# How I limit current flow to load?

#### Note Theeraphan

Joined Jan 15, 2018
21
Hi everyone,
I try to design circuit to limit current flow through the load
Condition:
-Current source : 0-30mA
-need to limit load current: 0-10mA

#### dl324

Joined Mar 30, 2015
17,003
If the current source is adjustable from 0-30mA, why can't you just use the 0-10mA portion of the range?

#### Note Theeraphan

Joined Jan 15, 2018
21
If the current source is adjustable from 0-30mA, why can't you just use the 0-10mA portion of the range?
I assume for abnormal case, if someone supply current more than maximum load spec (more than 1mA)
current flow through load should be 1mA, the rest of current from source will be managed by some limit circuit

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#### wayneh

Joined Sep 9, 2010
17,498
Depending on what you need (do you really need variable current in the range of 0-1mA? and what sort of voltage does the load need), there are different solutions. I'd use an op-amp based constant-current controller because I've built one before and know how it works. But you might get away with other simpler tricks.

#### Note Theeraphan

Joined Jan 15, 2018
21
Depending on what you need (do you really need variable current in the range of 0-1mA? and what sort of voltage does the load need), there are different solutions. I'd use an op-amp based constant-current controller because I've built one before and know how it works. But you might get away with other simpler tricks.
yes, I really need variable current in the range of 0-1mA, load is resistive so voltage depending on current that flow through it
and this voltage will connect to ACD

#### ebp

Joined Feb 8, 2018
2,332
"shunt" in the following means to make the current go around something via another path, not a resistor used to develop a voltage proportional to current

When the source "wants" to deliver more than 1 mA is it OK with being prevented from doing that or must it be allowed to deliver up to the full 30 mA?
How accurate must the 1 mA limit be?
Is there any source of power available for a control circuit other than the current source?
What voltage is expected across the load when the current is 1 mA?
What voltage range would be expected from the source?
How fast must the limiter respond?

If the source must be allowed to deliver 30 mA then clearly you must use a shunt to limit the current through the load rather than series circuit.

If precision of the 10 mA is not critical, constant current diodes (actually based on JFETs) could be used - they are essentially "on" at some low resistance until their nominal operating current is reached. Such diodes are a bit expensive and there are very few manufacturers anymore. The diode would be in series with the load, so the source would not be able to deliver more than the rated diode current. Some adjustable voltage regulators and references can be configured to do an essentially similar function, though the voltage burden would be higher.

Again depending on accuracy and allowable voltage burden, a single bipolar transistor can be used to make a shunt. The accuracy is subject to initial tolerance and varies with temperature.

High precision will demand a voltage reference and may require an external power supply depending on allowable voltage burden.

#### wayneh

Joined Sep 9, 2010
17,498
yes, I really need variable current in the range of 0-1mA, load is resistive so voltage depending on current that flow through it
and this voltage will connect to ACD
In that case I'd definitely look to the op-amp circuit. You'll use a reference voltage on one pin, the load voltage on the other, and the op-amp will drive the current through the load to produce the reference voltage across the load. What sort of voltage range do you expect to see when the load is drawing 1 mA?

#### crutschow

Joined Mar 14, 2008
34,686
What is the maximum voltage of the source and what is the load voltage at 1mA.

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#### Note Theeraphan

Joined Jan 15, 2018
21
In that case I'd definitely look to the op-amp circuit. You'll use a reference voltage on one pin, the load voltage on the other, and the op-amp will drive the current through the load to produce the reference voltage across the load. What sort of voltage range do you expect to see when the load is drawing 1 mA?
load is variable resistor 100-1k Ohm
expect output voltage range 0.1-1V (@1mA)

#### Note Theeraphan

Joined Jan 15, 2018
21
What is the maximum voltage of the source and the what is the load voltage at 1mA.
it is current source and load is variable resistor 100-1k Ohm
expect output voltage range 0.1-1V (@1mA)

#### Janis59

Joined Aug 21, 2017
1,855
For any aim of limiting current exists a few standard methods:
1) put the resistor in series with voltage source
2) organize the LDO with current sense negative loop
3) use the PWM circuit with negative current sense loop

#### Bordodynov

Joined May 20, 2015
3,182
See

#### wayneh

Joined Sep 9, 2010
17,498
Here's a crude sketch of the op-amp approach I'd use. The component values are very rough but approximate your situation.
R2 is shown as a fixed 10K but this is where you'd put a 10K variable resistor to set the non-inverting pin voltage to 0-1V.
R10 is your load. The op-amp will maintain the voltage on top of R10 at the same voltage as on the top of R2, 0-1V.

#### crutschow

Joined Mar 14, 2008
34,686
it is current source
Yes, we've established that.
My question still is, what is its maximum output voltage?

#### Note Theeraphan

Joined Jan 15, 2018
21
Yes, we've established that.
My question still is, what is its maximum output voltage?
maximum output voltage is 1V

#### ebp

Joined Feb 8, 2018
2,332

#### crutschow

Joined Mar 14, 2008
34,686
maximum output voltage is 1V
I'm referring to the maximum current source voltage when it's open circuit (no load).
I would imagine that's more than 1V.

#### Plamen

Joined Mar 29, 2015
101
If the current source is adjustable from 0-30mA, why can't you just use the 0-10mA portion of the range?
The simplest is a N channel JFET with adjustable resistor G to S, with + wire from D, - wire from G (actually from S via resistor).
JFETS have a natural current limiting properties, with value adjustable by biasing. In this case the resistor in the S acts as current sense resistor and the voltage drop across it is applied to G. This is a current negative feedback further adding to the constant current capabilities of the JFET.
Note that the current could be adjusted up to the 0 bias current of the JFET. Pick a type with Drain current at 0 bias above your requirement.
Note that JFETs have substantial part to part variations in their current at zero bias (and at any bias) i.e. not suitable for production with tight tolerances.

#### shteii01

Joined Feb 19, 2010
4,644
current divider would be the cheap and easy, but may not be efficient.