They don't get made because of the huge variation in current demand by each user... I think you will have to roll your own high side ( or low side) FET switch If you're that desperate.After a full throttle crash into the ground, there won't be much left of anything, trust me, I know. Fancy buying a 124" Missie kit : :)

Could you use a current sensing MOSFET?

Cheers

Accelerometer perhaps?

joe

Because a stalled motor doesn't have much back-EMF, so you can have high motor (and device) currents with modest supply current. If you just sense supply current you've made a battery saver, not an expensive-part- of-the-system saver.

Perhaps that has been thought of, tried, and rejected. Do you have an answer to my question?

Yup. There are quite a few switchers with integrated FETs that offer it, and a very few switchers with external FETs that offer it.

Bulk and weight are bad in and of themselves. Lossy parts induce bulk both in the requirement for more battery and in the requirement for more cooling (all model airplane electronic speed controls assume a ready supply of cooling air, but you still have to route it past the ESC).

If you're flying over grass with an internal combustion engine powered plane, the powerplant survivability is far greater than with an electric. Even when the airframe is pulverized, people would still like to pluck the drivetrain parts out of the wreckage and reuse it.

On Sat, 17 Oct 2015 07:59:33 -0500, Tim Wescott Gave us:

What gets broken? The propeller or the motor?

There may be a miniature inertial shock activated breaker out there somewhere.

Or you could devote an actual mechanical servo to throwing a mechanical switch you control from the handset.

On Fri, 16 Oct 2015 19:43:06 -0700 (PDT), George Herold Gave us:

The spindle drive FETs, of course.

On Sat, 17 Oct 2015 08:01:29 -0500, Tim Wescott Gave us:

Use a GaN FET

feed it with a programmable controller

On Sat, 17 Oct 2015 08:07:00 -0500, Tim Wescott Gave us:

Perhaps an investigation into all these drones in use may yield some tricks. You could actually use a drone circuit and leave off the extra props, OR just make a model B-29 Enola Gay RC plane an use all four. :-)