Stop motor when current rises - Page 1
Forum section: electronics / beginners

author: DarkZero
author_rank: Regular Contributor
timestamp: April 29, 2012, 09:45:02 am
content: Hiis there any way to make a circuit that will stop the motor when current rises. for example 12v DC motor runs and u stop it by hand current rises and then circuit stops him and only after reset will it start again and is there any way to make it stop at certain current level.thanks in advance !

author: ejeffrey
author_rank: Super Contributor
timestamp: April 29, 2012, 10:01:55 am
content: A fuse?You can also use current sense resistor, a comparator, a flip-flop, and a MOSFET.

author: Psi
author_rank: Super Contributor
timestamp: April 29, 2012, 10:06:54 am
content: A current sense resistor, a pot and an opamp/comparator could be connected together to give you an output that went high when current exceeds a threshold set by the pot.This output could be used to switch the motor on/off.You would need some lag in the system so that the high current when the motor initially starts doesn't trigger the limit.

author: DarkZero
author_rank: Regular Contributor
timestamp: April 29, 2012, 01:39:24 pm
content: so u meant something like this ?http://imageshack.us/photo/my-images/85/41053080.jpg/or i'm totally wrong

author: eevblogfan
author_rank: Frequent Contributor
timestamp: April 29, 2012, 03:31:01 pm
content: heyI am curious , when motor start's up , can be some massive amount of current , dont you need to delay the opamp's slew rate/response ? (for example put some capacitanse in the parallel of the sensing leads )have a nice dayand fun with the circuit

author: Zero999
author_rank: Super Contributor
timestamp: April 29, 2012, 05:15:49 pm
content: Quote from: eevblogfan on April 29, 2012, 03:31:01 pmYes you do, a simple RC circuit will do that.It depends on what you want to do. Do you need to stop the motor when the current exceeds a certain limit or do you need to limit the peak current?The attached circuit uses a comparator and a monostable vibrator to stop the motor for a few seconds if a certain current limit is exceeded. It won't limit the peak current so, if you're worried about damaging the driver MOSFET, you need a separate circuit.

author: Kremmen
author_rank: Super Contributor
timestamp: April 29, 2012, 05:21:12 pm
content: Quote from: DarkZero on April 29, 2012, 09:45:02 amWouldn't it be curious if there wasn't a way to do that?It can be done of course. It is not too difficult but will require soldering together an integrated circuit and some basic components.The attached circuit should work. This is the idea.-the motor M1 is controlled by relay K1 (contact K1.3).-the relay coil (K1.1) is energized firstly by N-fet Q1 which is turned on by comparator U1.1. More on that soon. Q1 provides the ground connection to the coil.- When pushbutton S1 is closed it provides 12V to the relay coil and the relay closes. S1 also forces the fet to stay on as long the switch is depressed. This takes care of any starting current spike.- once the relay has closed its own contact K1.2 will keep it closed and the motor will run.- Motor current is turned into a voltage over R8. R8 needs to be a small value in order not to affect motor operation. For that reason amplifier U1.2 raises the level to something comparator U1.1 can handle.- RC filter R6C1 will remove any spiking and average the motor current to avoid false trips.- U1.1 compares the amplified actual current value to a reference set by R3. Once the actual exceeds the reference U1.1 output goes low turning the fet off. Relay K1 releases and the motor stops. When the current goes to zero, U1.1 again turns the fet on, but the relay is off so nothing happens.- Once S1 is closed again the cycle repeats.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: April 30, 2012, 03:42:14 am
content: heythank you for replay ,Quoteoh , I meant to ask a question from curiosity , I don't have the need for that circuit , but thank you anyway

author: DarkZero
author_rank: Regular Contributor
timestamp: April 30, 2012, 05:29:34 pm
content: Thanks for the reply guys !im going to try your schematic Krenmmen maybe ill get it to work

