Dynamic Braking with minimal components [SOLVED] - Page 1
Forum section: electronics / projects

author: anishkgt
author_rank: Frequent Contributor
timestamp: December 30, 2018, 12:39:30 pm
content: 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 ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 30, 2018, 03:42:08 pm
content: 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

author: anishkgt
author_rank: Frequent Contributor
timestamp: December 30, 2018, 03:51:11 pm
content: 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 5secondsor 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.comEquipments: DSO104z, Hakko FX888D, Brymen BM869sSent from my iPhone using Tapatalk

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 30, 2018, 03:55:50 pm
content: How about using a relay (NC contacts) to switch a resistor in parallel with the motor when it's not powered?Tim

author: anishkgt
author_rank: Frequent Contributor
timestamp: December 30, 2018, 03:57:35 pm
content: Quote from: T3sl4co1l on December 30, 2018, 03:55:50 pmYea I did think of that but wasn’t sure of the rating of the contacts when the motor terminals short.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869sSent from my iPhone using Tapatalk

author: anishkgt
author_rank: Frequent Contributor
timestamp: December 30, 2018, 05:04:05 pm
content: here is the one with a relay with lesser BOM.

author: Heartbreaker
author_rank: Unknown
timestamp: December 30, 2018, 09:20:37 pm
content: Quote from: anishkgt on December 30, 2018, 05:04:05 pmIf 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 31, 2018, 04:09:29 am
content: D1 needs to be between the K1 coil and the motor, otherwise the motor powers the relay coil and nothing happens.Tim

author: anishkgt
author_rank: Frequent Contributor
timestamp: December 31, 2018, 04:28:21 am
content: 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.comEquipments: DSO104z, Hakko FX888D, Brymen BM869sSent from my iPhone using Tapatalk

author: anishkgt
author_rank: Frequent Contributor
timestamp: December 31, 2018, 05:54:02 am
content: Quote from: T3sl4co1l on December 31, 2018, 04:09:29 amIs this what you suggested here what about back emf from the coil ?

author: james_s
author_rank: Super Contributor
timestamp: December 31, 2018, 06:09:27 am
content: 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.

author: anishkgt
author_rank: Frequent Contributor
timestamp: December 31, 2018, 06:29:11 am
content: Quote from: james_s on December 31, 2018, 06:09:27 amYes that is what am trying to do here. Will change the title as well.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869sSent from my iPhone using Tapatalk

author: anishkgt
author_rank: Frequent Contributor
timestamp: December 31, 2018, 08:19:36 am
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: December 31, 2018, 07:14:13 pm
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: January 01, 2019, 08:57:14 am
content: Quote from: james_s on December 31, 2018, 07:14:13 pmThey'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.

author: spec
author_rank: Frequent Contributor
timestamp: January 01, 2019, 11:08:32 am
content: 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.

author: bson
author_rank: Unknown
timestamp: January 01, 2019, 11:34:26 pm
content: 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 fitsyourneeds. 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.

author: Dave
author_rank: Super Contributor
timestamp: January 02, 2019, 03:55:00 am
content: Shorting the motor will not make it freewheel, but brakeveryhard. It could fry the motor, the diode and fuse together the relay contacts, if the mechanical part has enough inertia.

author: amyk
author_rank: Super Contributor
timestamp: January 02, 2019, 04:08:32 am
content: Quote from: bson on January 01, 2019, 11:34:26 pmWhat's wrong with something thatdoesn't work?It's calledback-emf for a reason...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2019, 05:21:43 am
content: 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

author: james_s
author_rank: Super Contributor
timestamp: January 02, 2019, 05:49:44 am
content: 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.

author: anishkgt
author_rank: Frequent Contributor
timestamp: January 02, 2019, 06:27:24 am
content: I have seen this feature on my cordless Dremel. I have the pcb out will post some pic of it.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869sSent from my iPhone using Tapatalk

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2019, 08:02:41 am
content: Quote from: anishkgt on January 02, 2019, 07:33:41 amYou can't link images from another forum...Tim

author: anishkgt
author_rank: Frequent Contributor
timestamp: January 02, 2019, 08:04:09 am
content: It is the same one that I had done. I don’t have the pics in my work desktop.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869sSent from my iPhone using Tapatalk

author: anishkgt
author_rank: Frequent Contributor
timestamp: January 02, 2019, 08:12:57 am
content: Here are some pics from my DremelThe schematic is reverse engineered, i think i've got it correctly.

