Post by cye on Apr 27, 2014 2:25:52 GMT -5
OK, after having been asked several times about the mod required to the Velleman 12v thermostat to convert/hack it into a crude differential temperature controller, and having forgotten it several times over, here it is in writing lest I forget it again.
For those who don't mind an an hour's soldering, the basic thermostat kit costs about £6 from Maplin. Once assembled and with the minor adaptation described below, this then becomes what is probably the most affordable differential temperature controller available.
The circuit needs a 12v dc power supply(such as a 12v battery) and triggers a relay which is then used to switch on/off a 12v solar hot water pump. A potentiometer/knob is also provided which enables some degree of adjustment to the triggering temperature difference ( DeltaT), enabling one to set it so it switches the pump on when the panel is say 4 or 5 degrees hotter than the tank.
I know Caveman has some ideas on how to make the circuit better (more robust), but here goes with the basic mod as a 'starter for ten':
STEP 1 - Purchase the following
- (a) Velleman 12v Thermostat Kit MK138
- (b)One extra NTC10k thermister. I.e., exactly the same as the one already included in the kit (shown at section 12, SK1 of the attached schematic). The reason you need two thermisters (temp sensors) as opposed to one is because this is a differential temperature controller which will compare two temperatures, i.e., tank temperature versus panel temperature, activating the relay (switching on the pump) whenever the panel temp is X degrees greater than the tank temperature.
- (c) A small connector or terminal block suitable for fine cables.
- (d) some fine cable, such as bell wire, CAT5 cable, or alarm cable. This will be used to connect the two sensors to the circuit board.
- (e) some epoxy resin, such as the 'twin tube' resin glue available in Pound Shop, or if you prefer branded products, Araldite, Bison, etc
See attached circuit diagram and instruction manual of the basic Velleman kit.
STEP 2 -
Solder up the kit ALL EXCEPT for resistor R4. Instead of R4 (which we are now replacing with the extra thermister we have bought), solder in two small fly leads (two strands of cable as per part D above) in place of the two legs of resistor R4 and connect the two fly leads to your extra connector block (part c above)
STEP 3 -
Connect your extra NTC10K thermister (part B above) to the connector block (part c) mentioned at STEP 2
STEP 4 -
Mount your board in a suitable enclosure, such as a plastic project box from Maplin, and old lunchbox or even a margarine tub.
STEP 5 -
Weatherproof your thermisters by applying a liberal coating of epoxy resin.
CAUTION - The Velleman circuit does not have reverse polarity protection. I.e., If you accidentally wire +12v to the GND and the ground to +12v, then you will blow the board.
I will try and get some photos and load them up shortly
For those who don't mind an an hour's soldering, the basic thermostat kit costs about £6 from Maplin. Once assembled and with the minor adaptation described below, this then becomes what is probably the most affordable differential temperature controller available.
The circuit needs a 12v dc power supply(such as a 12v battery) and triggers a relay which is then used to switch on/off a 12v solar hot water pump. A potentiometer/knob is also provided which enables some degree of adjustment to the triggering temperature difference ( DeltaT), enabling one to set it so it switches the pump on when the panel is say 4 or 5 degrees hotter than the tank.
I know Caveman has some ideas on how to make the circuit better (more robust), but here goes with the basic mod as a 'starter for ten':
STEP 1 - Purchase the following
- (a) Velleman 12v Thermostat Kit MK138
- (b)One extra NTC10k thermister. I.e., exactly the same as the one already included in the kit (shown at section 12, SK1 of the attached schematic). The reason you need two thermisters (temp sensors) as opposed to one is because this is a differential temperature controller which will compare two temperatures, i.e., tank temperature versus panel temperature, activating the relay (switching on the pump) whenever the panel temp is X degrees greater than the tank temperature.
- (c) A small connector or terminal block suitable for fine cables.
- (d) some fine cable, such as bell wire, CAT5 cable, or alarm cable. This will be used to connect the two sensors to the circuit board.
- (e) some epoxy resin, such as the 'twin tube' resin glue available in Pound Shop, or if you prefer branded products, Araldite, Bison, etc
See attached circuit diagram and instruction manual of the basic Velleman kit.
STEP 2 -
Solder up the kit ALL EXCEPT for resistor R4. Instead of R4 (which we are now replacing with the extra thermister we have bought), solder in two small fly leads (two strands of cable as per part D above) in place of the two legs of resistor R4 and connect the two fly leads to your extra connector block (part c above)
STEP 3 -
Connect your extra NTC10K thermister (part B above) to the connector block (part c) mentioned at STEP 2
STEP 4 -
Mount your board in a suitable enclosure, such as a plastic project box from Maplin, and old lunchbox or even a margarine tub.
STEP 5 -
Weatherproof your thermisters by applying a liberal coating of epoxy resin.
CAUTION - The Velleman circuit does not have reverse polarity protection. I.e., If you accidentally wire +12v to the GND and the ground to +12v, then you will blow the board.
I will try and get some photos and load them up shortly