12 volt power source?

One needs 12 volts before applying the load if one needs a precisely regulated power supply!

Aww Klaus, just apply an expensive piece of equipment that will burn out if more than 12 volts is applied to it - it will detect the higher voltage quicker than you can say "B****R!"

Ray, charge an automotive battery to maximum charge and it will hold near enough 13.8 volts, even under a light load.
Greg.P.

13.8 volts isn't the voltage of an automotive battery charger, it is the maximum voltage an automotive battery will hold. An (decent) automotive battery charger is normally several volts higher. (circa 16 volts DC)

Your train power supply is unregulated, other than the rectifier and rheostat. Even at "12 volts" the output is in pulse form reaching close to 17 volts peak. That difference (17 - 13.8v) allows charging current to flow even when full battery charge is reached. A regulated 12 volt supply would only half charge your car battery.

On Fri, 21 Dec 2007 21:38:47 +0100, I said, "Pick a card, any card" and "Klaus D. Mikkelsen" instead replied:
Multimeters don't apply much of a load by design. My claim was based on a statement that the voltage on a 12 volt supply read more than 12 volts. Made by another poster, not me. I know for a fact, and based on your experiences stated below, that this is common with some poorly designed or special purpose power supplies. They rely on the load to have full regulation. It saves the manufacturer money.
Not at all. Anything above 10MOhm is not considered a load.
I've only got 40 years. Tubes were on the way out and transistors were very expensive back then. Integrated circuits were only dreams but they did exist when I started out. Built my first S-100 machine from scratch using a brand shiny new, Z80 (Soooooo expensive!).
I only do design work for myself now. I teach a bit at the local tertiary schools here in Perth whenever I'm asked. I am a former third party rep for MicroChip but gave it up when I moved to Australia from San Diego. I truly love embedded control and use any excuse at all to drop a PIC into a project.
Well done. -- Ray

On Sat, 22 Dec 2007 10:08:19 +1300, I said, "Pick a card, any card" and Greg Procter instead replied:
Go and measure you car battery. It's only 12 volts. If your charger didn't output 13.8 volts or more, it would discharge the battery. That's the nature of lead-acid batteries. No kidding, Greg. I'm really trying to educate you here. Just accept it for now and verify it when you get a chance. Ask a lead-acid battery manufacturer. Go to Jaycar or your local battery shop. Anywhere at all except inside your own head. -- Ray

On Sat, 22 Dec 2007 10:06:03 +1300, I said, "Pick a card, any card" and Greg Procter instead replied:
False. -- Ray

On Sat, 22 Dec 2007 10:18:03 +1300, I said, "Pick a card, any card" and Greg Procter instead replied:
Correct. Or, if it's only a 12 volt supply, it will actually drain his battery. -- Ray

On 12/21/2007 1:04 PM Greg Procter spake thus:
Isn't that like the old thing about using a $15 transistor to protect a 25-cent fuse?

On 12/20/2007 11:23 PM Ray Haddad spake thus:
Data point: I measured my car's battery voltage (in good shape & fully charged). 12.5 volts (both my multimeters agree).

Sounds about right!!! A "NORMAL" 'Train Power Supply', will charge an attached 'capacitor' to close to 20 volts, because of the peaks in voltage available for a very small amount of time.
Chuck D.

Ray Haddad skriver: We totally agreee......
Shitty powersupplies depend on load, but regulated doesnt.
Darn - Z80 was in my youth (I'm "only" 35:-)
Did you ever get the ZX spectrums ? it was british build and based on the Z80A. They keyboard was rubber and was later used as "anti slip mats in showers" :-)
Neat :-) Thanks. Another of my proud job was a late night, we were pretty drunk, when my friend pulled out an DVD player with defective switchmode powersupply. It took about 10 minutes and then it was running again - typical error, toasted power diode and dead capacitors..... The next morning i had headache......
Klaus

Greg Procter skriver: No........ Definitly not
Did you read the link I gave you, Im happy to give it to you again:
full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V.
Klaus

On 12/21/2007 2:55 PM Klaus D. Mikkelsen spake thus:
I still think the Z80 is one of the most underutilized and underappreciated machines ever built in the late 20th century. All that power and speed in such an unassuming package. Like those totally kewl alternate register sets and the instructions that swap them with the regular set, for high-speed interrupt processing ...

On Fri, 21 Dec 2007 23:55:37 +0100, I said, "Pick a card, any card" and "Klaus D. Mikkelsen" instead replied:
Most here seem to be hunting the cheapest of the cheap and probably buy the wall wart type. More than half of those are only partially regulated relying on the equipment on the load side to finish the regulation. Awful practice, that, but it does make for cheap power supplies when you need them.
I nearly used an 8080A but the Z80 came available and I switched. All wire-wrapped. What a headache that was. Tedious but fun.
I had a Sinclair ZX80, the membrane keyboard type. Designed a 64k memory expansion for it and published the design in the old Popular Electronics magazine sometime in the '80s. I forgot which name I published under for that one. I use pen names mostly to avoid flame wars here on USENET over something I publish.
I picked up a dead, second-hand, rack mounted P4 Quad Core PC used for 6 months as a server before it failed. It was failing big time according to the seller and I verified that. But, his company had replaced it with a bigger and better server with 4 terabytes of hard drive. This one had 4 320GByte ATA/IDE drives, 2 500GByte SCSI drives, 2 SCSI controllers and a 4 port RAID controller. On inspection, I found a lightning damaged burned trace at the DMA controller chip and fixed the unit. It's a screamer with more hard drive space than I can use in a lifetime. It cost me a whopping $10.00 yesterday. I wiped all the drives clean as promised.
Now, what shall I do with it? I think I'll sleep on it. It has sharp pointy corners but with a thick enough blanket I can at least have a nap on it.
Oh, rats. I meant to send this by e-mail but it's too much bother to retype it or cut and paste it. So, it goes on the newsgroup with a copy by e-mail. Sorry it's so horribly off topic, guys.
Well, I can make it on topic easy enough. I believe I'll use that new PC to automate my trains. There. On topic now. -- Ray

[snip prior chit chat] LOL
PS: I "built" me a new PC with an MSI board (1000MHz FSB) and a 2.4GHz Intel Dual Core, 2GB 800MHz DDR, two 250GB SATA, 256MB PCI-X video card, and a floppy drive. Still feel uneasy without a floppy drive. Had to put it in a tall tower, else the CPU fan wouldn't fit under the PSU. Just plugged all the bits and pieces together, turned it on, installed a brand new copy of XP Pro/SP2, copied a pile of zips and install packages from a CD, and starting playing with it. Erm, I meant _working_ with it. ;-)
Fastest machine Ive ever used.

On 12/21/2007 3:37 PM Ray Haddad spake thus:
I've taken apart lots of wall warts (to cannibalize the parts within) and don't remember ever seeing any with a regulator. Just a transformer, a bridge rectifier (or two diodes) and filter capacitor(s).

On Fri, 21 Dec 2007 16:05:21 -0800, I said, "Pick a card, any card" and David Nebenzahl instead replied:
Your point? -- Ray