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Current Limiting Circuit Doubt

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Advanced Member level 1
Jan 28, 2011
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Hi all,

I am trying to implement a current limit circuit using a Relay. I am following the circuit mentioned by TI instruments as attached. As I dont have the exact circuit components, i have replaced the diode in the circuit with 1N4007. The relay used is Song Chuan (832A-1A-C-BH-12VDC) and for the 12 V supply, I am using the Hi Link 12V (5W) supply.

However, when using 75k diodes and 12 V Zener along with a diode (1n4007) as mentioned in the circuit diagram, the circuit does not work.
But simply connecting the 12 V supply and 1N4007 (as freewheel) works. What could be the reason? The rise time for the 12 V supply is 5ms. Is 5ms enough to limit in-rush current?


  • Figure 5_Input rectifier stage of PMP10948.jpg
    Figure 5_Input rectifier stage of PMP10948.jpg
    647.6 KB · Views: 98

It won't work, the 75k resistors in series with the relay are far too high. the coil current is only .3mA.

Agree with G4BCH. Can't work with 75k resistors. The value is already in the TI link, apparently a typo.

According to the relay data sheet it has a 400Ω coil and requires 30mA to operate so, as noted, the relay cannot operate.

If 12V2 is 12V from the power supply output, then I suggest moving C109 to the relay pin 4, and changing the 75kΩ resistor to 75Ω (or one 30Ω resistor).
That would give about a 30ms relay delay as the 12V comes up which should be sufficient to hold the relay open until the surge current to charge the supply filter capacitors has essentially stopped.

And I see no reason for the two relay coil suppression diodes, as the 12V supply will relatively slowly go to zero when the power is turned off, so no significant coil transient will be generated.
Such transients are only generated if the relay coil circuit is suddenly opened when it is energized.

The circuit looks like it was generated by a neophyte engineer at TI.

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