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Old 20th January 2007, 07:03 PM   #1
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Default Buying power supplies

Hi DIYers,

I'm on the committee of an IEEE student branch at my university. One of the things we're trying to organise is an electronics lab outside of the university for the students to use without all the problems involved in using the university's own labs.

I'm looking at getting a grant for some lab equipment and was going to buy, along with other stuff, power supplies. I'm more of a valve type person, so I know exactly what I'll need for that, but I was having trouble deciding on what to get that will cover the lower voltage ranges.

I was going to buy / build (probably the latter) a +/- Jung Super Regulator for low power testing of things like headphone amps and preamps. But it's more complicated than the HV supply as I figured that some people might want to work on power amps and the supplies would probably get pressed into robotics or making coil guns to excite everyone, where they'll need to manage some significant current output. The first brings with it the problem of needing rail to rail supplies for some work, whereas the second and third would likely benefit from a single higher power supply.

Could I ask for some suggestions of a voltage / current band for solid state audio work? Please point out if you mean rail to rail.
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Old 20th January 2007, 08:06 PM   #2
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you can get open frame supplies real cheap from suppliers like jameco. you might want to look at +/- 12v, +/- 15v for most opamp prototyping,
and +/- 30v to +/- 100v at 10 or 20 amps will cover most other needs.

i once built a "screwdriver magnetizer" using a 40vdc supply, and about 5 or 6 100,000uf capacitors, a 200amp relay and a coil of heavy guage wire. on a single discharge, the device would magnetize a screwdriver so well, that it could pick up a whole handfull of screws. you could also put those little phillips bits that you get at the hardware store, into the coil, and shoot them across a large room. even though i used a 200 amp relay, the contacts had to be burnished every 20 or so discharges. it was an open frame relay, so it could be discharged by hitting the actuator with a screwdriver handle. the coil was about 30 turns of 10 guage wire around a 1/2 inch diameter cardboard tube about 6 inches long. i thought about replacing the relay with large mosfets, but once magnetized, the screwdrivers our company used only rarely needed to be re-magnetized. the 40v supply recharged the capacitors over a 30 second period through a resistor.
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Old 21st January 2007, 12:56 PM   #3
jaycee is offline jaycee  United Kingdom
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The Jung Super Regulator is mainly focussed on fairly low voltage, low current, with very low noise. It's not really suitable for a bench supply.

For most lab use, an LM317/337 based circuit would do. To boost the current a bit you could use external pass transistors, something like the MJ15003/4 pair.

In the LM317 data sheet there is an example schematic for a 5A constant voltage/constant current regulator using the LM317, an opamp and an external pass transistor. This could be useful. It also shows how to use an external pass transistor to increase current handling.

The people who want to build power amps can still use this sort of bench supply for testing. Obviously they won't be able to pump out 500W into 4 ohms with it, but that's not what using a bench supply is for. When they are sure that their circuit is working OK on the bench supply, they can move to the conventional unregulated supply.
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Old 21st January 2007, 03:46 PM   #4
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Thanks for the replies guys...

The jung super reg was only for doing critical listening work on things like headphone amps. Don't worry.

Here's how it is... the university has it's own labs that are actually extremely up to date, since they've just gone through some refurbishment. There are fifty or more desks, each has it's own;

Tektronix TDS(cheapest) scope
Resistance box
dBv meter
Handheld analog multi
DMM (old box style, but still perfectly good - oldest thing on desk)
Pulse generator
Function generator
4 Channel 25Vdc supply at 3amps with options for series / parallel and grounding
Temperature controlled station

So the university it's self can provide quite a good set of equipment for basic testing. The plan is to look for gaps that the university can't fill due to the numbers they have to deal with and to then fill them with grant money and an engineering society.

Jed, the screwdriver thing is kind of the stuff I'm talking about. The idea is not only to provide a resource for serious work but to get the students a bit more excited about the course (which is otherwise extremely boring). Making a coil gun would have a huge 'ooooo!' factor and has the oppertunity to sneak some learning in on the side.... 'how far should the projectile go? how far does it? whats the efficiency (terrible ), etc'... or, 'have a go at building a valve amp for your guitar, here's how a valve works...'

I realised I've kind of worded the title of this thread incorrectly. What I basically wanted to ask was for a list of voltage / current requirements for solid state / chip power amps people have built, so's I can get a better idea of which band of supplies to focus on.

Let me have it! Spam me out with voltage / current specs!
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Old 21st January 2007, 06:35 PM   #5
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if somebody is building amps, the voltage/current ratios are determined by load impedance 2/1 for 2 ohm loads 4/1 for 4 ohm loads, etc...... because of reactive loads, etc... you might actually increase the current capability to 2/1.5, 4/1.5, etc.....

a "raw" supply with transformer, bridge, and caps would probably be best driven by a variac, so you can have a variable output voltage. any solid state method (i.e. triac dimmers, etc...) of varying the input voltage is going to introduce noise... open frame linear (and switchers) regulated supplies are inexpensive from companies like jameco, who sell "recycled" power supplies. maybe as one of the projects, the students could build the power supplies themselves. power supply design is as much an art as amp design, with it's own set of quirks and pitfalls.

you may also want to consider using older PC supplies, as they are dirt cheap, and (at least the older ones) have +/-5, and +/- 12v outputs.
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