# Need help desiging a 3.0A current limiting circuit

Hello,

First post here. I am developing several PCB designs concerning modeltrain electronics. I design in Kicad and I can succesfully order PCB with SMT assembly from JLCPCB.

One of my designs for a DCC central carries 2 H bridges. I want to supply the board by a 4A or 5A meanwell PSU @ 18VDC.

The 18V makes it’s way to the train tracks via an H-bridge. The 18V is also tranformed to 12V via a buck converter. And I use a linear supply of 5V to supply an atmega328. The 12V is used to provide power for 2 bus systems.

My design lacks one thing and that is a current limiting circuit. The main tracks often experience a short circuit. We allow this to last for roughly 50ms. This is to filter out rush-in current of some engines. My previous method of limiting current, was simply to use 3A or lower PSU.

During a short circuit, the PSU is essentially also shorted. The 18V drops to low levels and this also allow the 12V buck converter to drop far below 12V.

I want to solve these 2 problems

• I do not want the full 5A to run at the tracks during a short, despite the brief time. The current to the tracks must not exceed 3A for longer than 50ms.
• I do not want the 12V to drop below 12V, (so that 18V has to be higher than 13V)

I have a diode in place with 3 capacitor to allow the buckconverter to survive the short. If the 18V drops, the buck will go on for the 50ms.

Using 3 1206 shunt resistors of 0R1 I can measure the current of an H bridge and switch of the power. But I still need to limit the current to 3A. With a PSU that can supply 4A or 5A the 18V should not drop significantly during a short. And I believe that my PSU will simply last longer if you don’t keep shorting it all the time.

I want to limit the current to 3.0A and switch off when the current exceeds 2.9A. So during a short current cannot exceed 3.0A and I can run up to 2.9A continously.

I obviously found circuits online, but I do not know how the heat develops in the transistors if you keep them 100mA below the absolute maximum. Most circuits were for lower current ratings

Can somebody help me in designing a current limiter circuit which limits currents to 3.0A and can run continously at 2.9A without dying?

I would prever to use SMD parts, but if THT parts are needed, so be it.

Any help would be greatly appreciated.

Kind regards.

Bas

P.S.
I cannot yet add images in a post. I attached parts of the circuit, but it does not work.

Hi,

It might be worth having a read of SLYY168 from TI about protecting a power path. The components are proprietary for TI, but the theory is applicable.

Since you have headroom from the 12V you need to the 18V power supply, you could use a simple constant current source set to 3A.

18V Supply → Buck Converter-> 3A current source → DCC

You may need to raise the output of the buck converter a little bit to account for the constant current source voltage drop, depending on the power device. If it’s a BJT it’ll probably be around 0.7V. With a FET it could be close to zero.

In the event that the circuit draws less than 3A, a constant current source would raise the voltage trying to maintain 3A, i.e. it would deliver 18V to the circuit. I doubt this is what the OP wants.