Fixing the Variac……..Part 2


A few months ago we did a blog post about our variac. It was tripping the circuit breakers on our distribution board and we made some mods to the variac to reduce the inrush current. Our story generated a lot of interest and was picked up by Hackaday. We used a 2 terminal NTC inrush current limiter from Ametherm.

Following our story, the good folks at Ametherm and Rhopoint Components (our local distributor) were in touch with us. They gave us some great technical advice and did some calculations for us. They have a whole range of limiters and it turns out that we had actually chosen quite a good model for our application. They kindly sent us some samples.

There are many ways to soft start a load. We originally thought about adding a series power resistor and a shorting relay. We also had some suggestions around Triac and IGBT solutions. Each of these methods have their merits and can allow fine grained control of switch-on. Unfortunately there was very little room inside the variac case and we didn’t want some kind of external box. So we looked at these relatively simple 2 terminal limiters.

Carefully chosen, these components are an elegant solution. They can probably last the life of the application with no maintenance. Placed in series with the load, they have a relatively high resistance at room temperature which limits the initial inrush current. When current flows through them they self-heat and the resistance falls very quickly, so they have little effect on the normal operating current.

We have now done a rev 2 of our mod. This time selecting the model MS32 5R020 limiter. It has a lower cold resistance of 5 ohms which should keep the operating temperature down a bit. Originally, we connected the limiter using PVC insulated wire, solder and heatshrink. Given the relatively high temperatures, we have replaced this with tubular glass fibre insulation and ceramic connectors. We should have no temperature worries now.
One other gotcha we found, is that our distribution board circuit breakers are actually B-curve and not C-curve as we earlier thought. That makes them even more sensitive to surges. When we did our original mod, we didn’t do a full load test. We did a quick run with our heatgun (about 9 amps). This time we did a full load test. At 12A of output current and an ambient temp of 20 deg c. The body of the limiter measured 200 deg C, which is hot, but below the 225 deg C max rating of the device. This was quite an extreme test, but we’ll continue to monitor and do some tweaks if possible.
If you’d like to take a look at our variac, or do something else with electronics, our regular Electronics, Microcontroller and IOT evening takes place every 2nd Monday. Check out the events section of our website for details.

TOG Brewday (Saturday 4th February 10am)

TOG members and Capital Brewers return to TOG HQ this coming Saturday (4th February). Hearty brews are on the menu, in both brewing with guidance and instructions from the experts, and tasting craft beer made at previous brew days!

A very rough time-line of a brew day is as follows (a little earlier than usual to be finished before AGM):

  • 9:00am – 11pm: Heating water, water treatment and mashing grain
  • 11pm – 1pm: Boiling & beer tasting from previous brews and visitors beers
  • 1pm – 3pm: Cooling and clean up

Every brew day is slightly different with a mix of brewers, recipes and techniques.  The long term goal would be to get a full set of brewing equipment that TOG members can use themselves at the space and make your own beer, cider or wine whenever you like! All you need are the ingredients. We are already well on our way to achieving that goal with several sets of equipment for members to choose from.

Brewing Equipment in TOG

Its always a fun and engaging day, the cost is free (donations are welcome) so come along, ask questions and even help with the brewing!


Workshop Improvements

We have been improving our workshop a lot lately. Check out some of the work below.



We have found a new home for our air compressor and mounted it on our metal shelves.   We also mounted a air hose reel. It makes using air tools in our workshop very easy.



Label all the things. If you have a lot of stuff, soon you find yourself in a situation where you can’t find anythings. We have done a big sorting job of our stuff. So now most things have their own box.



We have long things like pipes  hat need to be put somewhere but they don’t fit in our current wood storage area. Long things now have their own area . aka Long things storage.

Drop by our upcoming Birthday Party to see a host of changes we have made to the space since our opening last year.

Win a laser cut pencil? TOG Turns 8!

Picture of 10 laser cut pencils, with information about TOG's 8th Birthday Party

With less than a week to go, TOG is holding a competition to celebrate our 8th Birthday Party. The party is taking place after 7pm on Saturday, 28th January in our space, 22 Blackpits. You can find the details on our meetup page.

About Us

TOG is a hackerspace based in the centre of Dublin. It is a shared space where members have a place to be creative and work on their projects in an environment that is both inspiring and supportive of both new and old technologies. In the past 8 years we have worked on everything from giant LEDs to Raspberry Pis, we have made a twitter knitter and a pizza oven.


The prizes are a personalized laser cut pencil and a surprise from our 3D printer. Entering is really easy. Simply Retweet this tweet or Share this Facebook post. 5 winners will be pseudorandomly picked from Twitter and 5 from Facebook, on Saturday the 28th, prizes can be picked up in the space.

Update:   To enter the Facebook competition you will also need to leave a comment on the post.

TOG 8th Birthday Party

8th-smallWell we’re at it again. Once a year we all have our birthday and TOG is no different. This month we have reached the ripe old age of 8 years. We’ll be celebrating in style on Saturday 28th January in Blackpitts. Last year we combined our January Open Social, birthday party and grand opening, into one monster bash. It didn’t disappoint. It was our biggest social event ever. The craic was so mighty, that we’ve decided to do it again this month for our 8th birthday party. So note that time and date….. Saturday 28th January from 7PM. There are strong rumours of cake, TOG beer and pizza.

If you’ve never been to one of our shindigs, you’ll find it a great alternative Saturday evening in town. Come in and look around. Kick the tyres. If you’ve never been in before, we’ll give you the grand tour of the space. Talk to members and visitors about projects or things you’d like to do. Ask about our plans for 2017. If you like what you see, ask about joining as a full member. What a great New Years resolution that would be!

Last year, we invited just about everyone that we could think of, and most of them came. Now if you’re one of the few who were invited last year and didn’t come, well we know who you are, and we won’t be taking any excuses this year. The dog ate your homework, the cat was sick, or you’re washing your hair just won’t do 🙂 So get yourself in on the 28th. The space stays open until the last member is left……usually the small hours of Sunday morning. The evening is free to attend for members and visitors alike. No need to book…. just turn up. You can drop in for 10 minutes, or stay the whole night. We have parking available. Bring beer, food, gadgets! Our doors are open from 7PM. Hope to see you there. Expect mighty craic.


Laser Cutting Pencil Jig

Last weekend a bunch of our members visited the annual BT Young Scientist & Technology Exhibition. It’s always great fun to walk around and to chat to all the students about their projects. It’s fantastic to see how Arduino’s and Raspberry Pi’s have just become tools for the students to carry out their project.


Also at the exhibition were the Stem Module. It’s a mobile truck filled with all sorts of scientific and fabrication equipment to help delivery education programs to school kids. While getting the tour of the truck, we spotted a laser cutting jig for making custom pencils. We really liked the idea, so we decided to give it a go.



Laser Cutting Jib by the Stem Module.

Laser Cutting Jib by the Stem Module.


So we started off with testing if cutting pencils would work on our Lasersaur.


After this successful test, we tried out a single line font from Robert McNeel and a jig to cut ten pencils in one go.


We are mostly happy with the results. Below you can see the side by side comparison. I think with a better quality  pencils and raising the lens of the Lasersaur would get better accuracy. You can download our design here.