Official Luthiers Forum!

Are there any electrical engineers here ("real" or seat-of-the-pants)? I have already spent too much time trying to figure this out, and too much money on the parts to start over...

I am attempting to construct a timed, thermostatically controlled, heating system with a silicone heating blanket. I'm in over my head here. I don't know if this is too complex to ask in a forum. Maybe I need to find an electrical engineer to help me with the schematic for this. Or, maybe this is easy for someone here, and I could get some help with this.

I have:

===========================

   Digital Timer: Omron H3CA-A H3CA
   Temperature Controller: Omron E5CS-R1KJX-F (E5CS)
   Solid State Relay: Crydom A4825 (plus a big 4"x5" finned heat sink)
   Silicone Heating Blanket: MEI M3658 (120V, 1080W)

============================

I had hoped to use this digital timer to close the AC circuit and allow the blanket to heat for a specific number of minutes (kind of like a darkroom timer), but it appears that the only 115V is input. (Input terminals are no-voltage only. Output terminals do not have voltage stated.) My first question is: Do I need to buy an additional switch, and let that switch get tripped by the timer's output? Or, can I pass one leg of the 115V through the COM output terminal?

Ideally, when power is applied, the circuit for the heating blanket will be on for the length of time specified by the timer, and the temperature of the blanket regulated by the temperature controller. Is this a difficult circuit to draw a schematic? Or, is my brain too small?

Any help will be greatly appreciated.

Dennis

_________________
Dennis Leahy
Duluth, MN, USA
7th Sense Multimedia

Just wait, Dickey will show up!

Serge

Hi Dennis .. university was a ways back, but I stil have the degree !!!

I would imagine the timer can be made to work with a relay, but personally I would ditch it and just get a rotary mechanical one - home depot, 25 bucks - it will be more than close enought for 10 or 15 minutes worth, and it can control 115 volts. easy too, one in, one out, done.

The temp control you have needs a thermocouple, the specs list a couple there, in order to feed the temp into it to control. You dont say you have one.

The temp controller also only controls up to 3A at 250V - thats only 3x250= 750 watts - you have a 1050 watt blanket, not good IMO !!!

You have too much blanket or not enough temp control !! A 550 watt blanket ( omega sfrg 636/2) is tons - it will scorch wood if left on full too long.

So the circuit you want is pretty easy to do, but with other gear than you currently have. Thats how I would do it. But I dont bother with temp control, just the timer, and a dimmer - heat up the 550w at full for 5 minutes while bending, turn down to about 3/4 for a 10 minute bake. Good enough.

_________________
Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

[QUOTE=TonyKarol] Hi Dennis .. university was a ways back, but I stil have the degree !!!

I would imagine the timer can be made to work with a relay, but personally I would ditch it and just get a rotary mechanical one - home depot, 25 bucks - it will be more than close enough for 10 or 15 minutes worth, and it can control 115 volts. easy too, one in, one out, done.

The temp control you have needs a thermocouple, the specs list a couple there, in order to feed the temp into it to control. You dont say you have one.

The temp controller also only controls up to 3A at 250V - thats only 3x250= 750 watts - you have a 1050 watt blanket, not good IMO !!!

You have too much blanket or not enough temp control !! A 550 watt blanket ( omega sfrg 636/2) is tons - it will scorch wood if left on full too long.

So the circuit you want is pretty easy to do, but with other gear than you currently have. Thats how I would do it. But I dont bother with temp control, just the timer, and a dimmer - heat up the 550w at full for 5 minutes while bending, turn down to about 3/4 for a 10 minute bake. Good enough.

[/QUOTE]
Hi Tony,
Yes, the blanket has a thermocouple (built-in) J-type, to match the temperature controller. And, I'm really hoping to use the parts I have, to go into the box I made, with holes cut for these components...

I know, I know, it would be easier to replace the cool digital timer with a mechanical one, but if I can make this work, I'd sure prefer it. I know I have more precision here than I need, but it would be a waste to throw out one or more components just because I can't figure some aspect of this out. I'm not sure if this is the stubborn Irishman or the frugal Irishman coming out...

As for the blanket's wattage, yes, this blanket has the range to cook at a much higher temp than what I need to bend wood, but it should also go to any temperature in the range I need. From what I understand, I will never need to go higher than about 325°F. I can get to 325° without pushing 750Watts, right?

I wish I knew something about electrical engineering! Dang it! I just hope I'm close to making this work!

