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I've finally gathered all the bits I need for making an attempt on a heat blanket.
A - 10 A circuit breaker form Jaycar electronics $6. May not have been necessary because the relays on the controller I've made are fuse protected but this is worth adding for an extra level of safety.
B - A light/ motor dimmer . This uses an SCR to reduce power not a resistive type that drops power across a variable resistor. The SCRs don't get hot as they rapidly switch the power on an off at the frequency of the AC supply - $34 jaycar electronics. This will be used for controlling the maximum temperature. The controller switch that I have built separately will be used to switch the blanket on and off to approximate different temperature ramp profiles.
C - The screw terminals out of a terminal block. These will be used for connecting the resistance wire to the 240 power input. Apparently ni-chrome wire is impossible to solder so crimping or screwing it is the only option. About $1 from jaycar.
D - 13.68 ohms/meter ni-chrome wire. This is the heating wire. If doing again I'd use slightly higher resistance and space them out more. To get the right power with this I needed to use spacing of only about 1cm. Would have been less work to be able to space them 1.5-2cm. I have 4 circuits in the blanket connected in parallel. Each is about 200 ohms total resistance and in parallel they around 50 ohms.
E - Extension lead with the wires bared ready for connecting the ni-chrome resistance wire. $14 for about 90m on ebay (US)
F - Fibreglass dry-wall tape from Bunnings $16
G - Mold Max 40 2 part RTV silicone rubber 1kg $52 from rowetrading.com.au in South Australia
H - Excess 6 oz s-glass that I had left over from previous board projects. 2 layers 150x50cm needed.
This will make a 1kw blanket.
A few notes on the calculations
The calculations for the blanket are pretty straight forward.
For a 100 C blanket the power needs to be around 1.1kw.
Power = V^2/R
Where R is the resistance of the wire in the blanket and V is the supply voltage. Mains supply is usually an RMS value. RMS is just and average value of teh AC supply that makes calculations for average power mirror the much simpler direct current calc as above. In Australia its 230 V RMS (but with a range). So the average power consumed ( converted to heat) can be calculated as 230x230/R where R is the resistance of the wire in the blanket.
In my blanket I have 4 separate circuits, each of which will be connected between the active an neutral wires on the extension lead. This arrangement is referred to as parallel connection. As each of the circuits (continuous wire section) has the same length and its made from the same wire they have the same resistance. when you connect these in parallel, the resistance of that combination is calculated by dividing the resistance of one circuit but the number of identical circuits. So, the more circuits, the lower the resistance, the greater the power consumed in the blanket.
In my blanket I have 10 lets of wire in each circuit. The blanket is 1.5m long so the resistance of one circuit is 10x1.51mx13.60 ohms/m = 205 ohms ( the 1.51 m is used because each length has some wire that connects it to the next length of wire in the pattern). There are 4 circuits so the resistance is 205/4 = 51.3 ohms
the total power is then
P = 230 x230 / 51.3 = 1031 W
For a blanket this size that is about 1 W/ sq. inch which from the charts posted on the skibuilder forum is about right for hitting 100C.
By contrast industrial strength blankets have between 3 and 5 W per square inch. 3 W is required to hit the allusive 200C ( at least according to what I can see on some of the sites for the production blankets) and of course the materials you use need to have higher ratings as well.
I'm taking a couple of mental health days so had a chance to get moving on the heat blanket.
First things was to put the fibreglass down and then hammer in nails in at 1.3 cm space at the top and bottom. The wire gets strung around the nails it it keeps them uniformly spaced and lets you keep tension on the wire while you fix it in place.
With the wire help in place with a small weight I then put stuck them down with pressure sensitive fibreglass drywall tape ($16 from our local hardware).
Then the messy part. The 2 part silicone rubber I've got is quite thick. This is a bad shot but it shows how viscous the stuff is by the way it holds shape. The pack says 40,000cps is the viscosity. Not sure what to do with this info other than tuck it away for reference when comparing to others in the future.
At this stage I'm not going to cove the area where all the connections go. The only reason being that my table is not long enough!!!! So I'm put the rubber down and will let it cure so that I can take it off the table and work on the connections later.
My approach was to put a 440gms (including part B) of the rubber over the wire/ drywall tape and then wet it out just like you would resin and glass. To minimise the amount I disturbed the wire alignment I only worked the rubber through in the long direction of the blanket. Similar to resin, their is an audible popping that goes along with air bubbles trapped being worked out of the lay-up.
It seems like using the fairly light fibreglass was a good idea as the rubber was able to flow through the weave fairly easily.
Then I placed another layer of the say 6 oz woven glass on top and used the fibreglass roller to squeeze some of the rubber already poured into the new layer. I was suprised how much came through, even just wicking up into the new layer. I then poured 220gm of rubber over the new layer and wet it out with the squeegee like before.
The total of 660 gm of rubber was a little bit light for the area when just laying it up by hand. The 200gm of rubber which I have left was intended for putting on the underside of the blanket when it comes off the table. However, when I took it off the table this morning, it seems that plenty of rubber got through to the other side, maybe because of the lighter weight cloth I was using compared to the projects on skibuilder.com. This makes me think the better idea might have been to pour the whole 1 kg at one time.
Off the table
Unfortunately a few wrinkles appeared on the underside of the blanket because the builders plastic I used to cover the work space had wrinkles in it. With a bit of pressure on top the rubber just accumulated. This won't be a problem for the way I'm going to use it ( post curing or using a bro-kite style mold with 2 sheets of c. 5mm plastic top and bottom ) as it won't be in direct contact with uncurred resin. However, it does kill that possibility unless I can do something with the remaining rubber.
in hindsight, a better idea would have been to either use self-adhesive vinyl for a release surface and put a bit of weight on top while it cured... oh well always next time.
Today I wired up the connections on the blanket
I used aluminium crimping tubes to connect the ni-chrome wire to short lengths of wire that can be soldered onto the power lead lines. As the ni-chrome wire is so fine I found it better to make a bend the end of it over is a loop and put the loop and the extension wires in the tube and then crimp. Made for a better mechanical joint as well.
Then I removed section of the insulation from the power lead lines (a disused extension lead) where I wrapped the lengths of wire around the exposed wires and will solder them. Solder's melting point ins near 180 C typically so the 100 C blanket shouldn't cause any problems.
Prior to solder, a quick check that the resistance is as expected. Any difference would mean the circuits where not wired correctly.
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