IN4007 role in the power supply circuit diagram

Hello sir

i got a circuit diagram of Stablised Power Supply for Prototyping below :



Here a diode D5 is connected in between pin no.1 & 3 of IC1 Lm317

Whats the role of this one ? Is it alright or the configuration need some changes ?

Too is it compulsary to connect that one ?? As i have made previous experiment without it so please comment . . thanks

it is not necessary

ohh kk thanks pradeep

Read protection diodes section in page 11 of the datasheet https://www.ti.com/lit/ds/symlink/lm117.pdf

P.S. the name of the diode is 1N4007 (not IN4007)

The aim of D5 is to protect the voltage regulator against reverse voltages. If the output is placed at a voltage higher than the input a reverse current can flow through the LM317 potentially damaging it. This could happens, for instance, when you switch the power supply off: if the output capacitor (C1) is discharging slower with respect to the input capacitor (C3).
Probably in your case is not really needed.

Albbg is right. It is also mentioned in application notes and datasheets of the LM317.

Why is the secondary of the transformer rated at "15V-0-15V" which is a total of 30VAC? Then the rectified and filtered voltage at the input of the LM317 is 42VDC or higher if the current is low! Instead the secondary should be 12VAC to 15VAC. Then the filtered but unregulated voltage feeding the LM317 will be a minimum of 15VDC which is fine for an output of only 12VDC.

Why is the main filter capacitor C1 value only 470uF? Then the ripple will be high when the current is more than a small amount.

Why is R2 as high as 220 ohms which is used with the more expensive LM117 as shown on the datasheet? Then some LM317 ICs will raise the output voltage when the load current is low. R2 should be 120 ohms with an LM317 then the minimum load current is enough to prevent the problem. But the other resistor values must be re-calculated.

Audioguru said:

Why is the secondary of the transformer rated at "15V-0-15V" which is a total of 30VAC? Then the rectified and filtered voltage at the input of the LM317 is 42VDC or higher if the current is low! Instead the secondary should be 12VAC to 15VAC. Then the filtered but unregulated voltage feeding the LM317 will be a minimum of 15VDC which is fine for an output of only 12VDC.

Click to expand...

On this point you are absolutely correct.

The transformer is incorrectly labelled '15V-0-15V' (as though it had a center tap), whereas the symbol shows the secondary has a single winding.

Since the transformer is labelled wrong, please measure its output voltages with and without a load.