author: Zero999
author_rank: Super Contributor
timestamp: January 02, 2019, 12:32:40 pm
content: Quote from: anishkgt on January 02, 2019, 08:12:57 amIt looks wrong to me.Q1 should be P-channel, otherwise it will do nothing. The current will just flow though the body diode. D1 should be in parallel with the motor, so it acts as a freewheel diode.

author: anishkgt
author_rank: Frequent Contributor
timestamp: January 02, 2019, 01:48:43 pm
content: My previous schematic was incomplete as the PCB was not clear. I've removed all the components and looks good now.D1 is a dual diode with common Anode. The working is like this when in the OFF State as seen in the schematic, current flows from M2 through D1 and into the M1 of the motor. Making it a dead short aka Dynamic Brake.In the ON State current flows through M1, through M2 and D1, when the Gate of Q1 is pulsed it conducts and completes the circuit.

author: anishkgt
author_rank: Frequent Contributor
timestamp: January 02, 2019, 07:38:25 pm
content: A quick update i just tested this circuit and the braking is hard stops very fast compared to just shorting the terminal of the Motor. I've omitted the MOSFET part of the circuit as i already have a pwm controlled module to control the speed of the motor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2019, 07:45:36 pm
content: If that stops fast enough then what was this thread about in the first place?...Tim

author: anishkgt
author_rank: Frequent Contributor
timestamp: January 02, 2019, 07:47:39 pm
content: Without the D1, the stopping from max run to a complete stop took about 10 to 20 seconds. With it takes less than 5 Seconds.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2019, 07:49:10 pm
content: The diode doesn't do anything as shown.Tim

author: anishkgt
author_rank: Frequent Contributor
timestamp: January 02, 2019, 07:53:24 pm
content: D1 shunts the current due to the inductance of the motor armature winding (This is NOT the same thing as the voltage generates while still spinning. It is the opposite polarity to that.)Without D1 there would be high voltage spikes which here is shunt by the D1. It also helps to maintain the current through the motor during the PWM off time.Explanation from another user in another forum for the same question i had asked.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2019, 08:41:12 pm
content: Right, it catches freewheeling, but the inductance carries a tiny fraction of the rotating energy. The inductance is discharged within 10ms, then current drops to zero and it's spinning down with an open circuit.Tim

author: Zero999
author_rank: Super Contributor
timestamp: January 03, 2019, 11:00:33 am
content: Quote from: anishkgt on January 02, 2019, 07:53:24 pmAs mentioned above, that doesn't break the motor. The energy stored in the motor's inductance is tiny, compared to the mechanical inertia, which generates a voltage a little lower than the supply voltage, assuming it's running at full speed. Try turning the shaft with that circuit connected, then again with nothing connected and note that it makes little difference. Now try turning the shaft with the motor's terminals short circuited and note how much more difficult it is to turn. If it's a small motor, without a gearbox, the difference may not be noticeable, as the torque will be tiny but it will be with a larger or geared motor.Quote from: anishkgt on January 02, 2019, 01:48:43 pmThat looks right, but you've got the values of R1 and R2 wrong. 400Ω is far too high and will drop such a high voltage, hardly any current will flow through the motor. The letter L is used to denote milliOhms and R1 and R2 are marked 4L0, which means 4m0, so they're 4mΩ or 0.004Ω. See the links below for more information on SMD resistor marking.https://www.rohm.com/datasheet/PMR18EZPFV/pmr-ehttps://www.vishay.com/docs/20020/smdmark.pdfTry measuring R1 and R2 with a multimeter and they should read short circuit, as their value is well below the minimum resistance most multimeters can reliably read.The circuit short circuits the motor, which is fine for a load without too much inertia, as the motor can safely dissipate the energy in its windings, but it will be no good in an application such as an electric vehicle, which would cause the motor, wiring and switch to overheat, especially when going down a hill. In applications which require breaking a large mechanical load, the motor is briefly short circuited, the current monitored until it exceeds a certain level, then it's connected back up to the battery for long enough for the voltage generated by its inductance to be clamped by the battery, before being briefly short circuited again. The motor's inductance is being used as the inductor in a boost converter, with the load, being the battery and the supply voltage the back-EMF generated by the spinning armature.

author: anishkgt
author_rank: Frequent Contributor
timestamp: January 04, 2019, 09:22:39 am
content: Quote from: anishkgt on January 02, 2019, 01:48:43 pmIf I need to regulate the braking power would it be ideal to have a 4R 5W resistor in series with M2 and D1 ?Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869sSent from my iPhone using Tapatalk