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I'm going to add a second heating blanket to my bender, so that I can go to a
blanket/slat/wood/slat/blanket stack to get more uniform heating of the wood.
I use a PID controller with a thermocouple and relay to drive the single blanket
I use now. Can I use the two blankets in series, or am I better off to add a second
relay (which will require some shoehorning to fit in my PID box).

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Gene

Politicians and diapers must be changed often, and for the same reason- Mark Twain

don't go series use a parallel circuit .

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

As long as the total amps are not greater than what the relay can safely handle, and as John says, parallel the blankets.

Fred

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Fred Tellier
http://www.fetellierguitars.com
Facebook page http://www.facebook.com/pages/FE-Tellier-Guitars/163451547003866

Yeah....as long as the combined amperage of the blankets doesn't exceed the amp rating of the relay you can run as many blankets on one relay as you want.

Do you know the amp draw of your blanket(s)? You may find markings on the blanket that says 5w or 10w....which will be watts per square inch. Take that times the square inches of the blanket and then divide by 120(volts) to get the total amp draw of the blanket.

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

OK, Thanks!

They are both John's 6" x 34" blankets, which are 5 watts/in*2. So that should
give me 3.4 amps draw. I'll check my relay, I think it was good for at least 10 amps.

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Gene

Politicians and diapers must be changed often, and for the same reason- Mark Twain

8.5 amps?

I think Alan is closer if they are 5W per sq in. (6 x 34 x 5)/115 = 8.8, better check with John to make sure.

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Steve Smith
"Music is what feelings sound like"

Yeah.....dividing by 120volts.....that's an 8.5 amp blanket.

6 x 34 = 204 sq. in.

204 x 5 = 1020 watts

1020 / 120 = 8.5 amps

_________________
I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

Oops, my bad

You're right, 8.5 amps each, 17 total. So I'll need at
least a 20 A relay.

Thanks for the correction...

_________________
Gene

Politicians and diapers must be changed often, and for the same reason- Mark Twain

Look at all the Math wiz kids LOL / I want to add one thing about 2 blankets . Remember that wood is stupid and will bend when it gets hot . With 2 blankets you will have a heavy thermal mass. Watch the heat carefully so you don't scorch. Moisture will help but too much can be worse than not enough .
Wood is thermoplastic and will bend when the proper temp is reached . I bent over 200 sets this year. Lost 3 . 2 were high figure maple and one a lower end BRW . I have seen this method before and I know it will work but the pitfalls are scorching . Methodology of the bend process is equally important . Think PRESSING not bending .
Good luck and keep us posted , we all like to learn new things .

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

John, et al,

I appreciate your comments. And I'm laughing at myself for the math mistake. Been too many years
since either math or electrical engineering classes. It's the pits getting old

I want to try out the double blanket approach because I've had some buckling problems on several
highly figured woods. It seems to me that if I can control the heat, getting the whole wood mass
-especially both sides, up to equal temperature should help with more even bending. When bending
on a pipe, the compression side of the wood is always the hottest. I think that helps prevent buckling.
So if in machine bending I can get both sides to the same temperature it should help, since both sides
see both tension and compression. I've got the PID controller, so I should be able to control the temperature
pretty well- I'll watch it especially carefully until I get it dialed in. And I've wanted a spare blanket anyway,
in case the one I have dies on me.

_________________
Gene

Politicians and diapers must be changed often, and for the same reason- Mark Twain

keep us posted . I am sure there is a lot of interest on this .

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it