The next question is making a design with specified part numbers, and then sourcing them. This is where our difficulties start. As a sometime designer of audio gear, I have plenty of leftover parts from various prototypes, but you aren't in the same position. The problem is that we in Europe don't have electronics hobby shops in the same way Americans have Radio Shack. We generally have to order electronic parts by mail. Even if you have an account with RS or Farnell, the carriage can kill enthusiasm, and any remaining joy in achievement is usually destroyed by minimum unit packages that waste a lot of money.
So, please, just take it as read that you can't buy the components for the price of an entire circuit, given only that you shop right. So what you do is to buy a pre-built module, usually surplus from some other purpose where it was used in the thousands or even the millions. That means you have to trust someone else to have drawn a good circuit. But in fact most circuits of utility boards are good, because they are cookbook circuits, straight off the manufacturer's data sheet. The only two remaining questions then are the quality of the components and the quality of the workmanship.
http://www.ebay.co.uk/itm/LM317-DC-AC-4V-30V-Out-DC-1-5V-27V-1A-Converter-Fiber-Glass-PCB-Brand-New-/300694378839?pt=LH_DefaultDomain_2&hash=item4602c81957 shows a "LM317 In DC or AC 4V-30V Out DC 1.5V-27V 1A Converter" for around £4.26 (depending on Paypal's conversion rate on the day). You can't buy the components for that. Let's check out the circuit they used.
Hmm, that looks pretty good, perfectly standard, a cookbook circuit straight off the manufacturer's spec sheet. The input of AC or DC current is on the left, full wave-rectified by the four diodes (arrowheads with wavy lines across them), smoothed and buffered by a capacitor (two short parallel lines; it's a sort of battery device that works much faster though, for practical purposes discharging all its power in a very short space of time) and then the current is fed to the 317, which is the rectangulare yellow integrated circuit. The 317 is protected by a diode across it (above it in this piccie). The output voltage is set by the ratio between the two resistors R1 and VR1, the VR resistor being variable merely for convenience. The rest of the devices, of types already introduced, are simply protection and convenience devices. At the righthand side a controlled, fixed voltage, DC current, is delivered.
So let's look at the underside of the board and see if it is nicely soldered.
Okay, that looks like a professional job off a practiced assembly line. Note that the soldering points are widely spaced, a useful feature if you want to change anything.
Next we take a good look at the heat sink because it is one of the most critical components.
Looks big enough to offer us some substantial headroom. Notice also the beginnings of rust on its edges. This board is cheap because it has been sitting in someone's store, surplus production of a very commplace board. It does no harm though. We can also read 50V 470uF on the capacitor (tall blue and silver tower) facing us, and see that the designers and productionizers didn't skimp on the rating of the cap (in a cheaply made board, you'd probably find 35V caps in there, marginal headroom for those who push this circuit, as we won't do). We see also the red LED, a nice touch, and just that margin of constant current draw that will keep this circuit nice and stable. (It's actual purpose is as a protection device.) The 317 is the chip screwed to the heat sink, the potentiometer (commonly "pot") is the blue block with the screw on top as the adjustment knob, and the black block with the corner cut off contains the four rectifier diodes; it appears well sized to handle the 1W max we aspire to, and will probably never see. The stripey blue bar is a resistor and appears big enough to handle the heat it will radiate. The small red item next to it, half obscured, is a control rather than a storage cap. The green blocks are screw terminals for attaching your wires, much more convenient than solder patches or holes for those of you without a soldering iron or soldering skills.
It appears to be a standard design board, made to a good commercial standard, sold very cheaply as surplus. We can risk a fiver on it!
Those of you who have at least a multimeter, or a voltmeter, can buy this board. All the images are from the vendor I suggest:
http://www.ebay.co.uk/itm/LM317-DC-AC-4V-30V-Out-DC-1-5V-27V-1A-Converter-Fiber-Glass-PCB-Brand-New-/300694378839?pt=LH_DefaultDomain_2&hash=item4602c81957Those of you without a voltmeter or a multimeter, check my next post.
Andre jute
