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Laph's Easy Single Variac Fan Control

93 posts in this topic

...So anyway my first little project inspired by the discussions here on UK420 has been killing fuses so I decided to return it to its (as far as possible) unadulterated state. Took it down stripped out all the gubbins I'd added and put it back together again. It's working great, fingers crossed no more blown fuses. I salvaged the relay because some other goodies had arrived in the mail. Anyway, here goes......


Building a Single Variac Temperature Operated Fan & Heating Control


Gewiss 300x220x120mm weather-proof box £12

Wattbits 2A Open Variac £45

ATC-800+ Temperature Controller £29


Maplin 5A DPDT 240v power relay and base

2x single gang trailing sockets

4x 20mm cable glands

in-line 20mm fuse holder

strip of chock block contacts

assorted bolts, terminals, wire and a plug.

I've seen other people use the ATC-800+ on here and I always thought "I must get one of those and try it out" As far as functionality goes it's perfect for my needs this time and I love the huge temperature display (meh to hermies). It operates on a set point differential, you set the temperature you want, 21c for example. Then set the difference or sensitivity, 2c for example. When the sensor detects <19c it turns on the heating and at >23c it turns on the cooling. The unit has two single pole normally open contacts. For heating it is a simple task of connecting the earth and neutral to the supply and switching the live via the unit.

For the cooling I'm going to use that relay I salvaged from my last project to switch between the current from the variac and supply voltage. I'll only use one set of contacts on the relay to switch between the two as the neutral is common. The output from the variac will be connected to the normally closed (NC) terminal of the relay with the supply voltage on the normally open (NO) one. This will enable me to set the fan idling speed using the variac and swap to full power as soon as it hits >23c and the relay is activated by the cooling circuit of the ATC-800+.

If you are following this then it's important to match the size of variac to your fan, this is for a 10" box fan that's rated at 1.5A (it will say on the side) so I got a 2A variac but with smaller fans you can save money by getting a smaller variac.

The pesky thing about these variacs are they're damn heavy and awkward to hang as they need to be mounted on the lid of the box, a real pita come time to screw the box to the wall. To mitigate this somewhat after dithering a bit I've decided to put the majority of the wiring and components on the lid of the box. I'd also planned to have the display panel above the dial of the variac but after thinking about it I've eventually decided having the display beneath the variac is going to be better. It makes wiring up allot tidier, it's going to get marginally less heat there and it's closer to the bottom where the plug and sockets will be attached.

I began by marking out the lid with a ballpoint pen. Divide in half vertically and horizontally. If you've a square rule anywhere it's a godsend for getting the cuts and holes perfectly level and square to the box.


Measured the dimensions of the cut-out I'd need for the ATC-800+.


90mm high.. Nice easy mounting of the ATC-800+, just slide a flat head screwdriver blade behind the tab and the four mounts pop out. After the unit is in place pop them back in and tighten up the screws until they begin to bend inwards towards the case....


170mm wide. That gives me a centre of 85mm, mark the cut out onto one half of the lid of the box with a ballpoint.


The variac comes with a helpful list of dimensions that tells me that the bolt hole centres are exactly 92mm apart, so in the centre of the other half of the box, mark out a 92mm square and its centre.


Now it's time to try to avoid chopping off your fingers as you cut out and drill the holes in the lid.


Now insert the bolts to secure the variac.


Always a good idea at this point to stick it on and tighten up the bolts, then flip it and attach the knob just to make sure you can turn it. If you're a bit tight with the hole or your square's wonky it may trap the shaft against the case when you tighten the bolts fully. Better to find and fix now.


That's all for now, I shall return to do the rest tomorrow..... after I've been to get some crimp terminals.


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hi Laphroaig,

real nice guide dude, nicely laid out.

thanks for sharing,


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Was worried everyone would be groaning because I'm relying so much on pictures to document even the mundane stuff like marking out the box. But I'm going to persist anyway, got a late start tonight but should have it finished over the weekend.


