Can a constant current circuit be built using LDO?

小太阳yy

Can a constant current circuit be built using LDO? [Copy link]

 

Teachers, what is the principle of this constant current source circuit built with LDO? What is RL? Variable resistor? How to achieve constant current?

bigbat

Building a constant current source generally requires an adjustable LDO device, such as LM317, LM1117-ADJ and other chips. The fixed value chip should be a voltage source. If you want to make a constant current source, you need a variable "resistance" adjustment circuit.

maychang

"Can a constant current circuit be built using LDO?"

Yes, but the accuracy is not high. The first picture is an example.

maychang

"What is RL?"

It is the load that you want to supply constant current to. Of course, the load can change, and it is not controlled by you. The so-called constant current means that the current passing through the load remains basically unchanged when the load changes within a certain range.

maychang

"How do you achieve constant current?"

The working characteristic of this type of three-terminal voltage regulator chip is to keep the voltage between the output pin and the common pin unchanged. After connecting to the circuit as shown in the figure, the current passing through the load RL is equal to Vxx/RA, plus the chip's own current consumption (marked as Iss in the figure). Usually Iss is very small and can be ignored, so the current passing through RL can be considered to be Vxx/RA. If Vxx does not change, RA does not change, and the current passing through RL does not change, achieving constant current.

小太阳yy

maychang posted on 2022-7-1 12:22 "How to achieve constant current?" The working characteristic of this type of three-terminal voltage regulator chip is to keep the voltage between the output pin and the common pin unchanged. Connect...

Teacher, I have an idea. Currently the load is placed at the back end. Is it okay if the load is placed at the front end of the LDO? Is there any advantage?

Is the current flowing into the LDO equal to the current flowing out of the LDO?

maychang

小太阳yy posted on 2022-7-1 13:35 Teacher, I have an idea. Currently, the load is placed at the back end. If the load is placed at the front end of the LDO, is it okay? Is there any advantage? Flowing into the LDO ...

"Currently the load is placed at the back end. Is it okay if the load is placed at the front end of the LDO? Is there any advantage?"

I think it is possible to put the load at the front end (the positive end of the power supply to the input end of the three-terminal voltage regulator chip), although I have never seen such a usage.

As for the advantages, there may not be any. In the first picture, one end of the load is grounded (the negative end of the power supply). If the load is placed at the front end, one end cannot be grounded.

maychang

小太阳yy posted on 2022-7-1 13:35 Teacher, I have an idea. Currently, the load is placed at the back end. If the load is placed at the front end of the LDO, is it okay? Is there any advantage? Flowing into the LDO ...

"The current flowing into the LDO is equal to the current flowing out of the LDO, right?"

The current flowing into the three-terminal voltage regulator chip includes the current consumed by the chip itself (flowing out through the GND pin of the chip) and the current flowing out of the chip. The current consumed by itself is marked as Iss in the first figure, which is usually called static current.

maychang

小太阳yy posted on 2022-7-1 13:35 Teacher, I have an idea. Currently, the load is placed at the back end. If the load is placed at the front end of the LDO, is it okay? Is there any advantage? Flowing into the LDO ...

The second floor bigbat introduces the use of LM317. The advantage of LM317 is that its quiescent current is very small, the output voltage is adjustable, and the minimum output voltage is relatively small.

maychang

小太阳yy posted on 2022-7-1 13:35 Teacher, I have an idea. Currently, the load is placed at the back end. If the load is placed at the front end of the LDO, is it okay? Is there any advantage? Flowing into the LDO ...

This circuit is a modified version of a three-terminal voltage regulator. The chip has large power dissipation and poor stability, and is not as good as a dedicated constant current circuit.

chunyang

In addition to the accuracy problem, the biggest disadvantage of using LDO to build a constant current source is that it is affected by the input voltage range and its applicability is relatively limited. If you want to use LDO to build a constant current source, it is better to use a traditional three-terminal regulator, such as the 78 series or the adjustable LM317, which can tolerate a much higher input voltage than LDO.

Using a three-terminal voltage regulator to build a constant current source was a simple method commonly used in the past for DIY nickel-cadmium and nickel-metal hydride battery chargers. The materials are easily available and the circuit is simple.

chipsimi003

The load is placed at the front end of the LDO. This circuit can usually be used on LEDs, but there are certain requirements for the constant current value.