# DIY Heating Pad (Kanthal A1, 120VAC, Dimmer)

Discussion in 'General Electronics Chat' started by Guest3123, Sep 20, 2016.

1. ### Guest3123 Thread Starter Member

Oct 28, 2014
312
18
So I see they have heating pads, Flexible Heat Film, and Thermostats (HERPSTAT 1, VE-200, & Jump Start MTPRTC).

I'm very familar with Kanthal A1 Resistance wire.

I'm very familar with the resistances of Kanthal A1 Resistance wire.

So the chart shows 26 AWG highlighted. From the official site, listed in the link above the thumbnail.

It shows a resistance of 3.21 Ω/Ft.

I need it in Ω/mm.
12" x 25.4mm = 304.8mm / Ft.
3.21Ω / 304.8mm = 0.01053149606299212598425196850394 Ω/mm

Got it.

Now my Container, that I'll be keeping the feeder insects in needs an internal tempature of 75°F - 85°F.
The higher the tempature, "85°" ideal, the better, and the more likely they'll bread.

They are feeder insects for my Adult Female Chinese Praying Mantis (Tenodera sinensis)

They can also be used for other reptiles, bearded dragons, etc. etc. etc.

Anyways, winter is coming. It gets cold up here, and I want to learn how to do this before then.
I've already built a prototype heating pad, using an EVGA 430 Watt PSU, connected to the 12V rail.

The limit of the 12V rail is 35A, and I have an inline 30A fuse installed on that rail, not to mention, the PSU has built in fuse protection, etc.

That doesn't say crap about the fact that my title says 120VAC. Now does it.

I'm going to use 120VAC in the end.

The heating pad will be low wattage, VERY LOW.

With the pad that I made already, calculating the resistance of the Kanthal A1 26 gauge wire before installing it, I calculated for 2 Watts.

This is what I got.

So clearly, I know how long the resistance wire needs to be, in order to hit a target wattage.

So I wanted to hit 2 Watts, using the 12V rail on the EVGA 430 Watt PSU.
My main goal was to read 85°F (29.44°C) on the board.

I did it.

I also know how to program simple programs in Visual Studio 2010, coding in VB.NET.

So to do all these MAIN calculations, I made a piece of software.

Makes things almost too easy.

So now that all that's out of the way, I want to use 120VAC as my power source.

I would also like to use an Optional Dimmer & Transformer.
The Dimmer can be found here.
The Transformer can be found here.
The Transformer data sheet can be found here.

What do I need help with?
A. I need more voltage, for the container I want to use, and the area I want to cover. 24" (L), 13" (W).
B. Because the 26 gauge resistance wire needs to be 630" Long. to cover that area.
C. But it's only going to be half heated. So 12" x 13" (W). The screws will be spaced 1/2"
Here's the math (C).

12" (L) / 0.5" Screw spacing = 24 Screws, per side.
Each screw holds 2 Wires. One on each side of the screw.
24 screws x 2 wires each = 48 Wires.
48 Wires x 13" (W) = 624 Inches.

Now the screws are 0.09" diameter.
Let's see how much wire is wasted per screw.

Circumference of the screw shaft.
3.141592 x 0.09" Screw Diameter = 0.28274328"

It doesn't wrap around the hole circumference of the screw, only half of it.
0.28274328 / 2 = 0.14137164" Wire wasted per screw.

0.14137164 x 48 Screws total = 6.78583872" of wire wasted total per screw.

624" + 6.785" = 630 Inches roughly.

Q1. Will the dimmer go from 0% - 100%?
If I need the dimmer to go to 1 Watt, will it?

I need help with the Transformer I've selected.
It has six 0.25" terminals on one side (Primary), and six 0.25" terminals on the other side (secondary).

It allows 115VAC / 230VAC Primary. It outputs 48VAC on the other side, with a secondary current limit of 5.4A, with a secondary
current limit of 130 VA, which is Volts x Amps. 115VAC x 5.4A = 621 Watts..?

