Is there a normally closed mosfet or switch?
T Chap
Posts: 4,223
On a brushless dc motor there are 3 windings. If you short the windings(3 wires) together the motor becomes like a brake. On my motor driver board there is a method to short the windings in Brake Mode. However if there is no power to the controller the can be no braking. I am looking for an option to short the 3 wires together if there is No power but really do not want to use relays as I have seen failures, and a stuck relay would cost downtime. A “mosfet” - like switch would have less of a chance of failure IMO over many years of use. Any ideas on how this can be done? Ideally a prop with 3.3v would turn the brake off as needed.
Comments
Without power, one MOSFET gate is biased high whenever the motor tries to turn, limiting the source-drain voltage to the gate threshold. The body diode in the other MOSFET conducts. (If the body diode can't handle the braking current, you will want to add diodes in parallel to the MOSFETs. ) A high on the NPN from the Prop keeps both gates low, preventing braking. For this to work, the motor coils must be logic-ground-referenced when the motor is running.
-Phil
What current ?
There are NC Solid state relays, usually with modest currents, digikey goes as low as 90mOhm : Circuit SPST-NC (1 Form B ) 90 mOhms Load Current 3.25A
There are depletion mode mosfets, that conduct with 0VGS and are turned off by a negative voltage.
Search Digikey for MOSFET then filter using depletion. There is a IXTH16N20D2 with 73 mOhm @ 8 A, 0V spec. (not cheap)
You could try a design that uses the motor back EMF, to provide some gate drive, to standard cheap Logic level MOSFETS, but you might want to avoid too-drastic brake action at power off ?
- or even a storage-cap approach, so motor drive cell does not loose power as quickly as everything else.
The friction brake sounds more like what you need.
The wisdom behind electronic braking depends upon the application. I've had several where electronic braking was not only the best and simplest solution, but also completely safe. So, without knowing the OP's app, it would be wrong to prejudge the safety of his desired braking scheme.
-Phil
"can" vs "will"
Good point. High deceleration forces can be hazardous for both safety and equipment, however the deceleration can be controlled by placing a load on the motor instead of a dead short.
Dual shafts for encoder on top. To add the break you have to special orde the motors with the brake and special extended shaft for the encoder to sit on it. I have an in-line electro mag shaft brake but i don’t want to deal with the extra hardware. I also have a toothed sprocket mod I can add to the output of the gearbox with a solenoid drive latch. This works well but long term the power for the solenoid can become a source of failure or the solenoid itself can fail. Since I am looking for the most maintenance free solution the motor winding short method (no moving parts) is still something I want to try. There is no concern as above mentioned about things flying apart. I will look up a NC SS relay and see what can be found. Thanks for the input guys.
What about a UPS or some kind of other power backup that when activated shuts everything down and requires a manual restart.
I lean toward the relay over a Mosfet just because a relay would not be taken out by a spike or surge, and if the system power drops the relay will drop. Everything can fail, just a matter as Peter implies, what are the risks for your various solutions. I also would not use a short, especially with electronics, depending on the surge your device will generate, you could likely punch your FETs rendering the braking function inoperable. Go with an appropriately sized power resistor. Heck, gang and step them if need be, kinda an inverse of softstart, a soft-stop if you will. Worse case on a relay, the surge welds the NC contacts during the braking time and the fault manifests before the system can be started up again. Just my two cents from repairing systems where paranoia was left behind long ago!