Current Limiting Device

Thread Starter

qasimta

Joined Nov 15, 2022
14
Hello,

Is there a ready made device that can be used to limit the current being consumed by any load. Specifically, I would like to limit the current going out from a car battery for a home project. I'm looking for something that is adjustable where I can select what is the maximum current that can be withdrawn from the battery.

I appreciate your help.

Best Regards,
Tariq
 

D_racon

Joined Oct 4, 2021
11
currenLimiter.PNGThis is a example of a current limiter used in a GVS device, this one is very simplistic and meant for very low currents 2ma to 5ma.
but I remember that when I was building a GVS at home I could also just buy of the shelf current limitters, however most of them where meant for higher currents starting from 20ma and going higher. they where commonly used in displays so often you find them for something like 10ma to 20ma low to a few hunderd ma high and then many of then like 8, 10 or 12 on one chip. you can almost certainly get higher current ones.

so there are cirquits to limit the current, and you can also easily make them yourself, ofcource you shouldn't make it like in that schematic since that one is for extremely low current, but you can make or buy one for higher current.
but you might add a limiter for every module you have connected otherwise if you limit it at a to low current the voltage and one of the devices isn't limited and wants to draw a lot that might then cause a voltage drop after the limiter which would result in possibly some devices not getting power because all goes to that one that draws the most.

in the past before PWM powerleds and such tended to also be controlled and protected by a current regulator cirquit, basically the same as above but then for much higher currents.

these current limiters however are quite inefficient, so you can search for a high efficiency current limiter. or try to make one yourself, you can make a self regulating PWM controller which measures or calculates/estimates the current which is being drawn, you can do this with a extra resistor on the output which goes to ground. so you measure and/or calculate/estimate the current then you change the pwm signal(make sure it is a high enough frequency so that it doesn't interfere with your devices) the pwm signal is a more efficient way of limiting the current, not the peak current but the average current. if you add a inductor however you can really limit the peak current with the pwm signal since the inductance will cause resistance but that might also generate high voltage spikes which you might not want. if you use pwm you also might want to add capacitors somewhere to stabilize the power to the devices.

if you can use pwm then that is a great way of limiting the current because it is much more efficient and makes sure the voltage actually stays the same. a variac or clasical light trimmer works like that, it is a pwm controller which limits the current by rapidly switching it on and of and keeping it on or off longer.
 

djsfantasi

Joined Apr 11, 2010
8,676
Just curious… Why do you want to limit the current?

A device will only take what current it needs from a battery. If your battery can supply 100A and your device only needs 100mA, the device only takes 0.1A from the battery. No current limiting is required.

If you want to protect the battery or device in the event of a failure of either of them, then what you want is a fuse, as @ronsimpson has suggested.
 

crutschow

Joined Mar 14, 2008
31,129
I can understand why you may want have a variable current-limit similar to what many bench power supplies have.

Below is the LTspice simulation of a circuit using an op amp and a P-MOSFET that has an adjustable limit from 0 to about 2A:

The current limit (bottom traces) for settings of the current control pot (U2) to give 0A, 1A, and 2A (VRef / R5) is constant for the change in load resistance (top trace).

The output voltage (middle traces) changes with the load resistance to keep the current constant.
The output voltage essentially goes to the input battery voltage when below the current limit.

The current limit Ref voltage is referenced to the battery voltage so any variation in that will cause a proportional change in the current limit.
If that's a problem a Zener or voltage reference can be added to generate a stable Ref voltage.

Note that the M1 MOSFET will need to be on a heat-sink if the circuit is limiting the current for more than a few seconds.

1668545571174.png
 
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Thread Starter

qasimta

Joined Nov 15, 2022
14
You cannot maintain a constant voltage with a limited current. Your device may cease to function if you do this.
Thank you Papabravo. But I have purchased an adjustable power supply that I can fix the current and voltage -- see attached picture --. So, how does this power supply work. I'm interested in the same circuit but to be connected to a car battery.
 

Attachments

Thread Starter

qasimta

Joined Nov 15, 2022
14
View attachment 280775This is a example of a current limiter used in a GVS device, this one is very simplistic and meant for very low currents 2ma to 5ma.
but I remember that when I was building a GVS at home I could also just buy of the shelf current limitters, however most of them where meant for higher currents starting from 20ma and going higher. they where commonly used in displays so often you find them for something like 10ma to 20ma low to a few hunderd ma high and then many of then like 8, 10 or 12 on one chip. you can almost certainly get higher current ones.

so there are cirquits to limit the current, and you can also easily make them yourself, ofcource you shouldn't make it like in that schematic since that one is for extremely low current, but you can make or buy one for higher current.
but you might add a limiter for every module you have connected otherwise if you limit it at a to low current the voltage and one of the devices isn't limited and wants to draw a lot that might then cause a voltage drop after the limiter which would result in possibly some devices not getting power because all goes to that one that draws the most.

in the past before PWM powerleds and such tended to also be controlled and protected by a current regulator cirquit, basically the same as above but then for much higher currents.

these current limiters however are quite inefficient, so you can search for a high efficiency current limiter. or try to make one yourself, you can make a self regulating PWM controller which measures or calculates/estimates the current which is being drawn, you can do this with a extra resistor on the output which goes to ground. so you measure and/or calculate/estimate the current then you change the pwm signal(make sure it is a high enough frequency so that it doesn't interfere with your devices) the pwm signal is a more efficient way of limiting the current, not the peak current but the average current. if you add a inductor however you can really limit the peak current with the pwm signal since the inductance will cause resistance but that might also generate high voltage spikes which you might not want. if you use pwm you also might want to add capacitors somewhere to stabilize the power to the devices.

if you can use pwm then that is a great way of limiting the current because it is much more efficient and makes sure the voltage actually stays the same. a variac or clasical light trimmer works like that, it is a pwm controller which limits the current by rapidly switching it on and of and keeping it on or off longer.
Thank you D_racon for the detailed explanation. Unfortunately, this is not my background. I don't have the skills to build circuits. That's why I'm looking for a ready made device.
 

