Dc Motor Brake

[Zero999]

It will work without the diode and it isn't so critical, if you're using a mechanical switch, rather than a transistor.

The diode is still a good idea, because it reduces arcing on the switch contacts, thus prolonging their life and reliability.

I doubt you'll observe any visible difference between diode and no diode, unless you actually show the switch contacts to see if the arcing or put an oscilloscope across them measure the voltage.

In any case, diodes are cheaper than switches, so it's worth it.

[anishkgt]

Ah ok so the diode was actually to protect the switch and not the motor. Would a 1N007 be ok here ?

And to check the voltage at the switch , the probes should go at the anode and cathode, correct ?


Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D

[Zero999]

Your previous schematic is correct, except you'll need a resistor in series with the shorting contacts, to limit the current to a safe level.



I would have expected the negative pulse to be higher, than the results given by your previous experiment, but it will depend on the how fast it's turned off, which seems to be pretty slow. Going by the negative voltages, you've measured, it should be fine without a diode.
https://www.eevblog.com/forum/projects/dc-motor-brake/msg1293204/#msg1293204

[anishkgt]

but it will depend on the how fast it's turned off

as in the switch off time would generally be more than 30 seconds after switch ON. The blade has to reach its full speed before anything can be cut.

And a resistor in series, would that be just for test with the OSC or it has to be their as part of the circuit ?


Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D

[Zero999]

but it will depend on the how fast it's turned off

as in the switch off time would generally be more than 30 seconds after switch ON. The blade has to reach its full speed before anything can be cut.

You've misunderstood my comment regarding switching speed. It has nothing to do with how long the motor is turned on or off for. The switching speed, is how long it takes for the switch's contacts to physically open and close, thus switching the current on or off. It is a property of the switch itself.

The faster the switch contacts open, the higher the reverse voltage spike. In this case, the contacts open quite slowly, so there isn't much of a voltage spike.

And a resistor in series, would that be just for test with the OSC or it has to be their as part of the circuit ?

The resistor is there to limit the current through the switch and to control the deceleration of the motor, nothing to do with the test.

[anishkgt]

Thanks for clearing that. The resistor in series made it a lot more slower in stoping the blade. Without one was better. I tried an 8ohm 20w and that did not help. Not sure if I should go lower but even if I had I would have to get this from eBay.


Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D

[anishkgt]

Your previous schematic is correct, except you'll need a resistor in series with the shorting contacts, to limit the current to a safe level.



I would have expected the negative pulse to be higher, than the results given by your previous experiment, but it will depend on the how fast it's turned off, which seems to be pretty slow. Going by the negative voltages, you've measured, it should be fine without a diode.
https://www.eevblog.com/forum/projects/dc-motor-brake/msg1293204/#msg1293204

Here it is on the scope.  If i have interpreted it correctly, the spike is at 23.60v and less than 50ns and the whole duration after contact is 738ns.

[Zero999]

Is that without the diode?

It should be fine.

On second thoughts the lack of a resistor isn't that bad, since the blade doesn't have much mass. The motor also switches on, with nothing to limit the inrush and it's certainly no worse.

[anishkgt]

It is with this diode https://www.digikey.com/products/en/circuit-protection/tvs-diodes/144?k=P6KE27A&k=&pkeyword=P6KE27A&pv1291=136&FV=1c0002%2C1f140000%2Cffe00090%2C6540013%2Cb2801b3%2C1b1001f3&mnonly=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25

and no resistor.

To play it safe what would be the best inrush limiter ?

[Zero999]

Seems reasonable to me.

The inrush limiter can be as simple as, a series resistor which is bypassed by a relay, after a certain time has elapsed, or a transistor with an RC circuit, to make it turn on slowly.

See below.

RL represents the motor.  The MOSFET needs to be able to dissipate ¹/₂ of the power rating of the motor, for the time it takes to start.

[anishkgt]

That would take some time design and troubleshoot. I will just use a pwm controller like this https://www.ebay.com/itm/322417384785


Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D

[Zero999]

That would take some time design and troubleshoot. I will just use a pwm controller like this https://www.ebay.com/itm/322417384785


Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D

The link you've posted didn't work but I agree, a PWM controller would be a good idea.

[anishkgt]

Here you go. A Chinese by let's see how well it goes.


Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D

[anishkgt]

A quick update.

So if i've not misread the scope, adding a cap and a resistor has significantly reduced the spike at the switch. Is this correct ?

[Zero999]

A quick update.

So if i've not misread the scope, adding a cap and a resistor has significantly reduced the spike at the switch. Is this correct ?

Yes, but with 10K there will be very little braking.

[anishkgt]

Oh i got the connections wrong though the schematic was correct. Yes the 10k Resistor would be a bad idea. and the spike at the contacts pritty high. I had thinner wire that connected from the power source and that heated quickly and due to that amps was dissipated as heat.

Changed the wire to a thicker cable and now the motor has a bit more torque and as a result the spikes are higher too.