Burden Resistor On Voltage Regulators

[Zordon]

I am currently typing up a report and i was wondering why there is a resistor between the Comm pin and the output pin of the 7805 for example. I have three regulators in my circuit and they all have this 120hm: resistor.

At work someone told me its called a burden resistor and its used to keep the output steady. Is this true i havent been able to find anything online about it.

thanks

 

[Hero999]

You don't normally need a 120hm: resistor in an LM7805 regulator circuit but it's pretty standard in LM317 circuits.

Please post the circuit and we might be able to comment.

 

[Zordon]

sorry the resistors are on the LM317 and LM337, i was wondering what they are for.
this is a schematic of a dual tracking power supply. For a school project.

**broken link removed**

 

[Hero999]

The 120hm: resistor normally forms the top part of the potential divider in the standard voltage regulator circuit which is shown on the LM317 datasheet. Is also provides a minimum load and the regulator won't regulate properly, if its value is too high, which is why it's included in this circuit.

Another thing, the circuit on the datasheet if for the more expensive LM117 or LM217 which only requires a 240hm: resistor.

 

[ronsimpson]

Here is the data sheet for a LM317.
https://www.electro-tech-online.com/custompdfs/2008/03/LM117.pdf
Normally there are two resistors that set the output voltage. In your case R2 and a variable resistor across T4-T5 sets the "+" voltage.
In the case of the "-" voltage,R5,6 and U1 sets the negative voltage to track the positive voltage.

 

[shokjok]

Diodes D6 & D7 are reversed. You won't get an output voltage there.

 

[k7elp60]

Diodes D6 & D7 are reversed. You won't get an output voltage there.

Actually the voltage at the juction of D7 and R4 is approximately 1.4V negative. with a pot
connected between T4 & T5 will allow the LM317's output to be 0 volts.

 

[DigiTan]

I take it this precaution is optional on regulators with short-circuit protection?