Hugely fundamentally ignorant question.
If I have an electronics device that claims to take a power adapter of 12V
200mA, and I plug in one rated at 12V 800mA, will the device merely draw the proper 200mA (because the voltages are matched)?Or is there something about PA's that can somehow force too much current into a device?
The rating on the supply indicates what it can deliver to the device in terms of current. All this means is a connected device requesting current can not ask for any more than 800 ma. The device connected governs how much current will flow, the supply only indicates the amount it can give with out damage or shut down to it self.
Voltage of the supply must be close or exact to what the device requires. In your case you have a 600 ma reserve or power, so you are fine..
-- "I\'d rather have a bottle in front of me than a frontal lobotomy" http://webpages.charter.net/jamie_5"
It's not quite that simple.
For regulated adapters, you can reasonably assume the voltage will be as stated up to the rated current.
Many adapters are unregulated, however, and the voltage will depend on the attached load. Normally, an unregulated adapter will have the nameplate voltage under the nameplate load, but the voltage will be greater under lesser loads.
If the voltage is "too high" then it may force excessive current into a device, or cause component failure because of the excessive voltage.
Chuck
----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
----= East and West-Coast Server Farms - Total Privacy via Encryption =----
Chuck said something like:
If I were to check the nominal voltage with a voltmeter, it would show me the voltage supplied with essentially no load placed on it, right?
-- Very old classic: Three men check into a hotel: the room is $25 for the night. They each hand the bellhop $10 and ask him to bring back the change. When the bellhop returns with the $5 change, the men figure it\'s easiest math to give $1 back to each of them and leave $2 to the bellhop as a tip. Now each man paid $9 for a total of $27. The bellhop got $2, that makes $29. What happened to the last $1? Answer (rot13): Unir gb or pnershy ubj lbh nqq guvf hc. Gur guerr zra cnvq $27 gbgny, BHG BS JUVPU $2 jrag gb gur oryyubc.
Basically that's the idea.
See other posters' warnings about unregulated adaptors having a higher voltage when only partially loaded.
Graham
Right.
--
Best Regards:
Baron.
It may work, and it may not... Some crappy designs deliver the rated voltage only at (or near) the rated current, and as current drops, output voltage rises. As such a lightly loaded unit may in fact cause damage because of overvoltage.
Realize that these cheap wall warts are *not* regulated, and the output voltage is approxmate at best in most cases.
"Nominal" means "named", and you check it on the name plate. It is nothing more or less than the promise you're getting from the thing's manufacturer.
You can check the _actual_ no-load voltage at no load with a voltmeter, just by measuring what's on the end of the wire when the thing is plugged into the wall but isn't plugged into the device you want to power.
-- Tim Wescott Control systems and communications consulting http://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
A very frequently asked one, as well. And very frequently answered. Others gave real answers. I agree with the correct ones ;)
Actually, I believe. One kind of adapter that might be able to put more current into a device (by raising the voltage, of course) than is "asked for" is a constant current supply (battery charger for variable number of cells). But it's very unlikely that you have one. The plate would probably have to be wrong, too.
(???) Ok....I'm checking the named voltage as read from the name plate. I'm not sure how I misstated this. There is a voltage it is supposed to be, and I'm checking that supposed-to-be voltage.
You raise interesting (non contentious honest) questions within me noggin. I've heard that voltmeters are not equipped to properly measure a battery's worth because "it will measure the nominal voltage only" which is of course the no-load voltage which also happens to be the named voltage written on the side of the thing. Not sure if the two concepts are yet colloquially synonymous no-load and nominal....I'll trust you on this. But there is an acronym NV.....what would that possibly stand for if the definition were merely satisfied by naming something as "V"?
Ok, thanks.
For an unregulated power supply, like most wall-warts, the nominal voltage is the voltage it will deliver with some load - probably somewhere near maximum currents. With no load other than a meter, the voltage will probably be somewhat higher.
Batteries are a bit different from power supplies. For a dry cell battery, the nominal or marked voltage is the voltage delivered by a fresh battery. A used battery will show somewhat less, even with no load.
The "nominal" voltage of a lead-acid battery or an automotive electrical system is a sort of "random number" only roughly related to the actual voltage. The actual voltage of a fully-charged lead-acid cell is about 2.2 volts, so the "12 volt" battery in your car should measure about 13.2 volts when fully charged, and the voltage of the car's electrical system will go over 14 volts when the engine is running, and the battery is being charged.
-- Peter Bennett, VE7CEI peterbb4 (at) interchange.ubc.ca GPS and NMEA info: http://vancouver-webpages.com/peter Vancouver Power Squadron: http://vancouver.powersquadron.ca
Peter Bennett said something like:
Ah, ok, essentially each of the 6 cells in series then I take it?
-- Geez, I guess without your helpful "me too" he\'d be a little lost lamb?
--- Most small unregulated wall-warts and transformers have secondaries wound with wire with a resistance which will result in the specified nominal output voltage being obtained with the specified load connected to the output and nominal mains voltage applied to the input.
That means that with high mains and/or lower than specified output current the output voltage will be higher than nominal and with low mains and/or higher than the specified output current the output will be lower than nominal.
The reason for this is that the resistance of the secondary looks like it's in series with the secondary, so as current into the load increases the voltage dropped across the secondary resistance also increases, decreasing the voltage available to the load, like this:
View in Courier:
Vs Vl / / MAINS>---+ +--[Rs]--+ | | | P||S | R||E [Rl] I||C | | | ---+ +--------+
Typical regulation of small transformers is about 30% no-load to full load, so if we assume that's true for the transformer in your wall-wart, Vs will be 1.3 times Vl, or 15.6V, when Vl is at 12V.
With Il = 800mA, Rl will be equal to:
VL 12V Rl = ---- = ------ = 15 ohms Il 0.8A
and Rs will be equal to:
Vs - Vl 15.6V - 12V Rs = --------- = ------------- = 4.5 ohms Il 0.8A
So, to simplify things, we now have:
15.6V E1 | [4.5R] R1 | +--> E2 | [15R] R2 | 0VYour load is going to draw 200mA at 12V, so its resistance under those conditions will be:
E1 12V R2 = ---- = ------ = 60 ohms Il 0.2A
and the circuit now looks like:
15.6V E1 | [4.5R] R1 | +--> E2 | [60R] R2 | 0Vso the current in the circuit will be:
E1 15.6V Il = --------- = ------------- = 0.242A R1 + R2 4.5R + 60R
and the voltage across your device will be:
E2 = Il * R2 = 0.242A * 60R = 14.52V
Here's a data sheet for a typical smallish wall-wart which will illustrate the principle:
http://204.202.11.159/tamuracorp/clientuploads/pdfs/engineeringdocs/830AS12080.pdf
JF
http://204.202.11.159/tamuracorp/clientuploads/pdfs/engineeringdocs/830AS12080.pdf
_Nice_ link.
http://204.202.11.159/tamuracorp/clientuploads/pdfs/engineeringdocs/830AS12080.pdf
--- Yeah, they've got great data sheets!
Little problem with that one though, they've got the width of the mains prongs dimensioned out as 2.5" instead of 0.25"!
I just emailed them about it, so we'll see what happens. I've done business with them before and they've always been decent, so I'll bet they email me back with something like, "Oops... Thanks!" :-)
JF
Yes IF the power are really what the label said.
How do you know the powers are matched? Not all adapters telling your the truth. Use a meter, do not trust the cheap wall-adapters made by the Chinese.
Jack...
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.