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Pashune
Caps stifle innovation
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join:2006-04-14
Gautier, MS

Pashune

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Using AC adapter instead of DC adapter? And more questions..

Recently, I replaced the OEM DC 12v 500mA adapter going to my main Linksys wireless router adapter with a beefier DC 12v 1a adapter, thinking the weaker adapter was the cause of it rebooting occasionally. This should be fine, right?

But my guest router also uses 12v and requires 750mA. At the moment I only have a DC 12v 500mA feeding it. The reason? I don't have anymore stronger 12v DC adapters sitting around; only 12v AC. With enough time, could the guest router overload and kill the adapter? I only have the WAN cable connected to it and wireless enabled. Nothing else. The adapter is somewhat warm, but not scorching hot.

I did quickly test a 12v 1A ac adapter on the router and it powered it up just fine, but I'm afraid that long term exposure to the AC adapter could kill the router's circuitry. Could it really happen, and if so, how?

I also have a Motorola SB5101 modem sitting around that requires 12v 750mA. Connected to it is a somewhat large 9v DC adapter putting out 800mA. It boots it just fine and the adapter is barely warm. From what I've heard the SB5101 uses a 5v regulator and the LEDs actually draw more power than the circuitry itself. Will this be OK?

SparkChaser
Premium Member
join:2000-06-06
Downingtown, PA

SparkChaser

Premium Member

The best advise I could give you is STOP switching adapters. Plugging an AC adapter into equipment that says DC means you need a little education in electronics.

There are a number of variables to adapters. Most of the newer ones are 'switching' supplies and if it says 12V 0.5A that is what is coming out.

Some are transformers, they are heavier than the above and the voltage out typically varies with the load. This means that if you put a 12V 1A adapter on a 0.5A load the voltage might be up in the 14V range.

Others, that are plainly labeled 'AC out' SHOULD NEVER be put into a piece of equipment that says 'DC in'.

If, in fact, an adapter is causing problems, it may be that it is defective and not that it is under powered. In the past few weeks I replaced capacitors in 2 adapters and a connector on another. All because they were causing intermittent problems.

Pashune
Caps stifle innovation
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join:2006-04-14
Gautier, MS

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Pashune

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I suppose I'll do that then. Maybe my WRT54G was acting funky was due to something else..

aurgathor
join:2002-12-01
Lynnwood, WA

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Unless you know without doubt that the PS id defective, there's usually no need to use a different power supply. And as it was mentioned, a 12VAC supply is a very bad idea in place of a 12 VDC, although the opposite may work.

Most any modern electronics that has a 12VDC power supply will have an onboard voltage regulator in it, meaning that it may work just fine with a power supply that is not exactly 12V, depending on a few other things like the voltage rating of the caps, and whether it's a switching regulator or not.

cowboyro
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join:2000-10-11
CT

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Using AC on a DC device is a bad, bad idea. Some devices MAY have idiot protection (a diode to only allow for the right polarity), some may not... Applying AC will also make the device less immune to brownouts.
As far as the Motorola modem goes, it may or may not work fine. The LEDs don't draw more power than the circuitry, I can guarantee you that. You have a RF transmitter that is putting out quite some power... If you want to be sure check the actual power consumption by measuring the current drawn. The original adapter gives up to 9W, the smaller one 7.2W. Modern devices have switching power supplies so they will draw almost the same power regardless of the input voltage (as long as it's within operating limits): higher voltage-> less current, lower voltage-> higher current.
Again the same potential issue with brownouts by applying a lower voltage...

Pashune
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Gautier, MS

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How much variance would be safe? I've got some other DC power supplies laying around but they're higher than 12v. One is 14.5v and another is 15v. I will say I once saw a SB5101 plugged into a 14v power supply and it had no problem with it. I just tried one that's putting out 15VDC and 600 mA. From what I can tell that's 9w. Booted it up fine, but it was only a quick test.

cowboyro
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join:2000-10-11
CT

cowboyro

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said by Pashune:

How much variance would be safe?

That highly depends on the design of the device. Some expect the exact voltage and are equipped with switching power supplies. Others have internal switching regulators and can accept a fairly high variance.

Pashune
Caps stifle innovation
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Gautier, MS

Pashune

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Understood.

I'm sorry if I come off like an idiot when it comes to these things; I've been looking high and low on Google for a detailed explanation as to why you're not supposed to use an AC adapter outputting AC on a device that expects DC input.

When I was younger (We're talking before 10..), I would just match up whichever adapter would fit into the device. Most of the time it either worked or didn't work. Only twice have I blown something up. (Might be a big number to you guys though! )

The first time involved a cheapo pair of speakers expecting a 6 or a 9v input I believe. The adapter was putting out at least 16v from what I remember on the label. After trying to figure out why the speakers wouldn't turn on, I was greeted by some smoke and a pop...

