Limiting 12V DC current to 40A - Page 1
Forum section: electronics / projects

author: adauphin
author_rank: Unknown
timestamp: July 20, 2013, 02:43:01 am
content: What I'm working on is my son's ride-on toy and has dual 12V 21-turn motors, these motors are an upgrade from the originals and pull about 15-20 more amps when approaching half-speed or getting close to stall. There are two circuit breakers rated at 30A each, one at the battery, one after the speed selector before the motors.The new motors will trip the breakers on a small hill with two kids, I swapped the breakers for 40A ATC fuses and they only blow when bogged down in the grass and this is about at half the rated RPM and thus the high current draw.What I would like to do is limit the max current to about 35A, 40A max but I think 35A is safer. The problem is when pulling hills the watts will be roughly 500 so this makes chassis mounted resistors not only large, but expensive.I have some room to mount a regulating device but not a resistor like this..... just too bighttp://www.galco.com/buy/Ohmite/PFE5KR370EWhat options do I have? I guess I could use dual resistors rated at half of what I need.

author: adauphin
author_rank: Unknown
timestamp: July 20, 2013, 03:03:51 am
content: Actually I probably could mount an 18" long Ohmite resistor but cooling could be an issue. The area is enclosed so a fan would be necessary. How hot do these resistors run when absorbing about 15 amps on 12v?

author: C
author_rank: Super Contributor
timestamp: July 20, 2013, 03:05:23 am
content: How about thisAllegro 50 to 200 A Integrated Conductor Sensor ICs with galvanic isolationhttp://www.allegromicro.com/en/Products/Current-Sensor-ICs/Fifty-To-Two-Hundred-Amp-Integrated-Conductor-Sensor-ICs.aspxC

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 20, 2013, 03:35:51 am
content: Hack into the PWM circuit and have it automatically reduce the duty as the current rises.

author: IanB
author_rank: Super Contributor
timestamp: July 20, 2013, 04:21:32 am
content: Yes, you don't really want to use a series resistor when the motor is bogged down as this will limit torque and waste power. Imagine you're trying to climb a steep hill and someone tries to "help" you by sapping your energy with extra resistance...The answer is to change the gearing of the motor. This can be approximated electronically by adjusting the PWM duty cycle as Mike said, so that the motor gets full voltage, but limited power.

author: adauphin
author_rank: Unknown
timestamp: July 20, 2013, 06:35:00 am
content: I believe I understand the Allegro but does this tie into another device to limit the current? I'm not well-versed in EE but am a quick learner. Are we trying to cycle the current on/off rapidly to control power like an ESC?Here's a schematic of the wiring, from what I understand when in LO, both motors share the 12V and receive 6V, and when in HIGH, both receive 12V. The foot switch (accel pedal) is only an on/off switch, the shifter controls the motor bias.There has been installs of a hall-effect pedal from a scooter, as well as a compatible ESC, can the Allegro work in conjunction with these?Hall-effect pedal:http://tncscooters.com/index.php?route=product/product&path=65_79_83&product_id=346ESC:http://tncscooters.com/index.php?route=product/product&path=41_73&product_id=159The ESC listed is rated at 30A, may work but if I can get one of the 48V controllers to work with the pedal I may try that route since the current handling is much higher.Edited to add, this may be the more ideal ESC:http://tncscooters.com/index.php?route=product/product&path=41_75&product_id=186

author: Psi
author_rank: Super Contributor
timestamp: July 20, 2013, 08:09:22 am
content: What about a PTC (auto reset fuse).Digikey have one with a hold current of 30A and a full trip current of 50Ahttp://www.digikey.com/product-search/en?x=-1241&y=-73&lang=en&site=us&KeyWords=MHP30-36-T-NDI imagine if you used a PTC you wouldn't have to worry about it.Any loading above the hold current and the PTC would begin to act and bring the vehicle to a stop, then automatically recover a few minutes later after it cools down.Now that i think about it, If your poping 40A breakers you may have a fair bit more current than that flowing. You may want to spec a bigger PTC (or two in parallel).You might need to do some testing.Just be sure not to mount the PTC against plastic or it might cause melting.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 20, 2013, 08:47:13 am
content: Filament bulbs can work as current limiters as they have low resistance when cold - e.g. a few 12v halogen bulbs in parallel, connected in series with the load might be worth a try.

