Why is my LM-2940 CT12 regulator IC experiencing erratic function?

  • Thread starter Ravaner
  • Start date
In summary, a user has been experiencing issues with their LM-2940CT12 regulator, where the voltage output falls to 8 V instead of the normal 13.9 V. They have tried replacing the component, but the problem persists. They suspect it may be due to oscillation, possibly caused by a cheap electrolytic capacitor on the output. They plan to add a ceramic capacitor to improve stability. They also mention the importance of proper circuit layout and a robust ground. The "head" voltage is the raw supply at 13.9 DC. The IC is mounted on a heatsink, but it may not be effective in dissipating heat.
  • #1
Hello. I use for several months an LM-2940CT12 to supply 3 PCB's. All datasheet rules were respected.Max current is less than 200 mA. A few weeks ago voltage falls down to 8 V ant head voltage which is normally 13.9 V falls to 10.5 V and so regulation was out of order. I replace the component, and after few days, pb appears one more time. I precise that nothing has been changed on PCB's, and sometimes power works and head and then out voltages returns to correct values. How can we explain that erratic function of reguator IC ?
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  • #2
It could be oscillating. Is it properly bypassed? Does it get hot during normal operation? Did it get hot when the condition occurred? If you powered it down and up again did the condition persist? If you put your finger on the output did it change the voltage?

If it is oscillating it is probably because you used a cheap electrolytic on the output. You could parallel it with a 0.1 ceramic. There is a section in the data sheet regarding ESR of the output capacitor. (it wouldn't paste well)

Also, circuit layout can cause oscillation. the filter cap needs to be close to the part and grounds need to be robust.

What drives the "head" voltage. By head I assume you mean the raw supply is 13.9 DC.

What package are you using? Is it too small to dissipate the 1/2 watt and the part eventually fries? The SOT-223 is 175C/Watt so it would get pretty warm.
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  • #3
Thanks for reply. input cap (filtering) is 4700 µF, output cap 470 µF, distances to IC < 1". IC mounted on heatsink (not really usefull). Absolutely never any oscillation. For the time being it works perfectly ! Anyway I'll place a ceramic cap on output, may be effectively an ESR problem.

Related to Why is my LM-2940 CT12 regulator IC experiencing erratic function?

1. What is LM-2940 CT12?

LM-2940 CT12 is a type of voltage regulator that is commonly used in electronic circuits. It is designed to provide a stable output voltage of 12 volts, regardless of changes in input voltage or load.

2. What are some common troubles with LM-2940 CT12?

Some common troubles with LM-2940 CT12 include over-heating, output voltage fluctuations, and failure to regulate voltage properly. These issues can be caused by incorrect wiring, faulty components, or excessive load on the circuit.

3. How can I troubleshoot problems with LM-2940 CT12?

To troubleshoot problems with LM-2940 CT12, you can check the wiring to ensure it is correct and secure. You can also test the input and output voltages with a multimeter to determine if they are within the expected range. If the regulator is overheating, make sure it is properly heatsinked and not being overloaded.

4. Can I use LM-2940 CT12 with any input voltage?

No, LM-2940 CT12 is designed to work with input voltages between 7 volts and 26 volts. Using it with input voltages outside of this range can cause damage to the regulator or the circuit it is being used in.

5. How can I prevent troubles with LM-2940 CT12?

To prevent troubles with LM-2940 CT12, make sure to follow the manufacturer's guidelines for wiring and usage. Use proper heatsinking to prevent overheating and do not overload the regulator with excessive load. Regularly check and test the regulator to ensure it is functioning properly.