benchtop power supply

[ian weeks 1Participant @ianweeks1]

Dear All,

whist building the gear banjo in MEW issue82 I once again read the article in the same issue by peter rawlinson on electronics in the workshop. It took me back to the ‘radio and electronics club at school many decades ago.

I resolved to build the project at the end of the article as a ‘nostalgia trip’.

However I have a problem -the circuit shows 4 of the capacitors as 10 microfarad 35vw but the pictures show one of these as a physically much bigger beast of an electrolytic[ viz the one across the output of the bridge rectifier]. My problem is that a little bell is ringing in my head and saying that this capacitor should br bigger but my knowledge is so rusty I cant be sure and I can see no corrections to the circuit inn subsequent issues.

Any advice would be appreciated

Regards Ian

[ian weeks 1Participant @ianweeks1]

[Steve GarnettParticipant @stevegarnett62550]

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!

Edited By Steve Garnett on 10/09/2011 02:11:39

[ian weeks 1Participant @ianweeks1]

Dear Steve ,

Thanks very much for your advice and prompt reply.Its nice to get a bit of confirmation that not all the grey cells have gone. Also did wonder about the heat dissipation and bought heat sinks as you suggested. Thanks again Ian

[Anonymous]

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

 

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.

 

 

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

 

Regards,

 

Andrew

[Steve GarnettParticipant @stevegarnett62550]

Elucidation (hopefully…)

 

A larger capacitance at the output certainly decreases the magnitude of any transient response the regulator has to a change in load, but it also increases the device’s settling time – and the larger you make it the worse this gets. All the time it’s settling, it’s not providing a steady-state regulated output!

 

There’s a T.I. App note about it – the relevant bit is at the end of the first page, and at the end of it there’s the formula for calculating the optimum value for any given situation. Not that for most practical purposes it makes a significant difference – just sticking a 10uF low esr cap across the output is fine for the vast majority of applications.

 

[Anonymous]

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

[Steve GarnettParticipant @stevegarnett62550]

You might be amazed how little you can get away with on the reservoir (input) side as well. Only trouble with doing this is that you’re going to make the poor regulator work even harder. But hey, they claim that 7805s have an 85dB ripple rejection ratio…

 

On a more important note, the other thing that is missing from the circuit as shown in issue 82 (with an isolator switch on the input) is any form of reverse-bias protection diode across the regulator. If you switch off the supply and for any reason you get some sort of output over-voltage situation, you stand a fair chance of wrecking the device. One chunky diode with the cathode connected to the input and the anode to the output will take care of that though. For more details see the 7805 data sheet.

 

Ah, power supplies – don’t ya just love them? Earlier this week I won an eBay auction for an Anilam Wizard 211 3-scale head which was being sold as ‘not working’. It has a switch-mode power supply, about which there is no information, and it was very apparent that the major problem with it was that the bridge rectifier had got two shorted diodes in it. I now think that this takes the prize for being the one component that can cause the most collateral damage in a power supply! Three more diodes, the chopper FET, the current measuring resistor, the VDR on the input, both fuses, a couple of resistors that failed as a result of the current measuring resistor failing, probably the main smoothing cap (changed to be on the safe side), and the controlling IC. Changed the other two electrolytics in it as well, plus the start-up resistor.

 

Took a whole two hours to fix all that satisfactorily. But at least it works again now – result!

Edited By Steve Garnett on 10/09/2011 18:49:57

[Ian S CParticipant @iansc]

‘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

[Steve GarnettParticipant @stevegarnett62550]

It all depends what you want your power supply for, really. For many applications an unregulated supply is fine. Ian W. says he’s building his as a nostalgia trip, but I bet he’s got something else in mind really!

 

I just build them as I need them, and usually they are original designs for specific purposes. For general experimenting though I have a couple of Farnell triple-output bench supplies, plus a few external current-limiting boxes which save the odd disaster.

[Ian S CParticipant @iansc]

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

 

[Ian S CParticipant @iansc]

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

[Ian S CParticipant @iansc]

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. 

