Designing a General Purpose Heat Pump Controller

[buffalobillpatrick]

Arduino Playground - ArduinoPinCurrentLimitations

Arduino Uno, ATMEGA 328
NOTE: Although each I/O port can source more than the test conditions (20 mA at VCC = 5V, 10 mA at VCC = 3V) under steady state conditions (non-transient), the following must be observed.

The sum of all IOH, for ports C0 - C5, D0- D4, ADC7, RESET should not exceed 150 mA.
The sum of all IOH, for ports B0 - B5, D5 - D7, ADC6, XTAL1, XTAL2 should not exceed 150 mA.
If IOH exceeds the test condition, VOH may exceed the related specification. Pins are not guaranteed to source current greater than the listed test condition.

Pin SINK Current Limitations:

NOTE: Although each I/O port can sink more than the test conditions (20 mA at VCC = 5V, 10 mA at VCC = 3V) under steady state conditions (non-transient), the following must be observed:

The sum of all IOL, for ports C0 - C5, ADC7, ADC6 should not exceed 100 mA.
The sum of all IOL, for ports B0 - B5, D5 - D7, XTAL1, XTAL2 should not exceed 100 mA.
The sum of all IOL, for ports D0 - D4, RESET should not exceed 100 mA.
If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not guaranteed to sink current greater than the listed test condition.

What this says:
40ma maximum per pin
Pins (D0->D4 + A0->A5) can source 150ma total or sink 100ma total
Pins D5->D13 can source 150ma total or sink 100ma total

[jeff5may]

Originally Posted by buffalobillpatrick

jeff5may: "power is restored without losing its wits and memory"

Thats how they work without a battery.

The main goal in defining these conventions is so the unit can sense a power loss event on the mains. I don't know about you, but I want my unit to have a battery back up. When mother nature comes calling, the unit should not just reset.

By the way, what is IOL, VOL, VOH, IOH in the above statement? Just for clarity.

[buffalobillpatrick]

SainSmart 4 Channel 5V Solid State Relay Module Omron SSR AVR DSP Arduino | eBay

http://www.sainsonic.com/zen/albums/...0/05577d01.pdf

These have input resistor. At 5vdc input led current = 11.4ma +- 20%

[buffalobillpatrick]

jeff5may:

"The main goal in defining these conventions is so the unit can sense a power loss event on the mains. I don't know about you, but I want my unit to have a battery back up. When mother nature comes calling, the unit should not just reset."

Please explain why?

I think that after any power cycle or reset, the heat pump controller should behave exactly the same, as in: delay the compressor & water pumps 8-10 minutes.

In fact I wouldn't power up the controller (Arduino Uno) until a load (thermostat, aquastat, etc.) calls for heat or Air Conditioning.

[Ormston]

Originally Posted by buffalobillpatrick

SainSmart 4 Channel 5V Solid State Relay Module Omron SSR AVR DSP Arduino | eBay

http://www.sainsonic.com/zen/albums/...0/05577d01.pdf

These have input resistor. At 5vdc input led current = 11.4ma +- 20%

2A seems a little low to be useful, i know you could use this to drive a contactor but then you're very close to the lower limit of 0.1A (contactor would need at least a 23W coil at 230v).

These work very well, using one of the 8 relay boards myself although you need a separate 5v power supply to the board as each relay (after the opto isolator) requires 90ma.
They work backwards if anyone has trouble with them, drive the Arduino pin low to energise the relay.

New 4 Channel 5V Relay Opto Couple for Arm AVR DSP Arduino Uno R3 Raspberry Pi | eBay

[buffalobillpatrick]

ormston: your relays are obviously different than what I posted, we want minimum LED input current (standard relay term = coil current).

I wouldn't use an arcing contactor around flamible refrigerant, such as Propane, etc.

I would use 30 or 40 Amp SSR , low LED input current, well within the relays load current that I posted.

http://www.ebay.com/itm/CRYDOM-A2450...item338b2847dd

http://www.crydom.com/en/products/catalog/s_1.pdf

AC control input 5-10ma is all, AC output up to 50A
With this I wouldn't even use the small relays.

