pcb /relays

Hi iam kwame from africa.I   am an agricultural   engineering student building an incubator.I have finished building the machine but how to choose a suitable circuit board and install   relays to control the heater and cooler system on/off is my problem. I know next to nothing about how successfully install relays and contactors i need help in this direction.
   I NEED   A ERLAY BOARD   THAT CAN BE USED   WTH A   THERMOSTAT   TO CONTROL   A HEATER AND   COOLING   SYSTEM ;WITH A MINIMUM   AND MAXIMUM   TEMPERATURE   ALARM.THE   ALARM IS ALSO   SUPPOSED TO GO OFF WHEN   THERE IS A POWER CUT.THE ALARM    SOUNDS WHEN TEMPERATURE FALLS TO MINIMUM SET     AND   MAXIMUM SET POINTS.IN THE CASE OF THE LATER,THE COOLING SYSTEM MUST BE TURNED ON
              WHICH   RELAY BOARDS IS MOST SUITABLE FOR   MY SPECIFICATION?
           CAN   ANYONE   DESIGN A CIRCUIT/RELAY   BOARD FOR MY APPLICATION?IF KNOW ,WHERE CAN I FIND THE SAID PCB.
        WILL WELCOME ANY RESOURCES ON THE ABOVE SUBJECT SUCH AS OLD    BOOKS,CDs, etc.I WILL PAY FOR THE MAILING.
Wisdom Kwame   samlafo
Ghana

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HI AFRICA NEEDS TECHNOLOGY NOT AID!

How much current will your heater and cooling system draw?  Is it a 12 volt DC system, or a line voltage setup?  If line voltage, what are your line voltages there in Africa?  What kind of output do you have to trigger the relays?

My heater will draw 220- 240 volts of current.It is a line voltage ;alternating current ac.The heater is 1 kilowatt and i use   one horse power air conditioner for cooling.The temperature should be between 37 - 38 degree celsius to trigger relays.
Please help
kwameklu@gmail.com

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HI AFRICA NEEDS TECHNOLOGY NOT AID!

You might be able to modify a refrigeration thermostat and switching circuit since they generally operate at mains voltage (240VAC etc).

Your heater is a ~4 Amp current (1kW@220V = 1000/220 ~4A).
Your aircon is about the same (1 HP ~750W).

So your relay contact ratings need to be 5 Amp or higher, and rated and approved for mains applications and wiring (ie, 240VAC).

How you control them is a different issue.

Older mechanical refrigerator controls usually switch 240AC to the relay.
Many other temp sensing circuits use DC like 5V, 12V, 24V etc.


I'd suggest a low-voltage DC circuit around 12V since you will probably need to adjust the temperature to allow for thermal delays etc.

You haven't mentioned how much hysteresis you need (ie, heater on at 36C, off at 39C - OR - aircon on at 39C & off at 37C, etc) but I presume it is a large incubator since your are using airconditioners and heaters. (Often light bulbs are used as heaters, and most circuits are for heating only - not cooling as well.)


Your relays are the easy part - just any approved relay rated for 240VAC @ 5A (or 10A etc).
You are probably better off NOT mounting them on a PCB - or at least NOT the same PCB as the control mechanism unless it too is 240VAC.

Find a control circuit that does what you want, and that should determine what voltage the relay coils (solenoids) need to be.
Be aware though that the control circuit's voltage may not be too important - ie, if using a 3.3V or 5V control circuit, you can turn on 12V (or 24V etc) relays by ground switching them - aka "open collector" or "common ground" outputs.


I presume you will use a 12V DC battery backed control circuit since you want alarms when the power is cut?
(Or you you just use normally-closed relay contacts that are extended to another alarm system?)

Some thoughts on sourcing, try Maplin and Radiospares (RS Components, same group as Farnell, USA) For the boards, I'm sure I've seen something similar in both catalogues, on a humerous note Oldspark, you, us and probably the people in Ghana all run c230VAVC at 50Hz. We call them mains sockets, the US calls them wall outlets. The US is about the only country left running on 110VAC at 60Hz.
On a serious note Kwame, I absolutely agree with your bottom line.

I guess there are so few perfect countries... ha ha!

FYI - many Australians think was are still 240VAC, but we officially changed to 230VAC (50Hz) some years back. (Some places had above 240 - ie, ~256VAC - due to long distances; eg, Western Australia.)
So the US have a wall supply, and a wall outlet on the ends of extension cables, and ceiling drop downs, and car inverters with walls! Next you'll be telling me they don't wear thongs on their feet!


But seriously too - your serious comment.
Thanks. Sometimes I forget too - I only remember the wasted attempts and band-aides. (And now it's GM food... IMHO.)
Kwame - we can work through the complexities... and my bad expression.

Hi
thanks folks folks for all the expertise you put at my disposal. By the way   help me understand the following terminologies: Opto isolated 'IO' , FP48I/O Parallel Port Relay Board 4 Optically Isolated Inputs 4 Relay Outputs 4 Darlington Outputs , 4 channel opto-isolated IO relay board etc.
      When is it necessary to combine relay with contactors ?

Thanks
kwame


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HI AFRICA NEEDS TECHNOLOGY NOT AID!

Hi oldSpark
The hysteresis is as follows:the heater is supposed to be turned on when temp is 36 and off when temp is 37.5 deg celsius.Cooling system is activated when temp is 38 and off when temp is 37 .
Thanks

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HI AFRICA NEEDS TECHNOLOGY NOT AID!

Ok - you have some interesting control issues...
Heater... on @ 36 off @ 37.5 (deg celsius)
Cooling.. on @ 38 off @ 37.

So there is a situation between 37 & 37.5 when both heating and cooling are on....
As long as that is addressed by ambient temperature and the thermal delays of the system...


The opto-isolated is typically with 4N27 etc devices - it's a way of isolating (say) 5V or 12V sources from (say) 230VAC loads. IE - it provides galvanic (current) isolation between different power (voltage) systems of sources.
It's typically used where a hobbyist wants a computer of whatever circuit to control something on "licensed" systems - eg HV-DC or mains-AC systems.
It's similar to a relay in some respects (ie, the control voltage is independent of the controlled voltage/current).

Darlington is a transistor-transistor driving circuit for added "boost" (drive current or amplification).


You'd be best to consult Wiki for descriptions - they are pretty good.

EG - https://en.wikipedia.org/wiki/Darlington_transistor