Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
Resource icon

Standard Three Terminal Regulator Adjusts Down to Zero Volts 2016-11-20

Issue: Draft 07 of 2016_11_20
upload_2016-11-20_5-45-0.png
upload_2016-11-20_5-44-12.png


INTRODUCTION

Standard positive (LM317) and negative (LM337) three terminal regulators (TTRs), are dead convenient for making fixed and variable voltage power supplies to provide between 50mA and 7A, but, generally, they do not go lower than their 1.25V, floating reference voltage.

You can get around this by using one of the reference current regulators, like the Linear Technology LT3081/LT3083 families, but there are still two other problems to solve (see Addendum). Also, low drop out (LDO) regulators, like the LT3081/LT3083 families, are more difficult to stabilize in the frequency domain because of the voltage gain of their 'PNP' output stages.

The simple, low-cost circuit below resolves the three major problems. A commonly-used bridge rectifier is shown for the primary supply but all other configurations are also suitable.


2016_11_20_Iss1_ETO_LM317_PSU_ADJUSTS_DOWN_TO_0V_VER1.jpg

CIRCUIT DESCRIPTION
The power supply output voltage in Volts = 10.42mA * value of RV1 in K Ohms

TBC

NEGATIVE VERSION
A negative version of the power supply can be realized by simply swapping the positions of the three terminal regulators and reversing all polarized components (diodes and capacitors).

SIMPLIFICATION
For less demanding applications the following simplifications can be made
(1) R2: omit
(2) R3:replace by a link
(3) RV2: replace by a link
This will result in a worst case minimum output voltage of 100mV and depends on the relative reference voltages of the two three terminal regulators.

spec

ADDENDUM
PROBLEMS WITH ZERO VOLT OUTPUT
(1) Reference voltage cancellation
(2) Regulator minimum current
(3) Stability

LINKS

(1) LM317 (positive): http://www.ti.com/lit/ds/symlink/lm317.pdf
(2) LM337 (negative): http://www.ti.com.cn/cn/lit/ds/symlink/lm337-n.pdf
(3) LT3081 family (positive): http://www.linear.com/product/LT3081
(4) LT3083 family (positive): http://www.linear.com/product/LT3083
(5) LT3091 family (negative): http://www.linear.com/product/LT3091
  • upload_2016-11-20_5-44-12.png
    upload_2016-11-20_5-44-12.png
    1.2 KB · Views: 1,617
  • upload_2016-11-20_5-45-0.png
    upload_2016-11-20_5-45-0.png
    1.1 KB · Views: 1,533
  • 2016_11_20_Iss1_ETO_LM317_PSU_ADJUSTS_DOWN_TO_0V_VER1.jpg
    2016_11_20_Iss1_ETO_LM317_PSU_ADJUSTS_DOWN_TO_0V_VER1.jpg
    166.8 KB · Views: 1,900
Author
spec
Views
4,412
First release
Last update
Rating
0.00 star(s) 0 ratings

More resources from spec

Latest threads

New Articles From Microcontroller Tips

Back
Top