Edited By Ian S C on 11/09/2011 15:39:18

[michael howarth 1Participant @michaelhowarth1]

A few days ago I succumbed to a whim and bought a “12v DC 4 amp” power supply, thinking that it might power a 7.2v drill. It seems to produce “pulses” of power so that the drill continuously stutters on/off. I have little knowledge or understanding of these matters but cursory research indicates that it might be a switched mode device of some sort. I accept that it was probably a waste of money, but is there any use for it?

[Les Jones 1Participant @lesjones1]

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

[TerrydParticipant @terryd72465]

Posted by Ian S C on 11/09/2011 15:37:01:

…………….., they are 8′ and just managable, so I’v got that for when I can’t manage. Ian S C.

Edited By Ian S C on 11/09/2011 15:39:18

Phew,

 

I couldn’t manage an 8′ chuck even with a hoist. An 8″ maybe, I’d love to see your lathe Ian, it must be a whopper ,

 

Best regards

 

Terry

[Steve GarnettParticipant @stevegarnett62550]

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

 

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

[TerrydParticipant @terryd72465]

Posted by Steve Garnett on 11/09/2011 21:04:26:

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

 

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

 

 

Hi Steve,

 

Super photo but not entirely convinced, I think that an 8′ chuck would tax even that brute. Don’tcha just love those industrial archaeological pictures!

 

Best regards

 

Terry

[ian weeks 1Participant @ianweeks1]

Thanks everyone for the information and entertainment .Steve is right I do have another little project in mind . Years ago a friend gave me an old bush battery valve radio which he was about to throw out . I just couldn’t let that happen. It’s been at the back of the workshop staring at me for some time Before I can even look at it I would need Batteries [not made anymore] or a power supply . When read the MEW article I thought If I can cobble The bench power supply together, Will have new supply for mill table feed /useful 5v if ever needed and its not a world away from HT/LT supply to play with old radio and stop it staring at me.

Also I think I know that lathe in the photo -is it outside the foundry at the Ironbridge equivalent of Beamish by any chance??

Regards Ian

[Les Jones 1Participant @lesjones1]

Hi Ian,

Depending on the age of the old radio it will either need 2 volts for the heaters and 120 volts HT or 1.5 volts for the heaters and 90 volts HT. If the valves are B7G bases (7 pins with a a diameter of about 3/4″ and about 2″ high.) it will be the second type. The valve types will probable start with the letter D.

 

Les.

[Steve GarnettParticipant @stevegarnett62550]

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

Also I think I know that lathe in the photo -is it outside the foundry at the Ironbridge equivalent of Beamish by any chance??

 

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…

 

[TerrydParticipant @terryd72465]

Posted by Steve Garnett on 11/09/2011 23:27:46:

……………………………………….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…

 

Hi Steve,

 

You may be right, I wouldn’t quibble. I remember working at Davey Ashmore in Teeside and we were casting hopper bells for blast furnaces and I seem to remember that the charging bell was about 24 ft diameter and the loading bell about 12ft diameter. These were pit cast and then turned on what might be called vertical lathes (after much fettling and prepping) to create the sealing ring around the edge of the bells.

 

Fascinating stuff, and this equipment was destined for the steel plants of N Spain around St Sebastian I believe – if my memory serves me well , these were along with some enormous rotating molten steel torpedo railway trucks we designed for the plant in order to move molten iron from the blast furnace to the basic oxygen converters in it’s molten state without the cooling stage of pig iron production,

 

Nostalgia ain’t what it used to be (sigh)

 

Best regards

 

Terry

Edited By Terryd on 12/09/2011 00:06:54

[Ian S CParticipant @iansc]

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

[ian weeks 1Participant @ianweeks1]

Thanks les for the information and sorry about the delay in replying. had a good look at the old radio tonight and some stuff I had got together a few years ago in preparation for having a go at it . I did find a schematic for a power supply on the uk vintage radio repair and restoration website but it was for a 90v ht and 1.35v lt supply .it used a ‘voltage quadruple circuit’ which I didn’t undersatand so put it to one side for a long winters project

The radio I have is 135v ht and 2v lt

I will need to spend time relearning stuff I must have Known 40 odd years ago! ,or perhaps you know of a website that would have a ready made circuit diagram?

Kind regards Ian

[Steve GarnettParticipant @stevegarnett62550]

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.

Edited By Steve Garnett on 13/09/2011 23:12:54

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