Matching AC Adapter sizes

[dcapper]

My router 12V 1A AC adapter died. I have a 14V 800mA AC laying around. Will that work or should I just eBay one?

[mockingbird]

Re: Matching AC Adapter sizes

Is this a switching adapter or a linear adapter? If it's a linear adapter it has a large transformer inside.

[dcapper]

Re: Matching AC Adapter sizes

It must me linear. It's a little transformer. It's from a Cisco DPC3008 Cable Modem. The one I have that is 14v is a lot larger.

[stj]

Re: Matching AC Adapter sizes

little? then it's probably a switching psu and may (if your lucky) have a bad cap or 3 in it.
got foto's ?

[dcapper]

Re: Matching AC Adapter sizes

Here it is.

[mariushm]

Re: Matching AC Adapter sizes

Surely you mean DC output for both power adapters.

The Asus router most likely has some dc-dc converters inside which convert that 12v into some voltages used inside by the chips, like 2.5v or 3.3v
So the router should be relatively flexible regarding the input voltage, but there's some things you would normally have to be careful about.

If the router was really designed for 12v input voltage, then inside there's probably capacitors rated for maximum 16v. Also, some dc-dc converter chips can only support up 15v DC or some low-ish value.

So using a 14v power adapter may work fine, as long as you're sure that output voltage will never exceed 14v.
A lot of power adapters aren't very regulated, they output a slightly higher voltage when the device connected to them doesn't use a lot of power, and then the voltage goes down as the device connected uses more and more current.

Basically, as a fictional example - right when you plug the adapter into the router, as the router uses little current then, the power adapter may output more than 14v... and if it outputs more than 16v and the router uses 16v capacitors, those capacitors may be "stressed" and may be "damaged".

I'd say most adapters I've seen output the voltage specified on the label only from about 40-60% of the advertised current output.

You could use a multimeter and let's say some 12v car/bike lightbulbs or computer fans to gradually measure the output voltage with increased loads (add lightbulbs or fans in parallel to the adapter output)

You could also open up the router and snap a picture of the insides or tell us what's around that input power jack, what's written on the capacitors and chips on the board.
If you don't know how to find datasheets or you don't understand what those components do, we could tell you what's going on.

[stj]

Re: Matching AC Adapter sizes

the adapter in the foto looks like an electronic one - bust it open.

if it's glued, squeezing the sides near the joint in a vice can sometimes crack the glue.

[budm]

Re: Matching AC Adapter sizes

The one in photo is REGULATED SWITCHING power supply, most linear wallwart are not regulated, if you check the output without load it will have higher output, for example, the 12VDC 1A non-regulate linear power supply will have output at about 16V but it will be at 12V with 1A load.
So check the output voltage of the sub you want to use.

[mockingbird]

Re: Matching AC Adapter sizes

Hey guys, what's the calculation for amps usage when going to a higher voltage. Basically, I'm trying to figure out how many amps his equipment would use at 14V if it used 1 amp nominally at 12V.

dcapper - make sure you open the 14V adapter to check the lone input filter cap. These adapters are often 10-20 years old and have crusty bulged generic Chinese caps. If it's a 14V adapter, it will most likely be a 10-12.5mm 25V cap.

[dcapper]

Re: Matching AC Adapter sizes

I'll crack it open tomorrow and see what I can find. I appreciate the ideas to try.

[momaka]

Re: Matching AC Adapter sizes

Hey guys, what's the calculation for amps usage when going to a higher voltage. Basically, I'm trying to figure out how many amps his equipment would use at 14V if it used 1 amp nominally at 12V.

Simple.
You just equate the power. Power is given by P = V*I
So with 1A @ 12V, P = 12V*1A = 12 Watts
Now 12 Watts = 14V*#A, where #A is the number of Amps @ 14V
And you find that the current is 12 Watts / 14V = 0.857 A approx.

[stj]

Re: Matching AC Adapter sizes

except it's not because in the real world the psu will have losses that increase with increase between input and output voltage.

it's more likely it will still use 1A and simply generate more heat dropping the voltage.

[momaka]

Re: Matching AC Adapter sizes

Depends.
If the step-down in the router is done with a buck regulator, the loses will be nearly the same as before. I actually don't recall any router not use a buck regulator right up-front on the input - usually to produce 3.3V or 5V. After that, linear regs generate 2.5V, 1.8V, or whatever. Some of the older LinkSys WRT54Gs have two or even three buck regulators.

[dcapper]

Re: Matching AC Adapter sizes

Sorry I took so long to reply. Just as I was about to take apart the power supply, I noticed a nick in the wire. Repaired and all good. But I appreciate the info and calculations. I know what I can work with in the future now.