# large 60amp 12v ac-DC psu. adding current limiting by dropping voltage via volt trim pot

#### Tom7141

Joined Jan 31, 2023
1
Hello every one long time reader First time poster.

I want to add current limiting to a off the self 12volt 60amp ac-DC switching power supply for charging a battery bank. the off the shelf psu when it reaches its current limit it simply shuts down for a moment before going into a endless restart overload cycle. I have been managing this so far by turning down the voltage trimmer pot to bring the current within limits, then later bringing the voltage backup to the full 13.8-14.4volts to top off.

I have the parts to make the circuit quoted below that auto trims down the voltage as the current exceeds a set point but with a small change the shunt I have is a 100amp 75mv that is common on ebay and other places to fit the current I need to measure. apron testing I knew this mite be an issue due to the change of shunt but not sure where best to modify, the trimer marked U3 below lacks any sensitivity if moved off 0ohm almost immediately triggers limiting. would changing R1 give me more range possibly?

After some additional thought I believe the circuit below can give you a nearly full range current limit without any added power dissipation.
The circuit senses the voltage across the 10mΩ shunt resistor in the ground side of the load.
The Trim connection is the Trim input on the DC-DC converter.
(There is also a resistor to ground from the Trim input to determine the normal output voltage).

The load current across the 10mΩ shunt resistor is amplified by U1.
When this voltage (buffered/isolated by U2 and D1) exceeds the Trim reference voltage of 0.591V it will override that voltage, causing the DC-DC converter to reduce its voltage, thus reducing the current.
The adjust range is from ≈100mA to the current limit of the DC-DC (no added limit).

The one disadvantage of this circuit is that the output limit current is a very non-linear function of the pot position.
A logarithmic (audio taper) pot would thus be better for U3.

V1 is the input power to the DC-DC.

Note that the ground side of R_Shunt should be connected directly to the ground terminal on the DC-DC converter.

View attachment 158354

#### crutschow

Joined Mar 14, 2008
34,455
would changing R1 give me more range possibly?
Increasing the value of R1 would reduce the sensitivity.

#### Hymie

Joined Mar 30, 2018
1,284
I would experiment with a 10kΩ pot in series with R1, and adjust (along with the 100kΩ resistor) to achieve the desired sensitivity.