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LM317KCS From Texas Instruments

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Hi everybody

I am designing a power supply using LM317KCS,and from the datasheet I have downloaded from TI's technical documentaions,this regulator IC is capable of delivering 1.5 Ampere @1.25 to 35 volt output,considering the condition that the voltage drop accross the IC multiplied by the consumed current does not exceed 20 watts,the power dissipation.

I am using 15-0-15 volt transformer rated at 2 Ampere,and to avoid exceeding the power dissipation rated for the IC,I used a switch to select between 15 volt and 30 volt input to the IC,so when I need 10 volt output for example,I switch it to 15 volt input,but this is seriously impractical,so I tried many times "but failed" to design a circuit that senses the input-output voltage drop,and the current consuption,so that when the votage drop accross the IC exceeds 11 volt and the multiplication of both values exceeds 16.5 watts,a switching occurs from 30 volt input to 15 volts input to the IC,so that means a low voltage is needed at high current,and when I need higher voltages "say 20 volt" it automatically switches to 30 volt input.

How can you help me friends,as using external current booster transistor to overcome dissipation is excluded from my options,and some times I hear a relay clicking in commercial supplies when turning the voltage wiper to higher output voltages.

Best regards.


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There are a number of ways to do this. What you are looking for is automatic switching. This can be accomplished with transistors and comparators. If you are familiar with comparators, you can use them to switch the base of a transistor that is rated for what you are going to run through them. You would tune the comparator to the trigger voltage. A twist on this would be to use a relay instead of the transistor. Many relays are buikt with a normally open and normally closed channel, so you could get a dual purpose out of one device. With this, you could switch one signal on and the other off at the same time. Are you familiar with microcontrollers? You could build a voltage divider connected to the DAC on a microcontroller, then write the code to send a signal to a particular output pin when the reading is greater than X volts. The voltage divider is to protect the microcontroller because it cannot handle more than 5 VDC. This output pin would then switch on the transistor or relay. At the same time, you could have another pin switch off the other transistor, etc.

MP

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