Author Topic: Dynamic Braking with minimal components [SOLVED] (Read 4371 times)

anishkgt · « **on:** December 30, 2018, 12:39:30 pm »

Hi All,

I was looking for a simple braking circuit for a DC motor like the 775 rate 12-24V. here is a circuit that i had come up with. The components names are generic, yet to measure the reverse current from the motor.

Would appreciate if somebody could advise how ideal this would be ? Or should i go with an H-Bridge that has the breaking built into it ?

T3sl4co1l · « **Reply #1 on:** December 30, 2018, 03:42:08 pm »

What is the braking regenerating into? Just a resistor?

The EMF is in the same direction as rotation. If it was spinning forward, it will be braking forward as well. Only the current reverses.

Normally a synchronous bridge (half or full) is used, both for efficiency and control reasons, as well as because it is regenerative. Of course in that case, the supply must be able to absorb regeneration, which is fine for a battery supply, but will need an additional shunt regulation circuit if it is, for example, supplied from diodes and so has no sinking capability otherwise.

Tim

anishkgt · « **Reply #2 on:** December 30, 2018, 03:51:11 pm »

Not really regenerating into anything. Just used the term as the principle is what I was looking for.

I just need to stop the motor when the power to it is cut-off. The load is a circular saw blade. So do want it free wheel when a cut is done. Without the load it stops dead but with a load it takes some time but better than with out it.

Was looking for something more efficient, reduce the free wheeling to a minimum within 5seconds
or a few more.

I could do this with an arduino but don’t want waste an ic just for pwm. I have a pwm module from eBay lying around which I am planning to use here as the power source. So I don’t want to send the current back.

Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D, Brymen BM869s

Sent from my iPhone using Tapatalk

T3sl4co1l · « **Reply #3 on:** December 30, 2018, 03:55:50 pm »

How about using a relay (NC contacts) to switch a resistor in parallel with the motor when it's not powered?

Tim

anishkgt · « **Reply #4 on:** December 30, 2018, 03:57:35 pm »

Quote from: T3sl4co1l on December 30, 2018, 03:55:50 pm

How about using a relay (NC contacts) to switch a resistor in parallel with the motor when it's not powered?

Tim

Yea I did think of that but wasn’t sure of the rating of the contacts when the motor terminals short.

Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D, Brymen BM869s

Sent from my iPhone using Tapatalk

anishkgt · « **Reply #5 on:** December 30, 2018, 05:04:05 pm »

here is the one with a relay with lesser BOM.

Heartbreaker · « **Reply #6 on:** December 30, 2018, 09:20:37 pm »

Quote from: anishkgt on December 30, 2018, 05:04:05 pm

here is the one with a relay with lesser BOM.

If you have a concern about the relay contacts, why switch the motor current at all? Connect motor terminal 2 to the negative power supply terminal.

During the start of the break cycle the motor is free-wheeling with the relay as its load. If you move the relay coil to the other side of the diode you avoid that the motor drives the coil during breaking.

T3sl4co1l · « **Reply #7 on:** December 31, 2018, 04:09:29 am »

D1 needs to be between the K1 coil and the motor, otherwise the motor powers the relay coil and nothing happens.

Tim

anishkgt · « **Reply #8 on:** December 31, 2018, 04:28:21 am »

Thanks everyone. Since I don’t have H bridge, how fast would it brake compared to this design ?
Would it be like this or an H bridge would give better braking ?

Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D, Brymen BM869s

Sent from my iPhone using Tapatalk

anishkgt · « **Reply #9 on:** December 31, 2018, 05:54:02 am »

Quote from: T3sl4co1l on December 31, 2018, 04:09:29 am

D1 needs to be between the K1 coil and the motor, otherwise the motor powers the relay coil and nothing happens.

Tim

Is this what you suggested here what about back emf from the coil ?

james_s · « **Reply #10 on:** December 31, 2018, 06:09:27 am »

I believe the term you're looking for is "dynamic braking", at least that's what it's called in the context of railway locomotives.

anishkgt · « **Reply #11 on:** December 31, 2018, 06:29:11 am »

Quote from: james_s on December 31, 2018, 06:09:27 am

I believe the term you're looking for is "dynamic braking", at least that's what it's called in the context of railway locomotives.

Yes that is what am trying to do here. Will change the title as well.

Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D, Brymen BM869s

Sent from my iPhone using Tapatalk

anishkgt · « **Reply #12 on:** December 31, 2018, 08:19:36 am »

Based on the video, here is a schematic.

