Building a controller...help please

vinman · Post by **vinman** » Thu Jan 20, 2011 4:48 pm

thanks DBS. I went with the 20ga. I'll be wiring most of the controller this weekend. Having trouble finding a suitable main power switch. I've got 15A toggles for the individual blankets. But 30 amp double pole toggle is nearly impossible to find. I might just go with the type of switch Head monkey used.

Can I safely use a 25A switch in my set up if I'm connected to a 30A line? My blankets together will draw just under 22A.

Would a 10/3 wire dryer cord work for a main power cord running from my outlet to my control box?

doughboyshredder · Post by **doughboyshredder** » Thu Jan 20, 2011 7:45 pm

Vinman wrote:thanks DBS. I went with the 20ga. I'll be wiring most of the controller this weekend. Having trouble finding a suitable main power switch. I've got 15A toggles for the individual blankets. But 30 amp double pole toggle is nearly impossible to find. I might just go with the type of switch Head monkey used.

Can I safely use a 25A switch in my set up if I'm connected to a 30A line? My blankets together will draw just under 22A.

Would a 10/3 wire dryer cord work for a main power cord running from my outlet to my control box?

Yes on the 10/3.
As far as the main power switch, I don't have one. I just unplug it when not in use. I brought a neutral out and have 115 going to my pid's which is switched with a toggle, and 115 going to my hydraulics, which is switched with a toggle.

falls · Post by **falls** » Sat Jan 22, 2011 12:41 am

I think officially you are meant to run the actual thermocouple paired wiring from the quick release jacks to the PID. The whole basis of a TC is the pairing of 2 different wire types and if you then run just normal wire from the QR plug to the PID you are kind of depleting the purpose of the special wire pairing.
I just got a TC with a bit extra length wire and cut a bit out before reattaching the ends to the QR male plug.
The TC will still work without doing this, but I think that continuing the paired wire all the way to the PID is the absolute right thing to do.

On a side note I think I read some people were having trouble with malfunctioning TC's in terms of the temperature they were reading. I was reading the manual for my PID the other day and it said that having an unearthed TC in contact with a large metal area (eg. a cassette piece of aluminium) can subject it to electrical fields that can upset its readings. Anyone think there is anything in this.
My TC is a bare metal end type and just taped direct to an aluminium sheet. This sheet is grounded so maybe that is adequate?

vinman · Post by **vinman** » Sat Jan 22, 2011 6:36 am

yeah I guess that makes sense. I was going to buy an extension cable for my TCs anyway. I could probably use a length of that to connect the TC jacks to the PID terminals.

vinman · Post by **vinman** » Sat Jan 22, 2011 6:07 pm

I found K termocouple wire on ebay today for a good price that I'll use to connect the jacks to the PIDs. I'll also use these to make extensions for the TC themselves with this.

Also I started wiring my controller today. I should be done later next week once my female 220 outlets and fuses arrive.

I'm still looking for a suitable main power switch. But I think I might have finally found it. I have to double check with my electrician to see if it would work.

krp8128 · Post by **krp8128** » Sun Jan 23, 2011 7:20 am

Vinman wrote: I'm still looking for a suitable main power switch. But I think I might have finally found it. I have to double check with my electrician to see if it would work.

Home Depot has a 220v double pole switch rated for high amps, they are designed for water heaters. I'll check the rating on mine, IIRC the housing is green.

EDIT:

240v Double Pole, rated 30A for a heater.

http://www.homedepot.com/Electrical-Dim ... ogId=10053

vinman · Post by **vinman** » Sun Jan 23, 2011 8:03 am

right on, thanks. I think that is what I'll have to go with. It is not the style of what I was looking for but it will work.

doughboyshredder · Post by **doughboyshredder** » Sun Jan 23, 2011 9:28 am

falls wrote:I think officially you are meant to run the actual thermocouple paired wiring from the quick release jacks to the PID.

