# [Circuit Design] current limiter of 40A

Discussion in 'The Projects Forum' started by BobMarley, Mar 12, 2013.

1. ### BobMarley Thread Starter New Member

Mar 12, 2013
8
0
hello,
i need to design or to buy a circuit that limits the current to 40A.
i have searches in the internet for such a circuit and i have not found any. if somebody knows some place to find, please tell me.
my problem is the following~:
i have a battery :
http://hobbyking.com/hobbyking/store/__6237__Polyquest_2550mAh_4S_25C_Lipoly_Version_2_.html
and an ESC:
Phoenix Ice2 HV 80 , : http://www.castlecreations.com/products/phoenix-ice2-hv.html

and i need to limit the current to the motor (or load) to 40A. that is when the i have the system at work, and if at some time the load is requiring more that 40A, the current limiter must not allow to go to it more that 40A.

so how do i design such a system? can i do it with discrete components?

i thank you in advance for the help
best regards

2. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,791
976
Why not simply buy a properly rated motor and ESC?

3. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
Yes you can. A BJT is a current controlled current device. A FET is a voltage controlled Current device. You can measure the current in a load with a low value resistor such as 0.1 Ohms. 40 amps will produce a drop of 4 Volts.

So when you detect the over current condition you reduce the drive current or voltage to the transistor or FET so that it won't allow more than 40 Amps.

4. ### John P AAC Fanatic!

Oct 14, 2008
1,638
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A 4 V drop at 40 Amps dissipates 160 Watts! That will be some serious resistor.

I suggest a much smaller resistor or a Hall sensor. The resistor could be in the form of a known length of ordinary wire, where the voltage drop will be proportional to current. It won't be exact, but it might be good enough.

Then the transistor has a similar problem: the higher the voltage drop, the more heat it will produce. Maybe you could put a thermal sensor on the transistor and when it gets dangerously hot, it turns off completely.

5. ### #12 Expert

Nov 30, 2010
16,704
7,351
I have been very successful in using ordinary THHN wire as a 30 amp resistor.

6. ### BobMarley Thread Starter New Member

Mar 12, 2013
8
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Papabravo,"So when you detect the over current condition you reduce the drive current or voltage to the transistor or FET so that it won't allow more than 40 Amps. "
I need it to be automatically done.
John P, are there some hall efect sensors that i could use and that could somehow dont allow the load to have more that 40A?

well, basically i need a ciruit that is not allowing to pass more that 40A.
if the load asks for 50A for 10 minutes, the current limiter must not allow to pass more that 40A, if the load asks for 30A, the current limiter allows to pass all that current.

i found this thread, and kubeek, sugested there one current limiter, which is:
so i would like to know if that current limiter is fine for me.

and also, are there some currnet limiters avilable on the market that would do my job?
i have been looking in the rs electronics, farnell, texas instruments websites and nothing found.

7. ### John P AAC Fanatic!

Oct 14, 2008
1,638
225
No, of course not. You might as well ask "Is there a thermometer that will keep the temperature in my house comfortable?" A measurement element can certainly be part of a control system, though.

8. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
There was nothing in what I said that precludes an automatic reduction in driver current. In fact I assumed that was a requirement. What made you think otherwise?

9. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
There is a bit of a problem with a simple approach.

The motor controller is for a 3 phase/BLDC motor. Measuring current would require measuring all 3 phases, summing them, then limiting in some manner.

If you are measuring/limiting the current prior to the controller, a current limiter may reduce the voltage below the dropout voltage of the motor controller/ESC.

In other words, when the motor is pulling >40A, it's probably due to needing that much current to avoid a crash.

The best overall solution would be buying an ESC rated at the wattage of the motor you are using.

You may be able to adjust your transmitter limits so the max current isn't exceeded when the stick is at maximum position, or limiting in software setup what the transmitter is asking the ESC to do when the current is exceeded.

10. ### BobMarley Thread Starter New Member

Mar 12, 2013
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Papabravo, i have googled and searched in the forum, and yes i have found some people saying that using tbj vould do the job, but they also mentioned that it is for small currents, so can they handle 40A? and eventualy 11.1 V?
thatoneguy, i am going to use the Axi 2826/10 which is a brushless dc motor, so i think that i will not need at all a complex system for the limiter.

11. ### crutschow Expert

Mar 14, 2008
13,501
3,375
In this thread I show a current limit circuit that might work for you. You would need to change R2 to give the desired current limit and select a MOSFET for the required current and voltage rating. For long term operation at the limit current the MOSFET must be mounted on a heat sink.

What is the supply voltage?

12. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
Have you tried the current limiting feature built into the ESC?

Why use an 80 Amp Controller with a 40 Amp max motor?

13. ### BobMarley Thread Starter New Member

Mar 12, 2013
8
0
crutschow, thanks for the help. i will try to find suitable mofts, tbj and resistor to do my job, after that i will write here my solution. we are going to use a battery of 11.1V, like this one:
http://www.hobbyking.com/hobbyking/stor/__8915__ZIPPY_Flightmax_2650mAh_3S1P_30C.html

thatoneguy, yes, there are ESC that have the ability to limit the current when it is reached the 40A. we dont want to use such an ESC because:
1) it gets very hot when there are passing about 40A, so the ESC needs a very good refrigeration to keep it in the suitable conditions to work.
2) this ESC that is limited to 40A in reallity is limiting it to slightly bellow this value, it is something like 38,5. and i want to feed the load with all the permissible power.
3) all the ESC that we have found when they reach 40A they do not just allow to pass 40A. when they reach 40A the ESC cuts the supply,so there is not more power delivered to the load. i dont want to have such a situation given that i will have this limiter in a aeromodeling aircraft.
so i want to build something like a addon for the ESC, and this addon will not allow to pass more that 40A.
for example if the load will asks for 10 minutes 50A, the current limiter will allow to pass just 40A during all this time. if the load asks for 30A for 10 minutes, the current limiter will allow to pass in this case all the current, that is, the 30A.

14. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
Most of those issues will appear in a home built current limiter as well. Especially the heat dissipation problem. The other downside is that home built "simple" limiting circuits tend to fail more frequently. Thermal management alone is a headache at the power levels you are dealing with, as you discovered with the ESC.

The software for your ESC looked like it would PWM limit the current to a fixed value, that's the only way to do it without excessive heat. If there is a calibration problem, such as being set at 40 and allowing 38, set it to 42 as a limit?

15. ### BobMarley Thread Starter New Member

Mar 12, 2013
8
0
thatoneguy, the best aircraft ESC that i found and that can be configured are those from Castle Creations. the software that they have can configure the ESC not to ultrapass the 40A but it will cut off the current when it is reached that limit. And if my aicraft will be in air, i dont want in any case the current to be down.
i have googled very much looking for such an ESC that u describe but i have not found any one suitable for me. so i decided to try to design my own current limiter.

i found also at this company http://www.subtech.co.in/ a current limiter. but again it cuts off the current when it is reached the 40A. furthermore it allows for about 5 seconds the 40A to be ultrapassed, and i dont want also this characteristc.

do u know some home build current model(as u mentioned) that i could see if it is suitable for me?
some people told me that the the current chargers have this ability to limit the current. i have also seen some current chargers in the farnell website and i dont know how i could adjust the to my situation or get from them what i need.

16. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
Current limiting circuits are simple, if the heat can be dissipated and the current is low (< 1A).

When dealing with higher currents, pass transistors are biased to allow only enough voltage through so the current maximum isn't exceeded.

If putting a current limiter in front of the BLDC controller, the controller may think the battery is dead, get "confused", or simply stop working.

Attempting to limit the current in the 3 lines to the motor is more problematic. See this App note on BLDC control.

There are two major obstacles:
1) The motor runs on the difference between the 3 phases, not on the difference between each of the 3 wires and a reference ground. Transistors need an input that is "common ground" with the output, this makes the design not feasible, unless limiting is not done prior to the generation of the 3 phase power, i.e. In the ESC itself.

2) The signals are push-pull or bi-polar, AC in nature. Transistors only function in one direction.

17. ### BobMarley Thread Starter New Member

Mar 12, 2013
8
0
thatoneguy, thank you for the very usefull answer . it is very elaborated.
so, if i put the current limiter in front of the ESC, why it should think that the battery is dead? there are a lot of different batteries,
see for example this battery:
http://www.hobbyking.com/hobbyking/store/__11946__Turnigy_nano_tech_2650mah_3S_35_70C_Lipo_Pack.html

this battery has one property: --> Discharge: 35C Constant / 70C Burst
http://batteryuniversity.com/learn/article/what_is_the_c_rate

so in the case the current limiter will be assembled in front of the ESC, i think that when the ESC will ask for more current, it will not get it, given that there are also a lot of batteries that have low C parameter. and the ESC has no knowledge about the battery (you can check for example this ESC:
http://www.castlecreations.com/products/phoenix-edge.html
as u can see from the table of properties, it just works with a series of batteries acording to its specifications).

anyway, i would also to try to do my own ESC. in the document that u provided me from the freescale there are some ideas of how to build one, but i would like to know some other articles or books ,. etc from where i could see in more detail of how do make my own ESC.

18. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
It's Ohm's law based, sort of (when dealing with pulse power it's not as simple, but calculations can be done assuming a long enough time period).

I=V/R

Current is Voltage divided by Resistance. If we assume the load appears as a ≈250mΩ (0.250 Ω) resistor, the only way to reduce the current is to lower the voltage.

What is the working range of the controller input voltage? If you are "telling it" to go full speed, the PWM will be 100% for each phase, to limit the current to 40A, the supply voltage would need to be reduced to 10V. That was a rough estimate for a Thevenin equivalent of the ESC+Motor for the resistance, it's actually a bit more compilcated than that.

Building your own ESC is orders of magnitude more difficult that this topic, especially one with the features yours has. The cost would be higher per unit, physically larger, and generally not perform as well.

Microcontrollers in critical power applications (where failure could cause human injury) is an area I get nervous with. Inside the ESC, there is simply an amazing amount of work done behind the scenes that the user isn't aware of. The current limiting, software wise, with the built in microcontroller is probably the best bet. There may be an ESC around that when you program 40A, measures the input current, if it is 40A, the motor phases won't change in amplitude or duration until the demand is for less.

Again, I'd suggest: Getting a battery, motor and controller all mutually compatible with each other, using software limits for controlling the current, or limiting the range of control in the transmitter itself.

19. ### BillB3857 Senior Member

Feb 28, 2009
2,402
348
What am I missing? The OP has a 2550mAh battery and he says he wants to pull 40A for 10 min. Somehow that just won't compute for me.