UPS Shield for Raspberry Pi

General Information

June 3, 2019 by

Serzh Ohanyan

Index

1. Introduction
2. UPS Shield
3. Technical Specifications
4. The Real-Time Clock
5. The RS-485 interface
6. Discharge supercapacitors
7. Connecting two Industrial Arduino PLCs using RS-485 to open and close a relay

Introduction

In this post, you will find general information about the UPS Shield and the configuration that has to be done to the Raspberry Pi.

UPS Shield

The UPS Shield module is a supercapacitor based energy storage. It contains two independent DC/DC converters. The first one is a bidirectional step-up/step-down converter working as the interface between the stable 5V supply rail and the supercapacitor. During charging the converter works in step-down mode and transports energy from the external power supply to the supercapacitor. In case of a power failure, the load device (Raspberry Pi or another SBC) is supplied from the supercapacitor via DC/DC converter in step-up mode. The second DC/DC converter is a Front-end step-down converter. The only function is converting a high input down to 5.1V for the 5V rail.

Technical Specifications

General

Parameter	Value	Conditions
Max. Total Backup time Ttb	Min. 57 s Typ. 81 s	Operation until supercapacitors not discharged. Tamb = 25 ºC.
Backup time for Sort-term power failure Tst	Min. 22 s Typ. 31 s	Operation until supercapacitors not discharged. Tamb = 25 ºC.
Charge time (fast charge) = default configuration	Typ. 25 s	Operation until supercapacitors not discharged. Tamb = 25 ºC.
Operating time (slow charge)	Typ. 60 s	Operation until supercapacitors not discharged. Tamb = 25 ºC.
Operating temperature range (module)	-40 ºC to +85 ºC -25 ºC to +70ºC	Assembled PCB Including Push-Button and Screw terminals
Operating temperature range (supercapacitors)	-40 ºC to + 60ºC	Supercapacitor type: 22F / 2.7V / DxL = 16 x 26 mm

Input

Parameter	Value	Conditions
Input Voltage Range	7V DC ... 28 DC	at screw terminal
Average Input Current	1 A 0.5 A	Charge mode+RPi3 Booting at Vin = 12 V Charge mode+RPi3 Booting at Vin = 24 V
Current consumption	Typ. 15 mA	Normal operation (No charge mode, no Shutdown mode), Tamb = 25 ºC

Output

Parameter	Value	Conditions
Output voltage range	4.75 V DC ... 5.25 V DC
Output current range	0 Aavg ... 1 Aavg
Typ. output ripple	10 mVpp	At shutdown mode, Tamb = 25 ºC, ripple at fSW = 1200 kHz

Supercapacitor Interface

Parameter	Value	Conditions
Working voltage at supercapacitor contacts	1 V ... 5.25 V
Peak current through supercapacitor	7 A
Charging method	7.5 W (fast charge, default) = 1.5 A at 5 V 2.5 W (slow charge) = 0.5 A at 5 V	Constant power charging Constant power charging
Balancing	+/- 300 mA	Active balance circuit for two supercapacitors in series

The Real-Time Clock

The board contains a DS3231 integrated circuit communicating via I2C-bus with the RaspberryPi. A Lithium Coin Battery supplies the RTC chip when the Raspberry Pi is not powered on. We recommend using the CR2032 battery.

Parameter	Value	Conditions
RTC chip type	DS3231	Supported by the Raspbian OS
Battery holder	CR2032	3 V, Lithium
Battery Lifetime	> 20 years	at capacity 200 mAh
Communication	I2C-bus	Pins 3 and 5 of the GPIO interface (SDA and SCL), requires RTC driver installed on the RPi

In the file /boot/config.txt uncomment the line


dtparam=i2c_arm=on

and at the end add the line

dtoverlay=i2c-rtc,ds3231

The RS-485 interface

The UPS Shield module contains a 2-wire RS-485 (or RS-422) transceiver. The transceiver is driven by the Raspberry Pi UART interface on the GPIO14 and GPIO15 pins. We send and receive data by /dev/ttyS0.

In the file /boot/config.txt add the line

enable_uart=1

Parameter	Value	Conditions
RS-485 Interface Communication Type	2-wire, half duplex	Transceiver chip ISL8483E
RX/TX direction control	Controlled by GPIO24	Diver enabled by a low TX data bit Driver disabled 25 us after the end of a low TX data bit
Failsafe biasing	390R to GND 390R to 5V
Line Termination	120R	390R//220R//390R+2*10R
Protection	ESD to +/- 15 kV	No Surge/Burst protection
Echo-feature	RX always enabled TX enabled at data	Optional
Data transmission speed	0...250kbps slew rate limited	25 us transmission pause between TX and RX required

The driver-enable line follows the data with a short delay (25 μs) before disabling the driver. There is no need to adjust the time constant for different transmission speeds. The automation direction control works from zero to maximum transmission baud rate. The only requirement is a 25 μs transmission pause between sent and receives data. Thi short time interval corresponds to a time interval of about 3 data bits at 115 kbps. At 9600 bps the 25 μs time interval corresponds to a fraction of one data bit.

Instead of keeping the transmitter enabled for the entire transmission, the driver is enabled on the leading edge of the start bit or any logic low at the drivers in out. It also disables the driver 25 μs after the leading edge of the stop bit or any logic high at the driver's input. When the driver is disabled, the biasing resistors ensure the receiver's output is a logic high.

Discharge supercapacitors

The on-board discharge load should be used before unplugging the module from the carrier PCB. It helps to avoid sparking and excessive current flow when discharging the super-capacitors.

Parameter	Value	Condition
Discharge load	2 Ohm Meander PCB trace used as shunt monitor	at 25 ºC
Typical Discharge Time ar default 2x22F supercapacitors	1 min.	Short the pads R83 with a tweezer

Discharge supercapacitors