author: Kremmen
author_rank: Super Contributor
timestamp: April 30, 2012, 06:05:35 pm
content: OK, i hope you are comfortable assembling this circuit. It should not be that hard but here a couple of points that may make it easier:- First get an rough idea of the current where you want the motor to stop. Say it is 1 amp for the sake of argument. Next you will need to dimension the current sense resistor so that its voltage loss at the max current is a) something easily amplified with this simple cisrcuit and b) not too high to affect the motor operation. As a ballpark figure something like 100 mV could be a reasonable compromise. Thay would make the resistor value 0.1 ohms.- Next you need to decide the voltage level after amplification. In principle you can compare any voltages in the comparator but in practice they should not be to demanding for the sense amplifier to produce, yet high enough to be easily settable by the reference trimmer. I would bo for an amplification of 20-30 to produce 2-3 volts out from the sense amplifier.-The amplifier and comparator can be a single multiple amplifier package. The old standby, LM324 might work. There are better alternatives, so do your homework to see what works best for you. Input bias could be one thing to check.- The amp needs a supply of course and that would natiurally be the 12 V available. Do include a 100 nF supply bypass capacitor (ceramic) near the device supply pins.- The FET gate resistor should be something like 100 ohms and the grounding resistor between 1 and 10 k.- The diodes around the motor (not strictly necessary) and the relay coil can be vanilla 1N400x, the rest 1N4148 or similar, it is not critical.Good luck.

author: Zero999
author_rank: Super Contributor
timestamp: May 03, 2012, 09:42:10 pm
content: An interesting idea but it seems crazy to use a relay, unless this is a very large motor.Here's another idea which just uses a couple of transistors arranged as a bi-stable (Tr1 and Tr2) and another transistor to turn it off, after the voltage across the sense resistor exceeds the base voltage. The sense resistor could probably also be put on the other side of the Tr1 if you want the on voltage to contribute to the tripping characteristics.

author: Kremmen
author_rank: Super Contributor
timestamp: May 04, 2012, 05:24:30 pm
content: Crazy? Not at all. Of course there are numerous ways to replace the relay with something else, but hardly simpler. Relays are cheap, reliable and dead easy to get working right. Also the current measuring design is that generally used in motor control circuits (and PSUs). As such it is extendable to future cases as a general solution. The actual value of the sense resistor is decoupled from the limit circuit functionality so that no particular value of sense voltage is assumed. This frees the implementor to extrapolate the solution to any motor current and any allowable voltage loss, within reasonable limits.Most problems have many solutions. I considered mine a general solution to the question without making too many assumptions.

author: Zero999
author_rank: Super Contributor
timestamp: May 05, 2012, 08:14:50 am
content: Which circuit is simpler and more compact? The one with three transistors or the one with all those diodes, an op-amp and a big relay.Relays have their advantages and disadvantages over solid state components. They're electrically robust and have low on losses but are mechanical so will wear out and are more bulky.The circuit using a relay is better for large motors and the circuit with the MOSFET is better for small motors. The solid state solution also has the advantage of being modified to limit the peak current without tripping (i.e. acting like a constant current source to limit spikes) and it should be easy to integrate it into a PWM speed controller.One thing to note is that an electronic circuit shouldn't replace a fuse as MOSFETs tend to fail short circuit and it possible for a relay to become welded shut.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: May 05, 2012, 10:01:23 am
content: heyI think that the best solution is series pass n-mos , and crowbar circuit (as mentioned here )assuming that the mosfet would have been failed , the crowbar would have some delay ( in the order of mili sec or even less ) and when the crowbar would "punch" the fuse would tear ( a lesson I learn is- 12AH 12V lead acid battery can provide as high as 500A for 1-2 sec and as high as 1200A for a peake , so consider that in the crowbar SCR , or put some 0.12 ohm 10W resistor in order to limit that peake to about 120A or something like this , then put some 3 n-mos whose can handle like 80A peake each and I think you'll be within the S.O.A )note : I am a "begginer" . please take anithing in proportion here ! , oh and good luck with the circuit(BTW : I'd like to hear some criticism from the more experience in here - learn by mistakes)

