power extender

Power Extender

Hardware and software for a power and IO extension PCB with current monitoring and I2C interface.

This project has been generously supported by PCBWay. Read more about this project and also my review of the manufacturing on my blog: Design And Assembly Of An I2C Relay PCB

Usage

Use KiCad with my CustomComponents library to view and edit the hardware project, or download the fabrication files for PCB and/or PCBA manufacturing. I used KiBOM to generate the BOM. You can also download the complete schematics as PDF.

See the Arduino library documentation for usage with the Arduino framework. The library is also available on PlatformIO. Use Visual Studio Code and the PlatformIO plugin to edit the Arduino library and tests.

See the Raspberry Pi library documentation for usage with the Raspberry Pi Python 3 smbus framework.

Features and Specifications

• Power input via 5V and 3V3 pins
  • Min. 250mA @ 5V to actuate all four relays simultaneously
  • Less than 30mA @ 5V standby current
  • 3V3 voltage level is only required for I2C level shifting if the I2C signal level is 3.3V instead of 5V
• I2C bus for connection to a I2C master via JST header RPI
  • I2C bus passthrough via JST header PASS
  • Internal pullup resistors
  • Level-shifting 3.3V - 5V tolerant bus
• Four SRD-05VDC-SL-C three-lead (NC - COM - NO) magnetic relays
  • Each has a CC6902SO-10A inductive current measuring IC connected across COM
  • Galvanically isolated driver circuit using an optocoupler
  • Flyback diode protection
  • Red status LED indicates COM - NO actuation
  • 2oz thick copper traces enable high currents
  • Isolation milling reduces risk of shorting in damp environments
  • Maximum power: 10A, 250VAC, 110VDC
  • Contact load rating: resistive 7A @ 28VDC, 10A @ 125VAC, 7A @ 240VAC, inductive 3A @ 120VAC, 3A @ 28VDC
• One PCA9557 8-channel GPIO port (I2C address: 0x18), logic level 0V | 5V
  • GPIO channels 0Z - 3 connected to JST header DIGITAL
  • Relays are connected to GPIO channels 4 - 7
  • Maximum current (except channel 0Z): 25mA sink, 20mA source per channel, max. total: 100mA sink, 85mA source
  • Channel 0Z is high-impedance and open-drain (see datasheet for details)
• Two ADS1115 4-channel 16-bit ADC ports, voltage range 0V - 5V
  • All four pins of ADC 1 connected to current measuring ICs (I2C address: 0x48)
  • All four pins of ADC 2 connected to JST header ANALOG (I2C address: 0x49)
  • Configurable sample rate and gain amplifier
  • Max. 10mA continuous input current

For further specifications and ratings, all datasheets can be found in this repository.

Sensor Calibration

Due to the inherent nature of magnetic relays, the inductive current measuring ICs and also the ADCs experience a measurement offset while the relays are energized. It is recommended to measure the actual load at a distance from the board, in order to obtain reference values. These reference values should then be used to compute an offset or function for each channel and amount of relays energized.

For no load (floating) using an Arduino Uno, the maximum absolute measurement offset was found to be about as follows.

Amount of relays energized	Offset at energized channel	Offset at disabled channel
0	0.05A	0.05A
1	1A	0.7A
2	1.5A	1.3A
3	2A	1.8A
4	2.4A	N / A

Please note that these offsets are dependent on load and environmental factors.

PCB Details

17 unique parts, 60 SMT parts, 12 THT parts on a double-sided 90mm x 71mm PCB with 2oz copper finish.

Component cost per board: about $25 (Ignoring shipping, bulk discounts and changes in price over time).

BOM

Item #	Designator	Quantity	Manufacturer	Manufacturer Part #	Description / Value	Distributor	Distributor part #	Package / Footprint	Type	Notes
1	C11, C21, C31, C41	4	Any	TBD	CAP CER 100PF MAX. 5% MIN. 10V 0805 SMD	Any	TBD	SMD 0805	SMD
2	C1, C2, C12, C22, C32, C42	6	Any	TBD	CAP CER 100NF MAX. 5% MIN. 10V 0805 SMD	Any	TBD	SMD 0805	SMD
3	D12, D22, D32, D42	4	Shikues	1N4007F	DIODE 1N4007 SMAF	LCSC	C110856	SMAF	SMD
4	D11, D21, D31, D41	4	Everlight Elec	17-21SURC/S530-A3/TR8	LED RED 2V 0805 SMD	LCSC	C72037	SMD 0805	SMD
5	J1, J2, J3, J4	4	Any	TBD	CONN JST-XH PITCH-2.50MM VERTICAL MALE 5 PIN	Any	TBD	Through Hole	PTH
6	J11, J21, J31, J41	4	Ningbo Kangnex Elec	WJ128V-5.0-3P	CONN SCREW TERMINAL PITCH-5.0MM 3 PIN 20A	LCSC	C8270	Through Hole	PTH
7	K11, K21, K31, K41	4	Ningbo Songle Relay	SRD-05VDC-SL-C	RELAY SEALED SENSITIVE 5V 10A	LCSC	C35449	Through Hole	PTH
8	Q1, Q2	2	Shikues	BSS138	MOSFET N-CH SOT-23-3	LCSC	C112239	SOT-23-3	SMD
9	Q11, Q21, Q31, Q41	4	Nexperia	PMBT3904,215	BJT NPN SOT-23-3	LCSC	C8667	SOT-23-3	SMD
10	R14, R24, R34, R44	4	Any	TBD	RES 100 OHM MAX. 1% MIN. 1/8W 0805 SMD	Any	TBD	SMD 0805	SMD
11	R11, R12, R21, R22, R31, R32, R41, R42	8	Any	TBD	RES 200 OHM MAX. 1% MIN. 1/8W 0805 SMD	Any	TBD	SMD 0805	SMD
12	R1, R13, R23, R33, R43	5	Any	TBD	RES 1K OHM MAX. 1% MIN. 1/8W 0805 SMD	Any	TBD	SMD 0805	SMD
13	R2, R3, R4, R5, R15, R25, R35, R45	8	Any	TBD	RES 10K OHM MAX. 1% MIN. 1/8W 0805 SMD	Any	TBD	SMD 0805	SMD
14	U1, U3	2	Texas Instruments	ADS1115IDGSR	IC I2C 4 CH ADC VSSOP-10	LCSC	C37593	VSSOP-10	SMD
15	U12, U22, U32, U42	4	Cross Chip	CC6902SO-10A	IC CURRENT SENSOR 5V 10A SOP-8	LCSC	C350865	SOP-8	SMD
16	U11, U21, U31, U41	4	Sharp Microelectronics	PC817X2CSP9F	IC OPTOCOUPLER SMD-4	LCSC	C66405	SMD-4	SMD
17	U2	1	NXP Semicon	PCA9557PW,118	IC I2C 8 CH IO EXPANDER TSSOP-16	LCSC	C141380	TSSOP-16	SMD