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good work Laphroaig, nothing wrong in a pictionary build :yep:

im interested in making something similar, either using the atc800+, tc10 or a stc1000.

atb JH

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By providing pics you will hopefully swerve alot of the simpler questions mate.

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Bah no crimps, next day service seems to mean Monday not Saturday. So I've just been fiddling a little, I drilled two small holes in the front panel to attach the base for the relay and used a 20mm hole saw to make the three holes in the bottom of the box for the plug and two trailing sockets and one in the lid for the temperature sensor. You'll see that originally the relay began life attached to the back of the box, having it on the front just makes the wiring tidier.


The relay in the picture below is a mirage... caugh. It has since migrated to the front cover...




I'm still undecided where I'm going to put the terminal strip, I've put it on the bottom of the box but I'm beginning to wonder if it could live on the lid too. I'll get back to you on that.


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So I've been thinking about wiring this thing up, only wire and a strip of contacts to play with. Finally came up with the following. Now I'm never sure what I'm doing with wiring, so if anyone spots any issues please shout out. I'll try and detail it as thoroughly as possible. Anyway here's what I ended up with almost finished, just the last few jumpers to attach.


Bringing the power in from the right to the top of the strip of contacts. Right side (red with yellow stripe wire) is live. Left side (black wire) is neutral. Rightmost contact is connected to in-line fuse holder which will provide the live input for the variac. The earth wires are combined in that a little block I screwed to the back of the box. No earth is required inside the box.


On the long strip of contacts, going from right to left, the connections will be as follows:

1. (live) Variac - connects via in-line 2amp fuse to the "C" terminal of the variac to give a range of 0-240 volts.

2. (live) Relay - supply voltage to drive fan at maximum speed (terminal 3)

3. (live) ATC-800+ - power (terminal 7)

4. (live) ATC-800+ - cooling circuit (terminal 1)

5. (live) ATC-800+ - heating circuit (terminal 3)

6. (live out to Fan) - Relay (terminal 1)

7. (live out to Heater) - ATC-800+ (terminal 4)

8. (neut out to Fan)

9. (neut out to Heater)

10. (neut) ATC-800+ - power (terminal 8)

11. (neut) Relay - cooling circuit (terminal 7)

12. (neut) Variac - connects to the "A" terminal of the variac to give a range of 0-240 volts.


Only need to use one set of contacts on the relay.

1. Common output to fan.

2. Live trigger from ATC-800+ (terminal 2) cooling circuit

3. Normally open terminal - supply voltage for fan

4. Normally closed terminal - modified voltage from Variac (terminal "E")

7. Neutral for cooling circuit.

Hopefully the crimps I'm expecting will arrive in the morning and I can get this thing finished tomorrow.


Edited by Laphroaig
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looking good Laphroaig :yep: top documentation.

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Keeping an eye in this.

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hows it working Laphroaig ?????

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I luv crimps!


The sticky pad for the cable tie is far too high, I'll move that down to the bottom right hand corner of the ATC-800+. Have done the live cooling circuit in green and the outputs for the fan and heater in brown and red/yellow to distinguish them from the other wires coming back to the terminal strip.


All that is left to do now is to connect the sensor, that warning has me concerned about interference so I'm going to keep as much distance as possible between the sensor lead and other cables. The sensor is the thin twin cable in the centre without crimp terminals.


Sticky pads and cable ties to the rescue, not too tight, just enough to hold it in place because next comes ....


The foam, a couple of strips wedged in to provide a physical barrier to ensure the other wires are always separated from the sensor cable.


Trim all the ends and wire them up, don't have to be too particular, hehe I wasn't. Brown went to the fan, red/yellow the heater. All the reds and the green (I used because I ran out of red) go to the live side, there's five in total and that just leaves you with the three remaining black neutrals. I'm going to go back and even them up properly in a couple of days once my bootlace ferules arrive and move the sticky pad on the ATC-800+ down to the bottom of the case.


