Under Voltage Lockout Relay – Latching Voltage Monitor

  • Rajkumar Sharma
  • 1 Views
  • easy
  • Tested
  • SKU: EL126250
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This project prevents/disconnects the load when an under-voltage condition occurs. The circuit latches the output when input sense voltage drops below the lockout threshold. Lockout threshold voltage can be adjusted using the onboard trimmer potentiometer PR1.  Tactile switch SW1 is provided to reset the latch when input sense voltage is back to normal. The circuit consists of a MAX835 chip, 5V Relay, BC857 PNP transistor to drive the Relay, power LED, Relay/Output LED, and tactile switch. MAX835 chip features a level-sensitive latch, eliminating the need to add hysteresis to prevent oscillations in load-disconnect applications.

MAX835 micropower voltage monitors have a 1.204V precision bandgap reference, comparator, and latched output in a 5-pin SOT23 package. Using the latched output prevents deep discharge of batteries and disconnects the load from the power supply when Undervoltage is detected at input sense voltage. MAX835 has a push-pull output driver. Two external resistors set the trip-threshold voltage.

Operation

Connect an adjustable power supply to CN3 and apply input sense voltage in the range 3.4V to 8V, turn the PR1 trimmer potentiometer till the LED D3 is ON. Then the board is set for Undervoltage detection. For example, apply 4.90V to the CN3, rotate the PR1 so LED D3/Relay is on, the circuit is set for under-voltage detection threshold of 4.90V, now increase the input sense voltage to 5V and reset the latch using SW1.

Note 1:  Relay contacts can handle load current up to 7A, use higher contact relay for high current load. Relays up to 16A are available with the same PCB footprint (same dimensions)
Note 2: The project can also be used to prevent deep discharge of batteries, it disconnects the load when battery voltage drops below the set point, read the datasheet of MAX835 to configure the input voltage range of battery using PR1 trimmer potentiometer and resistor R5.
Note 3:  Input sense voltage range is 3.4V to 8V and can be adjusted using trimmer potentiometer PR1.  The circuit can be configured to detect higher voltage. To achieve that choose the right value for PR1 and R5. Read the datasheet of MAX835 for more info.

Connections

  • CN1 = Pin1 VCC, Pin 2 VCC, Pin 3 GND, Pin 4 GND
  • CN2= Pin 1 Sense Voltage Input, Pin 2 GND
  • SW1 = Reset Switch
  • D1 = Power LED
  • D3 = Function LED – Relay Output
  • PR1 = Trimmer Potentiometer Input Sense Voltage Adjust
  • CN2 = Load Connection, Pin 1 Normally Closed, Pin 2 Common, Pin 3 Normally Open

Features

  • Supply 5V DC @ 50mA when Relay is on
  • Load Up to 7A, which can be increased -> Read Note 1
  • Input Voltage Range 3.4V to 8V
  • On Board Tact Switch for Latch Reset SW1
  • Latched Output (Once Low, Stays Low Until Cleared)
  • Precision +/-1.25% Voltage Threshold
  • LED D3, Alert Monitor – LED is ON when output is Latched (OFF)
  • LED D1, Power LED
  • 4 x 3MM Mounting Holes
  • PCB Dimensions 45.25 x 29.53mm

Schematic

Parts List

NO.QNTY.REF.DESC.MANUFACTURERSUPPLIERPART NO
11CN14 PIN MALE HEADER PITCH 2.54MMWURTHDIGIKEY732-5317-ND
21CN23 PIN SCREW TERMINAL PITCH 5.08MMPHOENIXDIGIKEY277-1248-ND
31CN32 PIN MALE HEADER PITCH 2.54MMWURTHDIGIKEY732-5315-ND
41C110uF/16V SMD SIZE 1210 OR 1206YAGEO/MURATADIGIKEY
51C20.1uF/50V SMD SIZE 0805YAGEO/MURATADIGIKEY
61D1LED SMD REDLITE ON INCDIGIKEY160-1427-1-ND
71D21N4007 SMD DIODEDIGIKEYS1MBDITR-ND
81PR15K TRIMMER POTENTIOMETERBOURNSDIGIKEY3362H-502LF-ND
91Q1BC857 SMD SOT23-3ONSEMIDIGIKEYBC857CDW1T1GOSCT-ND
101RE1RELAY 5V DCCIT RELAYDIGIKEY2449-J107F1CS125VDC.36-ND
112R1,R6470E 5% SMD SIZE 0805YAGEO/MURATADIGIKEY
121R210K 5% SMD SIZE 0805YAGEO/MURATADIGIKEY
131R3100E 5% SMD SIZE 0805YAGEO/MURATADIGIKEY
141R4DNP
151R59.01K 1% SMD SIZE 0805YAGEO/MURATADIGIKEY
161SW14 PIN TACTILE SWITCHNKK SWITCHDIGIKEYHP0215AFKP2-ND
171U1MAX835 SOT223 5 PINMAXIMDIGIKEYMAX835EUK+TCT-ND
181D3LED SMD GREEN SMD SIZE 0805DIALIGHTDIGIKEY350-2044-1-ND

Connections



Gerber View

Photos

Video

MAX835 Datasheet

MAX834-MAX835

PCB

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