7812 Get Hot / Vin Voltage Drop

[yanco]

I have a circuit connected in this way, the input voltage supply from non-regulated adaptor is 18V 1000mA.


18 Vin o-------------7812 --------
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| R= 10ohm 5 watt
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Pin7 Pin 8 of BA6219 Rohm Reversible Motor Driver


My problem is why some of the Vin voltage drop to 15 V when the adaptor is plug into the circuit. Will this cause the 7812 get hot even when the motor driver is not working ?

Is that cause by low input impedance of the R=10 ohm, and how to resolve this as ROHM spec say that we need to have R=10 ohm as current limiting circuit to the input of Pin 7 and Pin 8.

But some of the same circuit different PCB board and parts, voltage drop only to 17.5 V. The 7812 will not get hot even when driver is working.

Thanks for any comments and inputs.

 

[awareof70]

Hello yanco,

Can't quite make out your diagram,looks like the 18V is going directly to pin 7.

The fact that the +18 supply is dropping to 15V indicates
that there is current close to or exceeding 1 Amp.


What is the regulated output during this condition ?

 

[yanco]

Yes. 18 V directly go into the Pin 7 of BA6219 (dc motor driver).

The regulated output is 12 V from the 7812. The question is why the current exceed 1 A. It suppose to be less than 1 A.

 

[melone]

Monitor the actual current going to the motor driver. It sounds like you might be getting current surges that are causing your power supply to drop.

 

[Chuckles159]

Sounds like you're in a bit over your head. The 18V adapter will indeed drop in voltage because of ripple on the output of the adapter. Most adapters are unfiltered and the voltage changes drastically with load. You appear to have 18V going directly to your motor driver IC and the input of the 7812. Then the output of the 7812 goes through a 10 ohm resistor to another pin of the motor control IC. Have you measured the current going through the 7812? If it is over 100mA, you will have to heatsink the 7812. Have you measured the current drawn by the motor controller itself from the 18V supply. If over about 500mA, you may need a bigger adapter and since the voltage is dropping, you definitely need more filter capacitor. The one in the 18V supply appears to not be big enough. Perhaps a 4700 uFd 25V cap across the output of the 18V supply would be good.

 

[schwarz]

10 Ohms is not causing low impedance as it is a series resistor. I looked at the BA6219B datasheet. Max. current is 60 mA.
Please check drop across 10 Ohms to see the actual current in pin 8. Pin 7 current can be measured with meter.

Secondly( should be first) check that 100uF cap recommended in operation notes across Vcc is connected close to IC pins as incorrect connection may cause supply voltage to drop.
Make sure of standard i/p and o/p caps at 7812.

Common adapters do see o/p voltage drop when load is connected. Check adapter by separately loading with dummy load.

 

[yanco]

I have solve the problem, one of the AND gate IC in the circuit is damage. The circuit get very hot, might be due to the IC suck lots of current from the 7812. I replace the AND gate IC and the 7812 getting normal already.