But first it needed a test drive!

Power on! Temperature displayed in big red friendly digits, immediately fan kicks in to maximum. Hit the grey ON/OFF button and the light of the same name on the opposite side of the display turns off and fan control defaults back to the dial of the variac. Nice touch, the room temperature is still displayed, you can even program the ATC-800+ when it's switched off. Turning the ATC-800+ off doesn't stop the fan, it simply prevents the fans switching to maximum or the heater from working making it behave as a standard variac. The fan can only be stopped by turning the dial of the variac to 0 or disconnecting the power. The ATC also retains the program so even after being disconnected from the power for some time your settings should be retained.

Okay with the manual settings out of the way time to move on to the automatic bit. The instructions for the ATC-800+ were written in Chinese and translated into English by someone who spoke neither languages, Google probably! Anyhow it's not too hard to figure out.

To enter setup mode:

Press the <set> button for 6 seconds. (the temp lamp on the right will illuminate and the display will change from the current room temperature to the set temperature)

To change set temperature:

Once in setup mode, press the black <temp> button once and then use the ^ and v to change the value.

Confirm change by holding down the <set> button for 3 seconds. (display will revert back to displaying room temperature)

To change temperature differential:

Once in setup mode, press the red <tem-dif> button once and then use the ^ and v to change the value. (the tem-dif lamp will illuminate and the display changes to the current temperature differential.)

Confirm change by holding down the <set> button for 3 seconds. (display will revert back to displaying room temperature)

During normal operation if you press either the <temp> or <tem-dif> buttons the current values are displayed.

For my test I set the desired temperature at 24c and the differential to 1c. Using the warmth of my hand to heat the sensor I cycled through the process from heat to cool and back again several times. It's now set at 21c in my shed with a 1500W heater and an old 10" duct fan blowing air through it at the sensor. Have done a bit more fiddling and 1c is a bit too low a margin although it works fine. I think I'll be aiming for a 2c differential and setting the temperature at 21c.

Have moved the clip down to the bottom of the ATC-800+ and the wires are behaving allot better, there's still a couple that are a smidgen too long that will need to be trimmed but provided it's still going strong tomorrow (and the shed hasn't burnt down overnight) it looks like the jobs a good one.

Will update once the bootlace ferules arrive.


Edited by Laphroaig
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Spot on mate. What heating will you use?

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Well it's destined for the big room, in there I've just a cheap, bog standard, pull cord operated, Dimplex 2kW wall mounted electric blow heater. I thought it was kinda apropos when I bought it.

Give it a couple more hours and hopefully the postman will have been with my ferules so I can finish it off and get it up on the wall. Coincidently I was pleased to see that the shed was still standing this morning!


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Hi Laphroaig,

This is a great thread mate with nice clear descriptions. I think I am probably going to try and copy it this one :yahoo:

Have you wired it exactly as you said in the above posts? I'm trying to look at the live wiring in your picture and it looks like your live ATC-800+ - cooling circuit from terminal 1 is going into 3 on the contact block (you said 4 in the earlier post). If that's so where is your live ATC-800+ - power from terminal 7 connected to? Or are my eyes just confused by the spaghetti :rofl:



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You've bloody good eyes. I have to admit that once I'd attached all the wires to the bits on the lid of the box I wasn't too pedantic about what order the wires went to the strip in the back. Which of the five live terminals each of the five live inputs used was completely arbitrary as were the three neutral wires coming from the lid when it came time to put it together.

The list I created earlier was just so I had the right number of terminals to avoid stuffing 2-3 leads into each so it all looked nice and relatively intelligible when it was finished (and as a reference for anyone who'd like to use it as a template to build their own). The order will no doubt change again when I put bootlace ferules on the end of all the leads going to the terminal strip in preparation to hanging it up on the wall.

I honestly didn't think anyone would notice. +1 for being observant :)


Edited by Laphroaig

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