I would use: Crydom D2450 or D2475 directly from arduino digital pin

[jeff5may]

Originally Posted by buffalobillpatrick

jeff5may:

"The main goal in defining these conventions is so the unit can sense a power loss event on the mains. I don't know about you, but I want my unit to have a battery back up. When mother nature comes calling, the unit should not just reset."

Please explain why?

OK, lets say you live in a rural area, served by a single, long power line, and that this heat pump draws power from that line. Every time a squirrel gets fried on the line, the power drops out for 2 seconds. Every time there is severe weather, power drops out for a few minutes. Every time the power utility works on the line, or some drunk hits a pole, or a tree drops a branch....you get the picture.

Without some sort of power sensing, the heat pump will act like any ordinary digital air conditioner or space heater, and stay shut off when power is restored. If you program the unit to automatically turn on after such an event, it will go back to its baseline temperature setting and mode, which may or may not be how the thing was set before the power went out. It would suck for the unit to be heating during summertime when you come home from work one day.

A standard digital thermostat does the same thing if the backup battery dies. When power comes back up after being lost, it boots up as if it were brand new out of the box. Any programming or configuration is lost. Some of the more advanced models have a "batten down the hatches" routine that stores the programmed parameters in non-volatile memory to avoid having to reprogram everything in case the backup battery dies while mains power is lost.

Originally Posted by buffalobillpatrick

I think that after any power cycle or reset, the heat pump controller should behave exactly the same, as in: delay the compressor & water pumps 8-10 minutes.

In fact I wouldn't power up the controller (Arduino Uno) until a load (thermostat, aquastat, etc.) calls for heat or Air Conditioning.

This does not qualify for standalone operation. In your application, the controller depends on another device in order to operate.

[Ormston]

Originally Posted by buffalobillpatrick

ormston: your relays are obviously different than what I posted, we want minimum LED input current (standard relay term = coil current).

I wouldn't use an arcing contactor around flamible refrigerant, such as Propane, etc.

I would use 30 or 40 Amp SSR , low LED input current, well within the relays load current that I posted.

Crydom A2450E Solid State Relay Input Vac Output Vac | eBay

http://www.crydom.com/en/products/catalog/s_1.pdf

AC input 5-10ma is all, AC output 50A

The point i was making is that the 4 ch SSR board you posted earlier has SSR's that can switch between 0.1 and 2A as in 0.1A minimum!

Also please be careful with SSR's , in industrial applications you would use a contactor as an isolation device then an SSR in series to provide the switching.
When you disconnect the load from an SSR the output can become live even though you are not triggering it, the ones around now may have improved as this in going back 6-8 years.

[buffalobillpatrick]

jeff5may: "This does not qualify for standalone operation. In your application, the controller depends on another device in order to operate."

Sooo, these heat pumps or a/c units operate on their own, without a load control device such as a thermostat or aquastat causing them to operate, seems strange to me.

I hope I never catch my boiler running without a "call for heat"

[jeff5may]

Originally Posted by Ormston

The point i was making is that the 4 ch SSR board you posted earlier has SSR's that can switch between 0.1 and 2A as in 0.1A minimum!

Also please be careful with SSR's , in industrial applications you would use a contactor as an isolation device then an SSR in series to provide the switching.
When you disconnect the load from an SSR the output can become live even though you are not triggering it, the ones around now may have improved as this in going back 6-8 years.

This is one reason I have been leaning toward using relays to control high current devices and contactors to do massive load switching. SSR's are cool and all, with their quickfast switching and no moving parts, but like their cousins (IGBT's), they tend to act unpredictably when things go awry. When the dookie hits the fan, they tend to run away and fail shorted. They seldom just quit working like relays do.

Another thing I dislike about SSR's is they are SPST. This means that to get two or three things to work together, they must be individually controlled in logic by their own SSR. As with a 4PDT defrost control relay, one can wire a number of devices to work with or against each other (or not at all) depending on what mode of operation the unit is in. That 5 dollar ice cube relay really kills a headache in a hurry.