I have an electric SPA (a.k.a. hot tub) that I would like to monitor and possibly control. Controlling the unit will require reverse engineering the control pad protocol so for now I will concentrate on monitoring.


  • Water Temperature
  • Ambient Temperature
  • Power Consumption
  • Timer Based Filter Cleaning Reminder
  • Wireless transmission of data to Misterhouse
  • PIC based w/ eeprom or flash

Unit will monitor water temperature, ambient temperature, and current consumption to provide data for research for how to best operate the unit. In addition the unit will proved a timer mechanism for filter cleaning reminders with a simple push button reset when cleaning the filter.

Unit should have as much autonomous operation as possible including local storage of sampled values.


Current Measurement

Adjustment of Current Transformer Ratios
DC Signal Conditioning for a CT
split core CT $12.50 - Use for mains
50:5 CT that I already own; can be used for SPA or HVAC
LVRC-.07RCT-ND .07 Ohm 5W resistor - 350mV/1.75W at 50A w/ 50:5 CT
LVR03R0700FB12 .07 Ohm / 3W resistor - Mouser
ACS754LCB-050-PFF - Allegro Hall Efect Current Sensor / $5.40 at Digi-Key but will require a PCB
Rocoil Precision Rogowski Coils

LPC2132 Based Ethernet Data Acquisition and Control System - Contains opto-isolation circuit for an SPI circuit

Power Measurement

It turns out to be quite hard to "accurately" calculate "power" directly from voltage and current readings. While I can do this directly in a microcontroller, there are chips avaiable that encompas all the processing functions and provide SPI interfaces for reading the registers.
Analog Devices ADE7753 - Has an application note AN564 with a complete design. There is also an evaluation board w/ schematic ADE7753EB

Digikey 296-20638-ND - 1W DC-DC Converter for isolated power

HCPL2630 @ DigiKey - $3.69; 2 channel; Opto-isolator
6N137 @ DigiKey - $1.12; 1 channel; Opto-isolator

Current Shunt Calculations
I - Primary Current
T - CT Turns Ratio
V - Shunt Voltage Output
R - Shunt Resistor
W - Shunt Wattage
P - Burdon VA

Shunt Resistance
V = Isec x R
V = (I/T) x R
V x T = I x R
R = (T/I) x V

Shunt Wattage
W = Isec^2 x R
W = (I/T)^2 x R

Burden Calculation
P = Isec x V
P = (I/T) x V

R based on burden
P = (I/T) x V ; R = (T/I) x V
V = P x (T/I)
R = (T/I) x P x (T/I)
R = (T/I)^2 x P

Shunt Resistance Calculation
for 50:5 CT w/ 2VA max
R = (T/I)^2 x P
R = (10 turns / 50A)^2 x 2VA
R = .08 Ohms
Common shunt values are .07 or .1; .1 will exceed 2VA

Full Scale Voltage
V = Isec x R
V = (I/T) x R
V = (50A / 10 turns) x .07 Ohms
V = 350mV

CT Burdon (VA)
P = (I/T) x V
P = (50A / 10 turns) * 350mV
P = 1.75VA

Shunt Wattage
W = (I / T)^2 x R
W = (50A / 10 turns)^2 * .07
W = 1.75W

Wireless Transmission

RWS-371 & TWS-BS3

Temperature sensing

DS18S20 - 1-wire
Need to determine the best way to measure the water temp. Too bad I can't tap into the existing controller that already has a calibrated sensor. Could epoxy a DS18S20 into a metal tube. I've also seen references to letting the DS18S20 package stick out the end of the tube. But where in the plumbing would I insert the tube?

PIC Boards


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Page last modified on Sunday 14 of February, 2010 11:34:04 CST by michael.