12 volt power source?

I don't know if this is a dumb question or if it just SOUNDS like a dumb question, but is 12 volts really 12 volts? I'm asking because I'm installing tortoise machines and in checking some of my old 12v DC power sources, I see that they actually measure about 16 v DC... even though they are labled as 12 volts.
It seems odd that three 12 volt sources would all measure about 16 so I'm wondering if a 12 volt power supply is actually something other than 12 volts... much like a 2x4 is really 1.5 x 3.5.
My concern, of course, is that I don't want to burn out a tortoise machine using a 12 volt power supply that is really 16.
Any thoughts?
Carter Braxton

Carter Braxton wrote:
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No. 12 volts was the figure set because car batteries were nominally 12 volts. They are actually 13.8 volts in their fully charged state. 12 volts is the maximum voltage set by such organisations as the NMRA and MOROP. MOROP further defines that model locomotives should run at scale maximum speed x 1.3 at 12 volts DC.
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Transformer voltage output will vary with load. One would expect a non-stablized trafo labelled 12 volts to show perhaps 16 volts at no-load and 10 volts at 1.5-2 times maximum rated load.
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Your tortoise machine should be ok on 16 volts, but 12 volts would be preferable.
The 16 volt AC accessory output commonly found on MR controllers came about because the old selenium plate rectifiers dropped the common transformer winding from 16 v AC to 12 v DC. (give or take a couple of volts either way and allowance for load variations, wind direction, humidity, temperature and monday morning hand windings)
Greg.P. NZ

On Fri, 21 Dec 2007 10:54:18 +1300, I said, "Pick a card, any card"
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Nonsense.
This is more correct. The load is the issue, not the 13.8 charge voltage. Unloaded power supplies will in fact go to the rail, the term used for the upper limit of the regulator. Loaded power supplies will be stable at 12 volts if they are not faulty. -- Ray

Ray Haddad wrote:
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Care to expand on that point? A standard automotive lead-acid battery in normally charged state is 13.8 volts.
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We're discussing unregulated power supplies at this point.
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'Regulated' power supplies ... You average power supply, those owned by (say) 90% of modellers using analogue control, does not include an electronic regulator.
Regards, Greg.P.

Greg Procter skriver:
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Nope http://en.wikipedia.org/wiki/Lead-acid_battery After full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V.
Klaus

On Fri, 21 Dec 2007 11:42:33 +1300, I said, "Pick a card, any card"
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Yes but he's not talking about a charger here or a battery. He was asking about a 12 volts. Don't confuse the issue.
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No, you are. Everyone else is probably on the same page.
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12 volts is 12 volts. Under no load conditions, unregulated or regulated power supplies will read higher. -- Ray

Ray Haddad wrote:
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I mentioned that because initially "12 volts" trains were commonly run from 12 volt car batteries - ie nominally 12 volts but actually slightly higher. That is the precedent which set the expectation for HO models.
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You're always on a different page.
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On Fri, 21 Dec 2007 14:17:00 +1300, I said, "Pick a card, any card"
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Fair enough, Greg, but the charge circuits are 13.8 volts while the battery remains very near to 12 volts. Seriously. -- Ray

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

Charles Davis wrote:
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I'll take a bet that the Bachmann 1/4 amp tin box/plastic box set controller is _the_ most common 12 v DC controller out there.
Regards, Greg.P.

On Thu, 20 Dec 2007 19:21:08 -0500, I said, "Pick a card, any card"
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I'm not nit picking, mate. The 12 volt battery consists of 6 cells each of which outpout 2 volts making the total 12 volts. The charger for that battery must be higher by 10% in order to charge the cells. A power supply, which is what the fellow is asking about, is NOT 13.8 volts even in Model Railroad Circles. A CHARGER is but not a POWER SUPPLY. Get it?
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Regulated or unregulated, a 12 volt supply always puts out a higher voltage until it has a load on it. Believe it.
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They still put out 12 volts, not 13.8 volts. -- Ray

Ray Haddad skriver:
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Say what ?
Whay do you then needs a regulator for.
Klaus

Klaus D. Mikkelsen wrote:
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To maintain the 12V under load.

"Wolf K." skriver:
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And without.
If you consider my Fluke 87 (voltage meter) as a load then noone knows if the 12 volt is more than 12 volts without load, because you can't measure it unless you connect a load.
Klaus

On Fri, 21 Dec 2007 19:16:23 +0100, I said, "Pick a card, any card"
replied:
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In some regulator circuits, a meter is enough of a load. In others, a meter just doesn't quite do the job. Look, Klaus, the real point I was making is that there is a significant difference between a 13.8 volt battery charger and a 12 volt power supply. Those wall cubes most people think are 12 volt supplies are sometimes chargers for internal batteries on some device or other. Using a multimeter will usually, but not always, separate the types for you. A load is part of the power supply circuit and when present your voltmeter will be more accurate than when not present. -- Ray

Ray Haddad skriver:
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But how do you then know the voltage unloaded ?
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Very bad design.
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I totally agree on that
Klaus

On Fri, 21 Dec 2007 19:41:16 +0100, I said, "Pick a card, any card"
replied:
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You really don't until you apply an external load.
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Clearly. The wall cube transformer/power supplies are often designed to be used with a specific item which does apply a load. Saves money. We all like those $10.00 answering machines but those wall cubes must be shaved down in cost to allow that kind of price. You get the idea.
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Lucky for me. You're good. Too good. -- Ray

Ray Haddad skriver:
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So how can you claim that the voltage is higher unloaded ?
My scope with the probe in x10 position loads the powersupply with 100MOhm - would you call that a load ?
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20 years in electronics repair/design/manufacture/support is the load on my back.......
Despite I'm not in the business right now, my latest repair was yesterday in my garage. A guy from one of the local garages mailed me and asked if i knew who in our town could repair on SMD level - my answer was Me or the local Vestas factory, so they came to me. Resolder of SMD chips on a dashboard controller for a Renault is now in my experiance list :-)
They called me today - it was a great success..........
Klaus

On Fri, 21 Dec 2007 21:38:47 +0100, I said, "Pick a card, any card"
replied:
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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.
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Not at all. Anything above 10MOhm is not considered a load.
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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!).
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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.
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Well done. -- Ray

Ray Haddad skriver:
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We totally agreee......
Shitty powersupplies depend on load, but regulated doesnt.
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Darn - Z80 was in my youth (I'm "only" 35:-)
Did you ever get the ZX spectrums ? http://www.worldofspectrum.org / AFAIR it was british build and based on the Z80A. They keyboard was rubber and was later used as "anti slip mats in showers" :-)
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Neat :-)
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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