So.. If it's only rated for 130VA (Watts), then.. idk. It's not really that important, but I'd like to learn about it if anyones willing to talk to me about the one I've selected. I would like to learn about it, for future projects, and also to learn a little about it. As easy as it might seem.

I'll use just the dimmer for this project. Seems easy enough.

So.. What do you think? Not safe I know.. I'll also use Felt for the top of the board, so the wires don't make direct contact with the
bottom of the Hefty 66-Quart Clear Tote.

2. ### cmartinez AAC Fanatic!

Jan 17, 2007
3,572
2,535
I've done a similar thing in the past. Heating a thin, short nichrome wire using 120 VAC.
In the end, what worked like a charm was a combination of phase control and PWM using mosfets. And also, a good thermocouple thermometer.
The real challenge will be, IMHO, developing a good PID control logic to slowly bring your bin to temp and keep it there.

EDIT: You have to use an isolation transformer for your project, by the way, in order to comply with the TOS of this place. Otherwise you run the risk of having this thread closed by the moderators.

Guest3123 likes this.
3. ### cmartinez AAC Fanatic!

Jan 17, 2007
3,572
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Question, how proficient are you with microcontrollers?

4. ### Guest3123 Thread Starter Member

Oct 28, 2014
312
18
I've worked with the the Arduino Uno.

Last edited: Sep 20, 2016
5. ### tcmtech Well-Known Member

Nov 4, 2013
2,034
1,653
As long as he isn't using any LED's in the project he's fine.

LED's are what make AC line power lethal apparently.

If it was me I would skip all the complicated control and just use common commercial heat tape (a lower 1 - 3 watt per foot rated one sandwiched in sand between two aluminum or stainless steel sheets) and a PID unit with a PT100 thermocouple buried right at the element level.

Pretty easy to maintain a few degree temperature range with that setup.

Last edited: Sep 20, 2016
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6. ### Guest3123 Thread Starter Member

Oct 28, 2014
312
18
Why do I need a Transformer?
The Rotary Dimmer can handle 600 Watts by itself.

Will the Dimmer not be able to adjust the voltage from 120VAC to 0VAC?

7. ### tcmtech Well-Known Member

Nov 4, 2013
2,034
1,653
The admin here has a phobia about LED's being powered by line voltage, and at times other things power directly by line voltage as well. Depends on what type of day he's having really, so all projects here have to include an isolation transformer in the discussion or the thread gets shut down.

The dimmer would be fine for controlling a resistance heater element.

8. ### SLK001 Well-Known Member

Nov 29, 2011
817
226
The dimmer will work, but not do what you want. You need a control network to monitor the heat output and adjust the power into your resistance wire so that your temperature doesn't get so hot that it kills your bugs. A simple triac controlled by a micro should do the trick.

9. ### Guest3123 Thread Starter Member

Oct 28, 2014
312
18
Ehem.. I mentioned a Transformer in my thread. At the top. I listed the transformer, linking to it, and the data sheet. So it will not get shut down.

It's a 120VAC - 48VAC transformer.

As for the dimmer, I kinda figured it would work, but I needed a little guidance. Thanks for letting me know it will work.

I will be going to get the dimmer from lowes in the morning, if everything tonight works in this thread. I will get the transformer, regardless if the dimmer works or not, just because to spend more money, and give myself freedom of having the option of working with lower AC voltage.

So, here's the thing, I just read on a page that the dimmer doesn't turn on until it reaches 50%, or should I say, until the knob is turned half way.

The webpage can be found here

I would like to start talking about the transformer I've listed in my OP.

So the dimmer will not turn on, unless it's turned passed 50%. That sucks. That means that if I've cut 26 gauge Kanthal A1 Resistance wire the to 630 Inches, and it's got a resistance of 173Ω, at 50% power that's 40+ watts.

That would melt the bottom of the enclosure. Just to turn the power on. I would have to pick up enclosure, or make sure nothing like "felt" is on top of the DIY heating pad, turn it on, and them put the items on top of it. What a pain.