author: Kremmen
author_rank: Super Contributor
timestamp: May 05, 2012, 11:29:47 am
content: Sorry but i don't understand why you say this or that is "better". To make that judgement you would need the baseline requirements for the circuit. There were 2 requirements:1: to make the motor stop.2: to execute the stop at a given current level, not specified in advance.That's it. I don't think that this would be an application where component life would be critical. At least that was not a requirement. Also it is not a high volume product where the smallest saving is significant.Both circuits do 1, but how does yours do 2? By changing the sense resistor? Is that supposed to be practical then?Sure, there are diodes in my circuit to ensure proper operation but so what. Their cost is next to nothing and their application couldn't be easier. Also there are relays the size of a sugar cube and cheaper than some regular to220 fets. So why would relays be only for "big motors". I have a stack of big Semikron and Infineon 400 amp IGBTs on my shelf and those are for big motors.And yes, the basic function could be accomplished with fewer components but that was not my main point. The circuit i made demonstrates the individual elements of the solution in an easily understandable way. The point then was that the OP might actually learn something instead of receiving a black box. I'm sure you think your circuit is a clever one and for a small enough application it might actually work. Coming from where i do, i would not consider it for motor control myself. But that is just me.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: May 05, 2012, 11:44:09 am
content: heythank youyou are right ... I shouldn't judge my own idea , I dont think my circuit is clever , but I know that's what I'll do for my own scooter ,anyway , thank you for the correction

author: Kremmen
author_rank: Super Contributor
timestamp: May 05, 2012, 12:01:13 pm
content: I didn't say it would not workjust that i would not use it. Whatever floats your boat, its not for me to judge.

author: dcel
author_rank: Regular Contributor
timestamp: May 05, 2012, 01:31:46 pm
content: Quote from: DarkZero on April 29, 2012, 09:45:02 amWell, since no one responded with the simplest solution, I guess I will.A resetable circuit breaker is the exact device you need.No need for anything else. A one component solution. Available from milliamps to hundreds of amps.Automotive industry have been using the auto-reset types for years, ie, window motors, seat mtrs, power mirrors, etc.You will want to source the manual reset version with a push to reset button. Choose your current rating and dont forget to check the data sheets for the dwell time ( time device takes to open at rated current).I designed one into a piece of outdoor power equipment to protect two linear actuators from operator abuse.The two motors were 25A max deadheaded and I wanted the breaker to open within five seconds, so 10A breaker was used. After about three consecutive actuations, the breaker would open within three seconds due to heating, again more info that will be in the data sheet.Hope this helpsChris

author: Zero999
author_rank: Super Contributor
timestamp: May 05, 2012, 08:55:24 pm
content: Quote from: Kremmen on May 05, 2012, 11:29:47 amYou're right, he didn't say what size his motor was so it was wrong of me to presume the MOSFET is the better solution.Quote from: dcel on May 05, 2012, 01:31:46 pmI agree and it certainly is the safest solution since it will be pre-approved and tested to trip reliably for a given fault current. You can also buy adjustable overloads which are good if you don't know how much current the motor will draw.Semiconductor circuits such as the ones discussed here should only be used in addition to a circuit breaker/fuse, not as a substitute. Given this one might say "What's the point of building a solid state motor trip?" The answer is they offer advantages such as being able to be reset, forcibly tripped or the current limit adjusted remotely or can do soft peak current limiting (i.e act like a constant current source for short overloads before tripping).

author: DarkZero
author_rank: Regular Contributor
timestamp: May 06, 2012, 10:31:20 am
content: thank you for all your solutions !I'm still going for Kremmen circuit because I am no expert in electronics and he offered understandable explanation i need something that will work at first then i can see how to refine it so i can learn some tricks along the way.Also i know about the devices that do the job but its not fun i like building circuits blowing them up or making them to work.