Then it happened again recently when I was trying to power up a portable DVD player expecting a 9v DC 2200mA input. I have NO 9v adapters this powerful so (like a moron you might say) I plugged it into a 12v AC 1000 mA input. It powered up, but the video was distorted, the speakers were buzzing, then I was shortly greeted with a terrible odor and smoke. I had two of these and they were both broken in one way or another, so it wasn't a big loss for me.

SparkChaser
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join:2000-06-06
Downingtown, PA

SparkChaser

Premium Member

said by Pashune:

Then it happened again recently when I was trying to power up a portable DVD player expecting a 9v DC 2200mA input. I have NO 9v adapters this powerful so (like a moron you might say) I plugged it into a 12v AC 1000 mA input. It powered up, but the video was distorted, the speakers were buzzing, then I was shortly greeted with a terrible odor and smoke. I had two of these and they were both broken in one way or another, so it wasn't a big loss for me.

That's why I said don't do it. As cowboyro See Profile said, it all depends on the design of the device as to what it can handle.

A for instance, one of the adapters I mentioned previously put out 5V and 12V. There was no regulator on the USB disk drive it went to, it wanted 5V and 12V. In fact, the 12V measured with a meter looked ok. I had to put an oscilloscope on it to see what was happening to the 12V. After seeing noise on the 12V supply, a simple replacement of capacitors fixed it.

It's all very device dependent.

Pashune
Caps stifle innovation
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join:2006-04-14
Gautier, MS

Pashune

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I got that part, but why is AC so dangerous to mix with DC?

That's what I'd like to know.

cowboyro
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CT

cowboyro

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It's like reversing the polarity 120 times/sec.
nonymous (banned)
join:2003-09-08
Glendale, AZ

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For AC use AC and for DC use DC. If it wants so many amperes use that size or maybe a little more.
For DC make sure the polarity is correct.
Keep the voltages the same.
A question. i rarely need a replacement adapter. But does anyone know a good cheap source if they are needed.
A couple stores I used to get them at locally have grown closer to generic we sell TV stores and have done away with the little stuff bit by bit. Now just expensive stuff like overpriced hdmi cables not the good solid cheap ones they used to have. Cables i still know where to get online and stores cheap if I need to. But occasionally I do need an adapter and just have never really looked yet to see where I can get one next time.

Pashune
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said by cowboyro:

It's like reversing the polarity 120 times/sec.

That makes sense now, and if a DC input device receives reversed polarity from AC input without an idiot-diode to protect it, it'll go boom! Or my guess is it'll kill the diode over time from excess amps/voltage?
nonymous (banned)
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The circuit would also only get half a wave I think. Plus someone will correct me here if I am wrong AC is a wave and DC is nice and flat. So AC with a diode would be half a wave and not even half a flat. So oddities may occur.
Its winter break time and I have tons of other stuff on my mind so I know I wrote that really badly.

tschmidt
MVM
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said by Pashune:

I got that part, but why is AC so dangerous to mix with DC?

Because the devices is designed differently depending on what type of power module it is used with.

1) If the device expects AC it has: rectification, filtering, regulator more filtering, active electronics.

2) If device expects unregulated DC it has: regulator, filtering, active electronics.

3) If device expects regulated DC it has: active electronics.

/tom

Pashune
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Pashune

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quote:
3) If device expects regulated DC it has: active electronics.

/tom
So if you connected an unregulated DC power supply to a device expecting DC regulated, you're potentially asking for trouble since the device has no filtering or caps to filter the incoming power on the board. Large jitters or dirty power can bring up glitches or screw up the device.

I think I'm starting to figure this out slowly but surely.

Thanks for the helping me out with this you guys!

SparkChaser
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said by Pashune:

That makes sense now, and if a DC input device receives reversed polarity from AC input without an idiot-diode to protect it, it'll go boom!

Exactly, (without a diode) half the time you are putting a negative voltage where it should be positive. (not usually a good thing )
said by Pashune:

So if you connected an unregulated DC power supply to a device expecting DC regulated, you're potentially asking for trouble since the device has no filtering or caps to filter the incoming power on the board. Large jitters or dirty power can bring up glitches or screw up the device.

Right again, plus the voltage may not be the voltage it should be.

r81984
Fair and Balanced
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Try using ddwrt micro if your router is doing weird things if you have not destroyed it yet.

J E F F4
Whatta Ya Think About Dat?
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Not replying to you - per say, the only time I've seen AC 12 VOLT output was for furnace or air conditioning thermostats. (simple transformer reduction 120 to 12 - 10:1 ratio of wrapped wire around the magnet)

In the future, remember that you can only have a small variable in output (DC) maybe ±10%. It's okay to have a higher output of A (or mA) but never lower. And of course, polarity must be correct, or else you can blow things up.

