Immersion Heater Controller

[brightasaspark]

Hi Folks – there has been a lot of info re Immersion Heater Controllers on this forum and as I was updating and drawing up the circuit etc I used for mine I thought I would share the info with you.

I realise that its design and facilities will never compete with the likes of Immersun but having had in service for 9 months or so, I can honestly say I am very pleased with its performance taking into account thealmost non existent summer and lack of meaningful sunshine. The complete cost using components sourced via eBay and RS Components and my own parts bincalculates to approx £40 - £45

The electronics element I have used is a circuit I recall I found via this forum, which had been designed and used by a Mathew Stevens in making up a Heater Control circuit for the production of Bio Diesel. This part of the circuitry is highlighted in pink on the drawing. If you search via Google etc using “ Mathew Stevens Immersion Heater Controller” you should find a PDF document fully explaining Mathew’s circuit and construction details. I have slightly amended his parts list and construction method for reasons, which I will explain later

My solar installation is 4kW working via a SMA 4000TL Inverter from which I have used the Multi Function (MF) relay facility to partly control the Immersion Controller. The MF relay is set for “Self Consumption “mode such that the relay operates when the Solar Panels are generating at least 1500 watts (Min On Power). The “Min On Power time” and “Duration Time” settings have both been set to 5 minutes.
The circuit elements associated with MF relay control are highlighted in green on the drawing – if you haven’t got any form of MF control this part of the circuit can be deleted.

Design Notes - the variable control provides the flexibility of powering a normal 3Kw Immersion from 250 watts to the full 3 kW. ( the 1 Megohm trimmer pot used by Mathew Stevens across the main control pot is not necessary and can be deleted)

Throughout, I have mainly used components that have 1/4inch(6.3 mm) spade connections – relays, main switch, triac etc. Using Yellow fully insulated female connectors on the ends of the wiring. This makes construction that more easy and component changes should they fail a doodle. For the Triac I used a BTA40 – 700B 40A version. This is a fully insulated TO3 cased device (like power transistors)that handles up to 700 volts at 40amps, so plenty of safety factor built in. I used a die cast aluminium case to house the controller and this acts as an effective heat sink to the triac. In my photo you will note a separate heatsink bolted to the side – this isn’t necessary and was a design overkill, asthe case only gets perceptively warm.

The small components – capacitors, resistors etc are mountedon a small Vero board, the high current paths being bolstered using 2.5mmcopper wire soldered along the Vero copper tracking strips.
In lieu of making a choke (power inductor) as per Mathews’sdesign I used a 22uH power inductor from RS components – see parts list.

For the MF relay control part, I used two Omron G7L – 2A-TUB –CB 120 volt ac coil relays. The reason for two over one 240 volt ac coiled version was availability and the fact that I could parallel up the two contact sets on each and by doing so hopefully extend the relays life. I included a 200ohm 5 watt resistor into the relay circuit to maintain the operation current to approx 10mA. The MF relay in the SMA inverter is designed to handle up to 1 ampso its life expectancy should be good?
The “MF” indicator simply lights each time the set inverter(1500 watts) is reached.
The “Override” switch allows the unit to control the immersion when there is less than 1500 watts available. Simply reduce controlto 1 kW setting and operate override switch – handy in the spring and autumnwhen light levels not so good.

I first tested the Controller using a 100 watt GLS bulb as the load and when all proved ok I used our 3kW Kettle as the load with anAVO meter in line to check the current and hence calculate the wattage per scale mark on the Potentiometer knob scale.

A satisfying little project that will service my requirements until Immersun and the like fall in price due to an exhausted market or competition from elsewhere.

Parts list with suggested sources – please re check stocknumbers.

Ebay - DieCast Aluminium case
BTA 40 – 700 BTriac -
Omron G7Lrelays –
6.3 mm Fully Insulated Yellow Female Spade Connectors

RS Components - Toroid Power Inductor 22uH Vertical – Stock no 7360964
Radial Polyprop Capacitor 33nF 1Kv –Stock No 1908494
Radial Polyprop Capacitor 150nF 630V– Stock No 1908393
Diac 2A 32volt - Stock No 7140521
Rocker Switch DPST Illuminated 20Amp – Stock No 6077450
Indicator Green 250volt – Stock No1951151
Quick acting LBC 15 Amp Fuse – StockNo 6686051
1 Meg Ohm trimmer Pot(discretionary) - Stock No 1542498

Home parts bin - Veroboard
Fuse Holders & 100ma fuse
470 Kohm Linear Pot. & Knob
Override Toggle Switch
Strip Connector (15Amp)
Cable & wire