Dennis

_________________
Dennis Leahy
Duluth, MN, USA
7th Sense Multimedia

Sorry to disappoint you Serge, just an electrician. This falls more into the electronics realm. I like the Jim Olson way, two wirenuts and an old extension cord cut off a defunct appliance. About a buck fifty if you buy a new cord and wirenuts.

I did use a little controller on mine. I wired a duplex receptacle to a 1000 Watt Commercial Dimmer with a heat sink. A standard house dimmer is only rated for 600 watts, and they need to be heavier than that for most heat blankets.

For those with heat blankets and no control, I understand some folks are using a Harbor Freight Router Speed Controller. Whatever anyone uses, they have to be rated to carry the voltamps.

Just buy the timer controller rig that LMI sells, and auction the stuff you have on eBay, then get on with guitar building.

_________________
Chris Nielsen
Soquel, CA.

Hi Dennis - OK, so the thermo is in the blanket, good. You still have an issue with the wattage though - you are right to get 25 out of the blanket doesnt need all 1050 watts - but to do that you are going about it the wrong way - the temp controller only knows on and off - whcih means all 115 volts and all 1050 watts, but in short bursts - the circuit will likely cook. The only way to control the blanket is to drop (variac) or chop (a rated dimmer) its voltage. Variacs (an auto transformer) reduce the line voltage to a level, like from 115 to 93 volts for a EVH Marshall brown sound. Most if not all dimmers dont drop the voltage they chop it up into bits of on and off, but still 115 volts. So the blanket controlled with an on/off circuit will want to draw all 1050 watts thru it, just for a shorter time to reach the 325 at which time the circuit shuts it off again until it cools some. IMO what you have isnt going to work for long until it roasts.

_________________
Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Check out this thread. You can see what I did about half way down (pic included)

http://www.luthiersforum.com/forum/forum_posts.asp?TID=2953& KW=Brock+bender#forumTop

I lifted this idea from someone... but I forget who...Brock Poling38736.8403935185

_________________
Brock Poling
Columbus, Ohio
http://www.polingguitars.com

Man you are over complicating this operation. To bend wood you need 2 things , heat and duration. The heat range is 275 to 375. That will bend 99% of all wood you will deal with.
   The actuall amount of time of heat is less that 10 minutes. I am usually finished within 5-7 minutes. This week alone I have bent abut 8 sets. Average of about 6 a week.
   If you need to use all the controls sure as you did get them but my heating blanet with a timer and contoller with a thermomter is about $175 complete.
   

Dennis,

Okay, a few things. Have you used a PID before? I have, albeit not for a bending blanket.

As others have said, PIDs work by turning on and off. So your blanket will get a full shot of electricity, turn off, turn back on, etc., maybe about 1/sec.

Second, the PID should be wired into a heavy duty relay, so the current handling of the PID is not an issue - all the current goes through the output side of the relay.

Third, it depends on the PID, but in general they have 2 modes of operation. The first mode brings you up to temp quickly, but this causes overshoot, meaning things get too hot. It takes awhile for things to settle down. The second mode builds temperature much more gradually, avoiding overshoot, but it takes a long time to get to temperature. Neither of these things is particularly desirable in bending.

When you program a PID, it runs several heat/cool cycles to determine the performance of the machine it is hooked up to. Make sure that you set it so it does not exceed the temperature of the set point, or you may drive your blanket to 500F and burn the place down. The programming will take a lot longer.

The accuracy of the PID depends on the programming, and will suffer if you try to control the circuit at a different temp. For example, if you program it to work at 300F, it won't hold 325F as well as it could if you programmed it to run at 325F.

Since you don't know EE, I am loathe to give you instructions on how to wire this thing up. You need to use the correct gauge wires, the correct cooling for the relay, a properly wired relay, etc., and then program the PID correctly to avoid burning the house down. If you are just following directions rather than truly understand how to calculate these things, a mistake will fly under the radar and kill you or burn the house down. It's really not that hard, but the consequences of a mistake, well...

Let me put it this way. I have a spare PID and some thermocouples hanging around, and I haven't bothered wiring this up. Not because I can't, it's easy peasy, but because I don't see the benefit in a non-factory situation, and because I think it will be slower than just doing it by hand. PIDs are designed for industrial applications where holding a temp within 1F is important, not for quickly ramping up temp over a 230F delta, and then holding it with an accuracy of 25-50F. Wrong application, IMO. But then I haven't tried to use a PID in that setting, so I may be wrong.

If you're trying to have some fun, go on ahead, but be careful, as these currents are lethal, but if you are trying to bend wood, just get the LMI setup and be done with it. IMHO.