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

Thanks in advance for your advice^^

 

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

 

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

 

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.

 

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

 

"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.

 

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?

 

What is the maximum voltage of the source and what is the load voltage at 1mA.

 

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)

 

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)

 

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

 

See

 

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.

 

it is current source

Yes, we've established that.
My question still is, what is its maximum output voltage?

 

Yes, we've established that.
My question still is, what is its maximum output voltage?

maximum output voltage is 1V

 

Ψ

 

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.

 

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.

 

current divider would be the cheap and easy, but may not be efficient.

 

Hi All
thank you for you suggestion