Electronics > Beginners
Foldback Current Limiting - an example
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iMo:
Here is a small simulation with calculation of the resistors for a Foldback Current Limiting.
Normally the current limiting with LM723 (or any other PSU with same current limiting wiring) works such the current flowing through the shorted output is aprox.

I_limit = 0.7V/R_shunt

With the Foldback Current Limiting the current during short is much lower, call it I_short.

Thus we have I_limit and I_short. After the output current approaches I_limit, it starts to drop down to I_short.

In the below example the I_short is aprox. 5x smaller than the I_limit.

Below is, as an example, a simulation with an LM723 and the calculation.
To find the right values is not straightforward, however.
The Rshunt is a much higher value as you would expect, and the both currents depend on the Rshunt value and R4/R5 ratio as well.
Excel sheet with the goal seeker is something which may help.

The simulation shows the calculation fits pretty well, with larger deviations when k < 0.8 ..


--- Code: ---      I_limit  I_short
Calc:  41mA    8.5mA
Sim:   39mA    8.7mA 
--- End code ---

More on how it works here:

http://www.eeeguide.com/foldback-current-limiting/

ledtester:
FWIW, this lecture on the topic might also be helpful:


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