The reason why things are AC (as in electrical lines) is that normal generators produce AC and AC can travel further on the lines without as much loss, as well, it is easier to boost or reduce voltage with AC.

If you want to try something fun with a battery, take a non-magnetized screwdriver, grab some shielded wire, and wrap it around (like a coil) the screwdriver (the metal part, not the handle) and hook it up to a battery. (I use a 9 volt, but I think 1.5v is fine) and presto, you have a magnetized screwdriver. You can also create you own motor, and rather than try to explain it, I posted the video below. You'll figure out pretty quickly how amazing electricity is, and why not to screw around with it.

»www.youtube.com/watch?v= ··· =related


In case you were wondering what is on the bottom of battery - those are magnets.

aurgathor
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said by Pashune:

How much variance would be safe? I've got some other DC power supplies laying around but they're higher than 12v. One is 14.5v and another is 15v. I will say I once saw a SB5101 plugged into a 14v power supply and it had no problem with it. I just tried one that's putting out 15VDC and 600 mA. From what I can tell that's 9w. Booted it up fine, but it was only a quick test.

I'd say up to 15.9V it's safe with 95% certainty, and that's assuming a linear regulator. With a switching regulator, I'd say it's 99%.

Pashune
Caps stifle innovation
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Pashune

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Yeah, the adapter is one of the old brick types, so I'm assuming linear.

But are you talking about the modem being linear/switching or the adapter itself?

I once heard a case where a bunch of D-Link routers were supplied with 12v adapters instead of 9v, even though the manual and router said it only accepted 9v, but it worked anyway.

tschmidt
MVM
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said by Pashune:

Would there be anyway to tell if it were linear or switching without opening it?

For a given power rating a switching supply is lighter and smaller then a linear supply.

Judging from this:
»www.motorola.com/Consume ··· 01-US-EN

And this spec sheet the modem draws 9watts
»www.motorola.com/staticf ··· _706.pdf

The wall wart is most likely a switching supply. A linear transformer supply would be more squarish. Have not kept up with Energy Star requirements but most likely a linear supply will not meet efficiency requirements.

/tom

Pashune
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Pashune

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I was thinking that too, but when I first got my hands on one of the SB5101's years ago, I clearly remember having to plug in one of those heavy wall warts that came with it.

And when I leased one from CableOne about 2 years back, it came with a pretty large 14.5v adapter that had one cord on each end. Never gave me any trouble.

Is it possible that Motorola could have switched to a different power supply design sometime back?

I'm just hoping it's 'safe' to use an adapter with a little bit of voltage variance in this case; Both the modem and adapter are barely warm at all, so I'm hoping that's a good sign.

tschmidt
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tschmidt

MVM

said by Pashune:

I'm just hoping it's 'safe' to use an adapter with a little bit of voltage variance in this case;

As long as the output voltage of the wall wart is the same as the input requirements of the modem you are fine. The differences between a linear and switching supply are cost and efficiency. The modem doesn't care how the voltage is created as long as the power supply delivers the correct voltage and is able to supply enough power.

No different then using a light bulb. The bulb doesn't care how the power is generated as long as it is correct.

/tom

Pashune
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Pashune

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Makes sense, but in this case it's getting 15 VDC @ 600mA, but the modem calls for 12 volts @ 750mA. Both of those are equal to 9w.

Something like an internal regulator or something in the modem itself would switch that to whatever the modem needs perhaps?

I apologize if I come off as confused or stubborn on this subject.

tschmidt
MVM
join:2000-11-12
Milford, NH
·Consolidated Com..
·Republic Wireless
·Hollis Hosting

tschmidt

MVM

said by Pashune:

Something like an internal regulator or something in the modem itself would switch that to whatever the modem needs perhaps?

Most likely modem has an internal regulator.

Digital and a lot of analog chips use 5 or 3.3v (sometimes even lower). If the input voltage rating of the device is something other then those two voltage it is a pretty good indication device has an internal power supply. The internal supply provides tight regulation near the load and may need to deliver multiple voltages. Often low level analog functions are isolated from digital circuity to reduce noise levels.

/tom

aurgathor
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As I've already said, any modern digital electronics that uses a power supply with a voltage greater than 5V, will have an internal regulator.

If it's linear regulator, higher voltage will result in higher power dissipation and possibly overheat, even before the max input voltage of the voltage regulator is reached.

If it's a switching regulator, the power dissipation will not change much, and will take higher voltage until something (likely a cap or a semiconductor) breaks down.