Papabravo

Joined Feb 24, 2006
19,612
It is a common misconception that you can control both the current and the voltage in a load. What you can do is apply a fixed voltage to a load and it will take exactly the amount of current that it requires. If the load requires more current, then the fixed voltage, current limited supply can provide it will reduce the output voltage and provide a constant current at a reduced voltage. This is NOT the same thing as being able to control both the current and the voltage independently. Clearly if the device requires a fixed minimum voltage for proper operation this arrangement will not work.
 

Thread Starter

qasimta

Joined Nov 15, 2022
14
Just curious… Why do you want to limit the current?

A device will only take what current it needs from a battery. If your battery can supply 100A and your device only needs 100mA, the device only takes 0.1A from the battery. No current limiting is required.

If you want to protect the battery or device in the event of a failure of either of them, then what you want is a fuse, as @ronsimpson has suggested.
Thank you djsfantasi. I need to limit the current because I'm doing electrolysis experiment. The solution might draw more current, but I want to gradually adjust the current until I reach a predefined maximum current even if the conductivity of the solution can allow for more current to flow.
 

Thread Starter

qasimta

Joined Nov 15, 2022
14
It is a common misconception that you can control both the current and the voltage in a load. What you can do is apply a fixed voltage to a load and it will take exactly the amount of current that it requires. If the load requires more current, then the fixed voltage, current limited supply can provide it will reduce the output voltage and provide a constant current at a reduced voltage. This is NOT the same thing as being able to control both the current and the voltage independently. Clearly if the device requires a fixed minimum voltage for proper operation this arrangement will not work.
Please excuse my ignorance as this is not my area of study. It happens that I need to use some electric/electronic devices from time to time. So, in the case of the adjustable power supply I have. If I set the voltage to 12V and the current to 5A and the load requirements exceeds 5A do you mean the power supply will reduce the voltage and will increase the current? if so, why is it called current limiting if it is not going to limit the current?
 

Papabravo

Joined Feb 24, 2006
19,612
Please excuse my ignorance as this is not my area of study. It happens that I need to use some electric/electronic devices from time to time. So, in the case of the adjustable power supply I have. If I set the voltage to 12V and the current to 5A and the load requirements exceeds 5A do you mean the power supply will reduce the voltage and will increase the current? if so, why is it called current limiting if it is not going to limit the current?
No. Go back and reread what I said in post #12. When a load requires current in excess of the current limit, the supply will reduce the voltage in order to maintain the current at the current limit. The reason it must do this is the other limitation on power supplies and that is the power out must always be less than the power in.
 

ericgibbs

Joined Jan 29, 2010
16,803
hi,
That is why I added this note
You will have to manually set the V/I output required.
He will have to adjust the Vou/Iout to give the required current, the eBay specification does also say CC.

Step-down DC-DC Buck 10A Constant Current Adjustable Module 200W Power Supply
 

Danko

Joined Nov 22, 2017
1,492
Is there a ready made device that can be used to limit the current being consumed by any load. Specifically, I would like to limit the current going out from a car battery for a home project. I'm looking for something that is adjustable where I can select what is the maximum current that can be withdrawn from the battery.
DC Motor Over Current Protection Device Current Limit Switch 6V 12V 24V 5A
US $7.66

Feature:
DC Overload Protector used for overcurrent protection, short circuit protection, motor blocked protection, auto-stop overload protection.
The input voltage is DC 6V-28V.
Output current: 8A max (resistance load); 5A max (motor load)
The Protected Delay Time: 0-20s
The Current Limitation: 0.2A-10A
The Reset Time: 1-600s (automatic mode)
Include Power-on LED and current-limit-protected alarm LED.

Operating instruction:
Before you attempting to wire the protector, make sure power is off.
WARNING: DO NOT REVERSE POSITIVE AND NEGATIVE POWER LOADS.THIS WILL DAMAGE THE CONTROLLER.
1. Connect wires refer to the "wiring diagram" (NO. n picture).
2. Install the jumper to"Auto" or "Manual" modes. (Default is automatic mode.)
3. Set parameters of Current Limitation, Delay Time and Reset Time (automatic mode).
4. Turn on the power.

Current Limit Adjustment:
When the red alarm LED is lighting, eliminate the overcurrent factors, it will automatic reset (automatic mode), or push the Reset Button, reset the power (Manual Mode).
Reset the current limitation pot should as follow:
1. Turn off the power.
2. Adjust the Current Limitation pot until the desired maximum current is reached.
3. Reset the power.
1668619580685.png
 
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