I thought about this a bit initially, but am convinced it doesn't really matter. I could be wrong though. I suppose it also may depend on the length of wire from your tc jack to your pid. My understanding is that the tc functions only at the head where the dissimilar metals are welded together. The only function of the wire is to provide voltage to and from this point. As the temp changes more (or less) voltage makes it through the head and comes back on the other wire. This is why plugs don't alter the reading.

vinman · Post by **vinman** » Sun Jan 23, 2011 10:12 am

Well, once I get up and running I can do some experimenting on this. The TC wire that I bought was ~$14 for 25ft of stranded 24ga K TC wire. Unfortunately the shipping was almost as much as the price of the wire. If you are interested search ebay for "thermocouple wire". There are a couple decent deals there compared to buying directly from Omega or Cole-Palmer.

krp8128 · Post by **krp8128** » Sun Jan 23, 2011 10:26 am

doughboy,

You are right and wrong, at the same time.

Thermocouples read at the tip, where the dissimilar metals are joined. You aren't applying a voltage to a thermocouple, the voltage is generated by the change in temperature of the dissimilar materials.

When you extend the t-couple with any old wire you are creating another t-couple (with unknown properties). This is then in series with the t-couple you want to read, and will throw off the temp reading in some way.

Use the correct extension wire for your thermocouple, along with the correct plugs and the correct polarity.

doughboyshredder · Post by **doughboyshredder** » Sun Jan 23, 2011 4:22 pm

Thermocouple wire is available in several different metallurgical formulations per type, typically, in decreasing levels of accuracy and cost: special limits of error, standard, and extension grades.
[edit]Extension wire
Extension grade wires made of the same metals as a higher-grade thermocouple are used to connect it to a measuring instrument some distance away without introducing additional junctions between dissimilar materials which would generate unwanted voltages; the connections to the extension wires, being of like metals, do not generate a voltage. In the case of platinum thermocouples, extension wire is a copper alloy, since it would be prohibitively expensive to use platinum for extension wires. The extension wire is specified to have a very similar thermal coefficient of EMF to the thermocouple, but only over a narrow range of temperatures; this reduces the cost significantly.
The temperature-measuring instrument must have high input impedance to prevent any significant current draw from the thermocouple, to prevent a resistive voltage drop across the wire. Changes in metallurgy along the length of the thermocouple (such as termination strips or changes in thermocouple type wire) will introduce another thermocouple junction which affects measurement accuracy.

Interesting.
Wondering how using 22 gage twisted copper wire has been effecting the readings. I doubt it has, but I will find out this week.

vinman · Post by **vinman** » Sun Jan 23, 2011 5:22 pm

How much error are we talking about? If if is a small percentage and both blankets are wired the same it might not make much difference to us.

Good read for TC errors

http://www.hogsden.net/Diagnostic%20for ... 0error.pdf

doughboyshredder · Post by **doughboyshredder** » Sun Jan 23, 2011 11:27 pm

Vinman wrote:How much error are we talking about? If if is a small percentage and both blankets are wired the same it might not make much difference to us.

Good read for TC errors

http://www.hogsden.net/Diagnostic%20for ... 0error.pdf

from that

The second common problem is the wrong type of wire. The extension wire used has
to be same K type TC wire or K type TC extension wire. You can’t use regular copper
wire.

Shit.

vinman · Post by **vinman** » Thu Feb 10, 2011 5:36 am

anyone have a good method to test a home built controller prior to hooking it up to expensive heat blankets?

I'm planning on having my electrical guy go over it with a voltage and amperage tester to make sure things are wired correctly.

but I also want to test me thermocouple extensions I made.

Basically I took a male TC plug and unscrewed the prongs from the terminals. I ran the fine wire TC into the now transexual plug and joined it with larger gauge TC wire at the terminal. At the end of several feet of TC wire I put a male plug on that will insert into the outlets on my control box.

I'm going to meed to prevent excessive bending on those wires as well anyone have some good strain relief ideas. I have some Kapton tape that I was going to use to tape the TC to the blankets maybe I could wrap the wires with that and then tape them to the body of the plug to prevent them from breaking....

doughboyshredder · Post by **doughboyshredder** » Thu Feb 10, 2011 7:09 pm

those little wires are surprisingly tough.

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Building a controller...help please