author: dr.diesel
author_rank: Super Contributor
timestamp: July 20, 2013, 09:11:13 am
content: You can get high current PWM controllers off ebay for ~$25 bucks. The ones used for pocket bikes etc.

author: Marco
author_rank: Super Contributor
timestamp: July 20, 2013, 10:06:21 am
content: Quote from: adauphin on July 20, 2013, 06:35:00 amNice price, but it has undervoltage protection ... so not much good for you unless you can reconfigure the batteries to provide 48 Volt.Since the motors aren't designed for 48V you're also looking at much higher peak currents and low duty cycles ... effectively it's only a 25 amp controller if you want to limit the voltage to 12 volt average to protect the motor.The only off the shelf unit I can find which suits all your needs is this one :http://www.motiondynamics.com.au/12v-48v-dc-pwm-with-current-limiting-single-direction-30a-or-50a-pcb-model.htmlPS. yes these all work like ESCs, they use PWM to control the average voltage the motor sees and have a shunt to measure the average current for current limiting ... in theory you could build your own, but it seems a bit ambitious for a first electronics project.

author: Paul Price
author_rank: Super Contributor
timestamp: July 20, 2013, 12:49:27 pm
content: Are you willing to build a few-parts simple switcher yourself that has low-power waste and just one 14-pin IC comparator, and a cheap but high current inductor and power n-chan MOSFET?

author: Marco
author_rank: Super Contributor
timestamp: July 20, 2013, 01:22:43 pm
content: I wouldn't use a plain MOSFET, dual BTN7971B boards can be had pretty cheap and the ICs come with build in current sensing and drivers (current sense output is not generally broken out, but that can be bodged). More importantly, they are half bridges so you retain the braking capability of the standard controller.Still need a PWM signal generator and some interfacing with the throttle pedal of course.

author: C
author_rank: Super Contributor
timestamp: July 20, 2013, 02:06:09 pm
content: Quote from: adauphin on July 20, 2013, 06:35:00 amadauphinFrom your description,The shifter could be a simple switch that changes the wiring of the motors from series to parallel with the accel pedal an on/off power switch. With the breakers to protect both the battery & motors from over current harm. Breakers were used in place of fuses due to the many repeated overloads. This is the cheapest way to get what you describe. This circuit has no low voltage protection for the battery at low battery charge which can shorten battery life.The next step up with best battery power savings is variable speed drive using a Pulse Width Modulation(PWM) controller. The fast On/Off of PWM lowers the voltage the motor sees changing the motor speed. This also lowers the motor current. The ESC is one version of this.Originally a pot or variable resistor was connected to the operators speed input & to the controller. The pot or variable resistor has a high failure rate so a hall-effect based version is used today. The hall-effect based control output could act like the original pot it replaced or a voltage divider. The site you link to does not supply this information.The controller's electronics will expect a specific battery voltage input range & will have a current limit. The controllers speed input will also have specific requirements. The cheapest controllers may be nothing more than a fast on/off switch that the speed input can change the ratio of on to off times. The better controllers do more & some will have a current sensor that is used to help protection( the on/off switch, the battery, the motor) and may change the speed input to act more like a cruise control of a car. All the parts connected to the controller must be selected to work together(battery, speed input, motor).The change from 30A breakers could cause the existing switches to fail or have a shorter life.With the proper information the Allegro current sensor's output could be used to reduce the speed input to a controller and by this reduce the current to motors. With the current 12 Volt battery, you will need a 12 Volt controller that can handle 2x max motor current or you risk the controller failing. This method reduces battery power waste. Some controllers will have the current sensor built in and not need the added sensor.

