Homemade power supplies

[ritch]

Is anybody good at making things like power supplies?

I'm looking for one that has an IEC 3 pin connector going in, so from a kettle lead, and outputs 12v on a Molex connector (the type that would power hard drives in a computer, but specifically for to make a couple of Joe Meek units into standalone preamps).

Any ideas? I've made the case for it, which has an IEC going in, so I just want something to convert to 12v now.

I've tried some of the random rubbish from china on ebay but they pop within a few days of using them!!

rich

[Bloodaxe]

[url="http://www.zen22142.zen.co.uk/Circuits/Power/psu.html"]A basic PSU[/url] is a simple enough beast... just needs:

A Fuse
A Transformer (240v to 12V)
A Rectifier (4 Diodes)
Some form of smoothing (A couple of chunky electrolytic capacitors)
A 12v Voltage regulator (either an IC or a Zener Diode)
Something to mount the bits on (preferably a PCB, but Vero or Tag Board are fine)

The main thing is to make sure that [u]all[/u] the bits can cope with the power you need plus a bit more. If you try to draw 500mA from a 100mA PSU it'll blow in no time at all (which is probably the problem you've had to date).

How much current do the JM units draw?

Pete.

[ritch]

Hi Pete

Thanks for the information! That's a big help. I'll give that circuit a go and see how far I get. I'm not 100% sure what current the meeks take - somebody has guessed at no more than 1000ma, but there doesn't appear to be much information online about it.

A couple of questions though ...

1. What should I search for as a transformer? Would this be the sort of thing? [url="http://www.bitsbox.co.uk/transformers.html"]http://www.bitsbox.co.uk/transformers.html[/url]
2. What is the "LOAD 100" on the right of the picture?
3. You mention a fuse - where is that in the diagram?

Scuse my lack of electronics knowledge!

Cheers

Rich

[flyfisher]

I wouldn't bother about building a PSU. It's not that difficult if you know what you're doing but it's a safety-critical item so the risk is not worthwhile, especially considering how cheap a fully-approved PSU is to buy.

Whatever you decide to do, you [u]must[/u] know the power requirements of the PSU if you're to avoid under-spec'ing the PSU so that it runs hot or shuts down, or fails. You also don't want to over-spec the PSU because that will just waste money.

The Joe Meek units must surely have a specification somewhere that will tell you their power requirements? If you really can't find the info and you don't have any other high-powered PSUs you could hook them up to a car battery (via the cigarette lighter) and measure their power consumption with a meter.

Once you have the power requirements it should be easy to find a suitable PSU to buy, especially for something so standard as 12 volts.

Then your only problem is to fit the required molex connectors. These are easy as long as you have the right sort of crimping tool. Don't be tempted to use pliers unless you want a bodge that won't give a reliable connection. Maplin will probably have the parts you need or an online supplier like CPC.

If you rarely do this sort of thing then you probably won't want to be buying special tools and test equipment so the best bet would be to try and find someone local who could help. A local guitar/amp tech would be a good place to start.

[Bloodaxe]

[quote name='ritch' timestamp='1322343033' post='1449790']
Hi Pete

Thanks for the information! That's a big help. I'll give that circuit a go and see how far I get. I'm not 100% sure what current the meeks take - somebody has guessed at no more than 1000ma, but there doesn't appear to be much information online about it.

A couple of questions though ...

1. What should I search for as a transformer? Would this be the sort of thing? [url="http://www.bitsbox.co.uk/transformers.html"]http://www.bitsbox.c...ansformers.html[/url]
2. What is the "LOAD 100" on the right of the picture?
3. You mention a fuse - where is that in the diagram?

Scuse my lack of electronics knowledge!

Cheers

Rich
[/quote]

OK... your last line worries me a tad, so you might be better off taking Flyfisher's advice & just buying a ready-made one and swapping the plugs.

But, to answer your questions...

1. No. The 6-0-6 will give 500mA total, and the 12-0-12 a mere 125mA per Secondary. To get a total 1000mA out, you'd need a 12VA transformer, but if each JM unit is 1000mA then you'd need a 24VA transformer with either a 6-0-6 or a 12-0-12 secondary - that'd give you 1A per secondary tap @ 12V.

2. As the "Load 100" label is right next to the symbol for a resistor, I'd take it to mean "Load = 100Ω".

3. The diagram doesn't show a fuse (or a mains switch for that matter), but it would usually live in between the switch & the mains side of the transformer (the "Primary Winding"). Value should be a tad more than the total expected load.

All these figures are based on your conjectural 1000mA consumption - which is quite high. What JM units do you have?

P.

[ritch]

Thanks

They're two of these:

[url="http://www.dolphinmusic.co.uk/product/1013-joe-meek-micromeek-mq1-computer-interface-mic-pre-amp.html"]http://www.dolphinmusic.co.uk/product/1013-joe-meek-micromeek-mq1-computer-interface-mic-pre-amp.html[/url]

They're meant to be in a drive bay in a desktop pc, but I have mounted two of them in a box so I can have them as a standalone unit (less interference etc). I can't find any specs for them.

Maybe I'll just keep hunting for an off the shelf 12v one then. They all either seem to be cheap from china, or overly expensive, or a wall wart. I need one with an IEC socket that can sit inside my box.

Cheers

[Bloodaxe]

Found this:
[url="http://www.joemeek.com/pdf/mq1quickstart.pdf"]http://www.joemeek.c...1quickstart.pdf[/url]
but it doesn't give any power specs.

The contact details at the bottom of the page may well be out of date, as the main Joe Meek website gives an e-mail address of [email protected] & they're in California. However, PMI Audio do seem to have a Torquay faction - 01803 612 700 ought to get them.

Pete.

[flyfisher]

Something like this?
http://www.sim-entry.com/12V_1A_power_supply/p163033_2132065.aspx
Assuming 1A is adequate of course.

[ritch]

Bought.

Thanks. I'll give that a go - it looks like it will fit in perfectly and dead easy to wire up.

Here's a couple of pics of it as it stands incase you're interested.

[flyfisher]

To avoid having to bother with making up the molex connectors, you could try buying a hard drive power cable splitter that has the molex connectors already fitted. A bit like this one: http://www.amazon.co.uk/gp/product/B000ITL4KI/ref=pd_lpo_k2_dp_sr_1?pf_rd_p=103612307&pf_rd_s=lpo-top-stripe&pf_rd_t=201&pf_rd_i=B0014BL9PE&pf_rd_m=A3P5ROKL5A1OLE&pf_rd_r=1EJ3BPVWAJHMZ58P3SMZ

Any local PC shop should have something similar.

Then just cut off the unwanted connector and wire it directly to the screw terminals on the PSU, leaving the two others to connect to the Joe Meek units.

[ritch]

Yeah - I'll do that. I've got loads of them lying around from old computers.

[flyfisher]

Good stuff. Sounds like the job's almost done - nice one.

I just hope 1 amp is sufficient!