My load is a circular saw blade running at 12v and 5A approx. Would it be ok to use the same P(https://www.mouser.com/datasheet/2/308/FQP17P06-1300953.pdf) and N(https://www.mouser.com/datasheet/2/308/FCP165N65S3-D-1315684.pdf) Channel MOSFET with 60V -17A and 650V 19A specs respectively.

james_s · « **Reply #13 on:** December 31, 2018, 07:14:13 pm »

I wish tutorials like that would just provide a proper schematic in the first place. So much easier to read than a photo of parts with a jumble of wires drawn over them.

Zero999 · « **Reply #14 on:** January 01, 2019, 08:57:14 am »

Quote from: james_s on December 31, 2018, 07:14:13 pm

I wish tutorials like that would just provide a proper schematic in the first place. So much easier to read than a photo of parts with a jumble of wires drawn over them.

They're obviously aimed at beginners, but I think this kind of thing only holds people back. I followed schematics right from the beginning and never thought about drawing circuits that way.

spec · « **Reply #15 on:** January 01, 2019, 11:08:32 am »

H anishkgt,

I would suggest that, instead of electronic breaking, you go for a mechanical brake as used on many petrol lawn mowers. All you need is a spring operated drum or disk brake and a hold-off solenoid. The brake could possibly be salvaged from a car or motorbike.

This approach has the potential for quicker and more positive breaking of the saw blade.

bson · « **Reply #16 on:** January 01, 2019, 11:34:26 pm »

Is there some kind of special graduated engagement required here or something?

What's wrong with something very basic?

Basically, let it flywheel freely (D2 can be any rectifier) by bypassing the load using the switch.

Starting with a naive, simple approach often works well, then improve and refine it to address shortcomings. The problem is these shortcomings are often use case or application specific, such as the specific motor and mechanical load, and when you co-opt someone else's finalized circuit you don't know what their use case or application is, or how well their approach fits your needs. So you end up reverse engineering it to find its operational limits, response times, dissipation, etc. Or, their circuit is general purpose enough to handle all imaginable shortcomings, in which case it's going to be ridiculously complicated, over engineered, and takes forever to reverse engineer and understand.

Dave · « **Reply #17 on:** January 02, 2019, 03:55:00 am »

Shorting the motor will not make it freewheel, but brake very hard. It could fry the motor, the diode and fuse together the relay contacts, if the mechanical part has enough inertia.

amyk · « **Reply #18 on:** January 02, 2019, 04:08:32 am »

Quote from: bson on January 01, 2019, 11:34:26 pm

Is there some kind of special graduated engagement required here or something?

What's wrong with something very basic?

Basically, let it flywheel freely (D2 can be any rectifier) by bypassing the load using the switch.

What's wrong with something that doesn't work?

It's called back-emf for a reason...

T3sl4co1l · « **Reply #19 on:** January 02, 2019, 05:21:43 am »

D2 is backwards. It's not inductive flyback, it's more like a capacitor equivalent to the rotor's inertia (and anything else hanging off it).

Tim

james_s · « **Reply #20 on:** January 02, 2019, 05:49:44 am »

I have an older cordless electric lawnmower that originally used a bundle of 18AWG wire as a braking resistor, connected across the motor by a mechanical switch that controls power to the motor.

anishkgt · « **Reply #21 on:** January 02, 2019, 06:27:24 am »

I have seen this feature on my cordless Dremel. I have the pcb out will post some pic of it.

Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D, Brymen BM869s

Sent from my iPhone using Tapatalk

T3sl4co1l · « **Reply #22 on:** January 02, 2019, 08:02:41 am »

Quote from: anishkgt on January 02, 2019, 07:33:41 am

https://forum.allaboutcircuits.com/attachments/img_1820-jpg.166948/
with the best of my knowledge i guess this is the schematic
https://forum.allaboutcircuits.com/attachments/capture-png.166949/
Looks like the FET is just a switch and the dual diodes are just blocking the back emf from the motor but where is brake part of the circuit.

Here is the remaining PCB in the dremel
https://forum.allaboutcircuits.com/attachments/img_1823-jpg.166945/ https://forum.allaboutcircuits.com/attachments/img_1824-jpg.166946/

You can't link images from another forum...

Tim

anishkgt · « **Reply #23 on:** January 02, 2019, 08:04:09 am »

It is the same one that I had done. I don’t have the pics in my work desktop.

Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D, Brymen BM869s

Sent from my iPhone using Tapatalk

anishkgt · « **Reply #24 on:** January 02, 2019, 08:12:57 am »

Here are some pics from my Dremel

The schematic is reverse engineered, i think i've got it correctly.


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Author Topic: Dynamic Braking with minimal components [SOLVED] (Read 4371 times)

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