The knob only needs to be turned half way (50%) to activate, after that, it can be adjusted. So half of what the image says. It's still too high if something is sitting on top of the pad.

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Last edited: Sep 20, 2016
10. ### Guest3123 Thread Starter Member

Oct 28, 2014
312
18
I need help with understanding the transformer please. I don't understand the terminals. It has 6 therminals for the primary, and 6 for the secondary. I don't know which ones two is for 120VAC primary, and which two are for 48VAC secondary. If I need to choose another, maybe cheaper transformer, here's the main page for transformers. I like Mouser, and Digi-key. I've never had a problem with the products from them.

Mouser Part #:
546-185F24-BULK (main product page)
Mouser Part #: 546-185F24-BULK (The Datasheet)

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11. ### tcmtech Well-Known Member

Nov 4, 2013
2,034
1,653
Sorry. Went to puplic sckool, can't read big wordes good.

12. ### Guest3123 Thread Starter Member

Oct 28, 2014
312
18
Didn't mean to offend.. honestly.

It seems Mouser electronics has screwed me. It's not rated for 48VAC on the secondary.

The product comes from Hammond Manufacturing. Which is here.

The actual PDF is here, on Hammond Manufacturing.

It says that in series, the.. damn it.. Just never mind, I'll find a different one. It's \$32 anyways.

In series, it's 24VAC, in Parallel, it's 12VAC. So how is this a 48VAC transformer? I understand that it has SIX terminals on each side.

but not even on Hammonds site, does it say which terminal is what. So.. Maybe it's best that I find another Transformer.

13. ### Guest3123 Thread Starter Member

Oct 28, 2014
312
18

Mouser Part #: 546-187F48 (Main Product Page)
Mouser Part #: 546-187F48 (Mouser Data Sheet)
Hammond Mfg. : Main Page

What are the lungs? What's what? It doesn't describe what terminals are what, despite the really clear drawing.
It's numbered 1, 2, 3, 4, 5, 6, 7, 8.

Much bigger image, http://i.imgur.com/tBgBEuf.jpg

14. ### SLK001 Well-Known Member

Nov 29, 2011
817
226
You DO NOT WANT to use a dimmer for your application!!

15. ### Guest3123 Thread Starter Member

Oct 28, 2014
312
18
a
A simple triac controlled by a μC should do the trick.

Alright. I'm listening. Please teach me. I've seen a video showing how it's done, but I'm not familar with a triac. I've heard of them, and seen them, but don't know jack about them.

While I'm waiting, even if it's not until tomorrow, I'll just wait, and watch some more videos about the Triac. Thanks so much.

16. ### Guest3123 Thread Starter Member

Oct 28, 2014
312
18
Is this acceptable?

17. ### Guest3123 Thread Starter Member

Oct 28, 2014
312
18

I'm using a Isolation Transformer, and an inline fuse in my circuit.

18. ### cmartinez AAC Fanatic!

Jan 17, 2007
3,572
2,535
I would't use a Triac for what you want. Triacs require a minimum activation current, and another minimum value holding current (it's called hysteresis). That is why dimmers first need to be raised to a certain level before they kick in, and then you can lower them only so much before they completely disengage.
Using a mosfet with the technique that I described would be much more precise.

19. ### cmartinez AAC Fanatic!

Jan 17, 2007
3,572
2,535
The way that I propose to use the mosfet would be to first rectify the AC source to DC, using a diode bridge, and then have the mosfet switch your resistive load using PWM and in sync with the pulsed DC. This can be accomplished with a zero-crossing detector connected to the MCU of your choice. The mosfet could be driven by a small, optically isolated floating power supply.

This way, you could use any gauge and length of heating wire that you want.

The Arduino that you mentioned should be up to the task.

20. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,791
1,103
..... or you could simply use an off-the-shelf aquarium heater. It's thermostat maintains a similar temperature to what you want.

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