A Homebrew 12V PCB Drill Speed Controller

[David G4EBT]

I’ve recently made a little PWM speed controller for controlling the speed of a 12V PCB drill, which might I thought interest others who make their own PCBs. I'd already got one that I made 25 years ago, but I work indoors and in an outdoor workshop, so another would be handy to have. Circuits for such things abound on internet and I found one for which, fortuitously, I had all the bits in my spares box, so it didn't cost me a penny to build. I developed a component layout from the circuit, which is attached, along with the PCB artwork I designed and the component overlay, showing an 'X-Ray' of the PCB track layout.

Though I've made a few minor alterations to the circuit, I claim no credit for it - I just designed the PCB and built the project to meet my own needs.

I’d been concerned that heat sinking might have been necessary for the N-Channel FET used in the design. It's an IRL530N mosfet (TO-220 package), which I understand can handle current up to 27A. It was chosen by the designer of the circuit as it has fast switching and low on-resistance. In this application, the PCB drill is unlikely to draw more than 2 Amps - 3 at most, and will be used for maybe up to half an hour, on load for drilling each hole in a PCB, which takes only a matter of a few seconds at a time, with a brief pause between each hole drilled. I did allow space round the transistor to fit a heat sink, but in the event, it runs cold.

It will be seen from the circuit that the 78L05 voltage regulator only has to provide the 5V for the 555 IC, so is more than adequate for that, (rather than a T0220 cased 7805).

I'm pleased to say that the completed controller works a treat - much better than the one I built years ago, and better than a commercial 'Como Drills' one. Like this one, they're both PWM designs, but at slower speeds they 'pulse' which isn't really much of a problem as the drill needs to run at fairly high speeds anyway. However, this design will slow right down to a stop with no jerkiness at any speed, and if I apply a load to the drill I can tell that the controller retains the full torque, whereas with simpler non-PWM designs, (not that this is a complex design), which basically just lower the voltage to the drill, the drill loses torque and may stall under pressure.

In addition to the circuit, I’ve attached a pic of the finished board, and a few other pics to show the construction, along with the front panel layout. The overall size was dictated by the transformer I had to hand, and the box that I made is 13 cms wide x 13cms deep, by 10cms high. (5” x 5” x 4”). Should anyone wish to construct one, in case the artwork for the PCB doesn’t print to the correct size, it will need to be re-sized to measure 10.5cms x 8.2 cms.

I guess it could be built on strip-board or tag-strips, but I assume that anyone who might be interested in a PCB drill controller will be into making PCBs!

Hope it’s of interest and use to someone out there.

I'll post a few more pics of the construction in another post.

[David G4EBT]

A few more pics.

[Ti Pwun]

Very nice. I love the boxes you build for these things.

[David G4EBT]

Thanks for your kind comments John.

The boxes are made from whatever scraps of hardwood I can get my hands on - that one was made from iroko.

I developed the habit because I think they give a vintage feel to my homebrew projects, evocative of crystal set boxes of a bygone era. They're a bit time consuming to make as you can imagine, but then it's a hobby after all. Woodworking/woodturning are my other interests so it's a way of combining the two hobbies. By making the boxes they can be 'made to measure' - I'm not constrained by having to use standard sizes as I would if I used metal or plastic off-the-shelf enclosures, which of course don't come cheap.

I generally use scrap double-sided PCB sheet for front panels - easy to cut and drill. Of course, anyone wishing to make the speed controller can just use an ABS box or whatever and there's really no need for a front panel, so it can be simplified if desired.

[Radio Wrangler]

The cabinetry is as important a part of the attraction of vintage gear as the electronics.

I guess you have a dovetail/comb jig like the Trend one? Equally impressive is your source of hardwood.

It's worth remembering that good hardwood can be recycled. I came across this when staying in a holiday apartment in the west country. The owner was a manager in a firm which did major refurbishment contracting and he bought scrap hardwood from them.

He used lots of ironwood, and told me that it was all recycled from old doors from Rampton! They couldn't use steel because that would make the place look like a prison, yet the doors had to contain some very dangerous people. Wooden doors were tested by a couple of beefy orderlies hurling lavatory pans at them. Ironwood worked. A far cry from some gorgeous cabinet-making.

David

[David G4EBT]

Originally Posted by Radio Wrangler

The cabinetry is as important a part of the attraction of vintage gear as the electronics.

I guess you have a dovetail/comb jig like the Trend one?

Thanks for reading the thread David, and for your comments.

I don’t use a commercially made jig by Trend, Axminster, etc – it’s one that I made from scrap offcuts of plywood from a design by the late Roy Sutton, which features in a 1995 VHS video, later on DVD, but currently unavailable as far as I can see. I have a cheap DIY type router fitted permanently to the jig, with a 6mm straight shank router bit in it. There’s little load on the router so nothing fancy is needed – each slot takes only a few seconds to cut.

For anyone not into woodworking, I think the main problem wouldn't be making the jig, but slicing the wood for the boxes down to about 8mm thick. for which a bandsaw is needed, and either a planer/thicknesser or belt sander to clean up the wood. The box I made for this project was 5" wide and was cut on a bandsaw with 6" capacity, then sanded smooth on a belt sander. Needless to say, protection from the hazards of dust inhalation is a must - not sensible to use your lungs as a dust filter!

I won’t elaborate further on here, to avoid straying too far off topic and away from the ethos of the forum as a primarily being for the discussion of vintage radio and electronics. However, for anyone who might wish to know more about how to construct and use a comb-joint jig, there are a couple of threads elsewhere that might be of interest, as follows:

‘A Router Jig for making Comb-Jointed Boxes’:

http://golbornevintageradio.co.uk/fo...d.php?tid=3745

The thread at the link below entitled ‘Making a Bespoke Wooden Box’ is by Colin, forum member ‘Retired’, which covers a rather splendid lidded box that he made for housing the 45 cast iron wave winder gears that he’d cut for an AVO-Douglas Coil Winder that he restored. The thread includes pics of the home-made Roy Sutton comb joint jig in use:

http://golbornevintageradio.co.uk/fo...ad.php?tid=898

If anyone would like further information on how to construct and use such a jig, please PM me.

(I’m not in a position to make boxes for anyone else due to time constraints).

Hope that's of interest.