I’ve looked at it – and I can see what you mean. As far as the value is concerned, yes it’s a typo – if you read the first paragraph on page 49, the author mentions a capacitor of 4,700uF/35v and that’s certainly more like what I’d expect to see as a reservoir capacitor in that position. Ideally you’d put in a high-ripple device too, but for a relatively small power supply that’s not seriously important.

 

Because all of these power supplies are regulated, they don’t need larger capacitors between the regulation and whatever they’re supplying, as the regulator does a rather better job on the output side than the capacitors would anyway. I think that every data sheet I’ve seen reflects this. In fact a good argument could be made that having a larger cap on the output makes it harder for the regulator to work properly.

 

One other point about the general purpose power supply as shown is that I’d want a heatsink for each of the regulators, not just the LM338 – the 7805 needs one as well. That poor device is being supplied with over 30v at its input, which is fairly close to the maximum allowed, and the author says that you can have up to 1 amp from it. It’s going to get pretty warm, and if its internal temperature reaches 150 degrees C, it will shut down. In this design, it could easily be dissipating 30W, so that’s going to require a fair chunk of metal, and good thermal contact being made.

 

The author shows it without one. Clearly he hasn’t been using it!

Posted by Steve Garnett on 10/09/2011 02:09:41:

 

Steve: I’m not sure I follow that; can you elucidate?

 

Regards,

 

Andrew

Ah, my mistake! I read it as referring to the size of the reservoir capacitor on the input of the regulator; and I couldn’t for the life of me see how that would affect the regulator.

 

Oh well, where’s the dunces cap?

 

Regards,

 

Andrew

‘Fraid i just use an un regulated supply. Originally it was the power supply for an old Zerox copier, the transformer is about 200 mm sq, as is its a single winding type(auto transformer?), there was plenty of room so I wound on another winding to give the 18 V that I wanted, its got a 45W bridge rectifier and 2500uf cap. Ian S C

My supply gives me 18 V around my workshop, it powers the table feed on the mill, the electrolitic rust removal bath, a small automotive portable air compressor, and an 18V battery drill, and a hoist mounted above my lathe on a rail, I use that when changing chucks, they are 8′ and just managable, so I’v got that for when I can’t manage. Ian S C

 

My supply gives me 18 V around my workshop, it powers the table feed on the mill, the electrolitic rust removal bath, a small automotive portable air compressor, and an 18V battery drill, and a hoist mounted above my lathe on a rail, I use that when changing chucks, they are 8′ and just managable, so I’v got that for when I can’t manage. Ian S C

ps.  Motor on hoist ex garage door opener, rated 36V. 

Hi Mick H,

I suspect that the starting current of the drill exceeds 4 amps causing the power supply to start shutting down. There are probably many uses for it. For example electroplating, anodising, powering 12 volt bulbs etc.

Les

Ha Ha. I think that this must be the one…

 

I think you could just get an 8′ chuck on it!

Posted by ian weeks on 11/09/2011 21:57:26:

 

Yes, it’s at Blists Hill. I took the picture in a feeble attempt to wind my bro-in-law up. At the time I took the photo, he’d had a lathe outside under a tarpaulin for well over a couple of years without looking at it at all. This was just to suggest to him what sort of state it might be in!

 

Turned out that his wasn’t quite as rusty as that…

 

And Terry, that’s a gap-bed lathe. If you size it by looking at the imperial bricks in the wall behind it (about 3 rows/ft), the faceplate on it has to be best part of 6′ diameter, so I reckon that an 8′ chuck is just about within its capability. You probably wouldn’t lose the chuck key in a hurry though…

 

OOPS, sorry to disapoint you Terry, its only a 8″ chuck, quite large enough for me. Got a mate just down the road how has a lathe with 16″ chucks, I think its a Pratt with about a 6′ bed,1930 vintage, hes just got a Colchester of a similar age, and not much smaller, have not had a good look at it. He seems to collect worn out vintage machinery, that includes tractors. Ian S C

2v DC for the heaters is relatively easy – although it has to be pretty smooth DC or you get to hear a lot of hum. Your real difficulty is the 135v, and although a quadrupler circuit would work fine, I think (there’s very little current needed), it’s not the way I’d do it. What’s probably easiest these days is to have a second transformer running from the secondary of the first one (the one that’s supplying your heater 2v), only connected so that the primary is the output. That gives you an isolated supply back into the hundreds of volts, and a simple rectifier, reservoir cap and zener stabiliser is all you are going to need to produce an adequate 135v supply from it. You can get four 33v 5W zener diodes from Rapid electronics for 19p each, and put them in series. All you need to do then is calculate the value of resistor you’ll need to drop whatever secondary voltage you end up with down to 135v, allowing for a few milliwatts through the zener chain, and you’re there.

 

If this isn’t clear from the description, I can sketch it out for you.