author: adauphin
author_rank: Unknown
timestamp: July 20, 2013, 03:01:20 pm
content: Quote from: Psi on July 20, 2013, 08:09:22 amI thought about re-wiring the entire setup using dual leads everywhere to reduce the strain on each individual wire and connector, thus going to multiple breakers was an idea. I could solder an additional female spade to the shifter buss where I needed an extra lead.The battery harness on the toy side of the battery gets warm when in operation, not hot so it hasn't concerned me yet, possibly high resistance in the small connector tabs and changing out that connector is an option as well.Quote from: Marco on July 20, 2013, 01:22:43 pmWould these boards handle up to 14V? I'm just basing that on a fully charged battery with some headroom.That looks like something I would be willing to try.Quote from: Marco on July 20, 2013, 10:06:21 amI didn't notice the undervolt protection, was only browsing but obviously that changes things. Could this be bypassed?The motiondynamics controller looks exactly like what I need, could I use a hall-effect pedal on this, or is it not needed? Darn thing is pricey though but it's worth a try or I could possibly get 10 other buyers to go in on it.What would it cost to build one, from scratch is it worth building or are they cheaper to buy as is? This is something I feel comfortable attempting, could even get used to the Tek 465B I have sitting aroundQuote from: mikeselectricstuff on July 20, 2013, 08:47:13 amThis has been considered and is easily worth a try, just need to keep the heat down to a minimum and a fan-cooled box could be built.Quote from: NiHaoMike on July 20, 2013, 03:35:51 amSounds fairly easy but this is where I'm poking around above my pay grade...lol However this is what I want to learn how to do.I appreciate all the help, I trust we can get this to work, There is an upgraded 6-pin shifter module that eliminates the excessive switches and wiring and may be what I need to tackle this project with minimal clutter.

author: adauphin
author_rank: Unknown
timestamp: July 20, 2013, 03:12:52 pm
content: One thing I need to mention is one motor is operated in reverse. The upgraded motors I believe have not only fixed timing, but the reverse motor is specifically a reverse motor so timing is likely optimized in that direction, the stock motors are universal.Looking at this controller could I use a hall-effect pedal and combine both controllers? I believe I would need one for each motor.http://www.ebay.com/itm/High-Current-DC-Motor-Driver-Module-Board-H-Bridge-PWM-Signal-Control-NEW-/321045121657?pt=LH_DefaultDomain_0&hash=item4abfc7ca79

author: Marco
author_rank: Super Contributor
timestamp: July 20, 2013, 03:51:57 pm
content: You can use one board for two motors you split the motor outputs between the two motors, and connect their other connections to ground, then you drive both inputs with a PWM signal just like the page tells you not to doThere are also boards specifically for two motors on ebay and the net with BTN7971B and BTS7960B ICs. The mode of operation described on the page allows for electronic reversing through a full bridge, but you can presumably use the existing controller (probably just a chunky two pole switch) to reverse the polarity across the motors to reverse so you can use both half bridges separately.As I said before, you need to break out the current sense pins from the board and add two resistors ... then you need to find the max of the sensed currents (two diodes and an opamp buffer) and smooth it with an RC filter for feedback for the PWM generator ... this would be usable with the hall sensor throttle pedal, but in the end you are going to need to build your own circuit for all this out of a TL494 and some opamps.PS. on second thought maybe reversing with the existing circuity is not too smart, it probably just provides the same voltage to both motors ... which will put the controllers in parallel which is not generally a good idea and won't allow you to sense current to individual motors to cut power when just one motor is stalling. Using two controllers with electronic reversing might be better (a high current four pole double throw switch isn't cheap either).

author: adauphin
author_rank: Unknown
timestamp: July 20, 2013, 05:12:22 pm
content: I ran a few tests today using my Fluke clamp and with both batteries connected I sent two kids up the driveway in LO and HIGH.My driveway is roughly a 8-10% grade near the top. In LO, it pulled 22-23 amps and was struggling near the top, about 1 MPH.In HIGH, it pulled 35-40A but near the top it was pulling 47-52A and just started to show signs of tripping the breaker, the 40A+ load lasted about 5-7 sec.Then I removed one of the batteries from the circuit and the HIGH load came down slightly to a peak of around 49-50A.Looks like I only need to dump about 12A which isn't too bad. I may try the light bulb dump first and see if it works.Edited to add....that's about 1 hour to exhaust the battery....may not be the best choice...hmmm.

author: adauphin
author_rank: Unknown
timestamp: July 20, 2013, 05:37:14 pm
content: Quote from: Paul Price on July 20, 2013, 12:49:27 pmI suppose it depends on price but yeah, I'm open to all suggestions.Could I build this to connect at the battery and let the rest of the wiring work as intended, with only limiting the current? I could add a 1-4V pedal if needed leaving all the shifting and reverse wiring alone.

author: adauphin
author_rank: Unknown
timestamp: July 20, 2013, 11:53:28 pm
content: What about using a series of 60A relays and have the stock wiring switch the relays? This would free up the load from the factory harness and be only used as a switching signal.Run some 10ga wire from the battery, through the relays, to the motors. Not sure how to draw it up but if we can still run LO through a series circuit then 12V through a parallel then I want to think it's possible.There's plenty of room to run extra wire and store a tray of relays.