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AC Adapter Question

eazy.erik

Jul 29, 2012
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Hello,

I am fairly inexperienced when it comes to electronics, so I have some questions.

First, I will tell you my problem. I am from the US and on a recent trip to Europe, I burned out the AC adapters for my baby monitors. I assumed (wrongly) since it was a power pack and not a direct plug, that it was able to handle 100 - 240 V, and it wasn't. No major damage but the adapters are inoperable. The output voltage for these adapters is 7.5V. The two adapters are identical, and one plugs into the base unit which listens and transmits to the reciever, and is constantly plugged in (with AA battery back-up). The other adapter plugs into a charging stand for the reciever, which has rechargeable AA batteries inside.

Upon returning to the states, I started looking for solutions. A universal adapter will no doubt solve the problem but needing two it is a fairly expensive fix. Spending 60 - 100% of the cost of the montior ($50, IIRC) is out of the question, if I'm spending that much I will just by a new baby monitor.

Looking through my box of old electrical junk, I found some wireless phone adapters. The plugs fit the devices, but it is 9V instead of 7.5V. However, it does work. The only percieved difference is I can hear a little static from the base unit when it transmits, and when the reciever is on the charger, the light on the reciever stays on constantly when before it just came on for approximately 30 seconds and then went out.

My questions are as follows:

1. What is the long term effect of using a 9V adapter for something that should get 7.5V? After about a week there doesn't seem to be any problems.

2. Any suggestions to buy cheap AC Adapters, if needed? I looked around on the web but couldn't find anything cheap. Currently I'm looking at discount stores while we are there for cheap electronic devices that might have the same plug and voltage required, but in the meantime I plan on using the 9V adapters.

Thanks in advance for your help!

Admins: this seemed like the best place to post my question, however, please move this to the correct spot if needed..

Erik
 

Harald Kapp

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The long term effect of using 9 V instead of 7.5 V is unpredictable. It depends on the construction of the baby monitor's electronics. It may work or fail...
Chances are that the 9 V adapter outputs a bit less than 9 V. You could reduce the output voltage further by adding 2 diodes in series between the output of the AC adapter and the plug to the baby monitor. Use 1N4001 or a similar diode.

Cheap adapters are on Ebay (e.g. http://www.ebay.com/itm/AC-100V-240...=US_Server_Power_Supplies&hash=item3378716fdb).
 

KJ6EAD

Aug 13, 2011
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There is some risk of overcharging the batteries, depending on the design of the charge circuit and your transmit range may be extended a little :D. I'd be looking in thrift stores since I get small power adapters for $1-2 each that way. Just watch out for excessive heating of the batteries.

Harald beat me to it but I was going to add that if the 9V units can be opened the two diodes could be installed inside the case.
 
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eazy.erik

Jul 29, 2012
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Harold, KJ,

Thanks for the advice! As for installing diodes, that might be a little outside of my comfort zone, are you talking about opening the adapter itself or something in the monitor? Sorry but not an electrician/electronics type, I work on aircraft for a living, but not the electronics.

It seems like I will just try to find a cheap replacement and let the monitors function until they break, if they do. I can always buy new ones. Thanks, again.
 

eazy.erik

Jul 29, 2012
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CD. This site looks great! I don't know how to tell if the plugs will fit my devices, though. Is there an easy way to figure it out? Also, the original, burnt-out adapter is 7.5V 150mA, and the two 7.5V options on this site are 30mA and 300mA. If I buy from this site, which one should I buy. I never really understude the whole Volts, Amps, and Watts thing that well.
Sorry for my ignorance, and I thank you for your help!
 

(*steve*)

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Voltage (Volts) should be as close as possible, current (Amps) should be the same or greater than the original part.

Only the required current will be supplied, any excess is in reserve and means the power supply is not working flat out (and should last longer).

If you're planning on traveling overseas then get devices rated for 90V to 220V (or higher) 50/60Hz.
 

CDRIVE

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I don't know how to tell if the plugs will fit my devices, though.


As Steve stated the current rating should meet or exceed your original model. This model exceeds your current demands, so it should be fine.
http://www.mpja.com/75VDC-300mA-Unregulated-Plug-Supply/productinfo/17558+PD/

The plug type is 5.5mm OD / 2.5mm ID, with the female center (2.5mm) dia being (+) positive. Measure the OD (male section) of your plug. If it matches 5.5mm then the center female should follow.

One note here though.. Center positive is fairly standard but not cast in stone. Check your power pack. Many have an image on them indicating the polarity of the plug.

Btw, 5.5mm is very close to the dia of a .22 Cal bullet. Got any laying around?
 

eazy.erik

Jul 29, 2012
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One note here though.. Center positive is fairly standard but not cast in stone. Check your power pack. Many have an image on them indicating the polarity of the plug.The drawing for polarity looks something like this (again, electric illeterate): (-)-C●-(+) (where the dot is inside the C)

Btw, 5.5mm is very close to the dia of a .22 Cal bullet. Got any laying around? No but I'm familiar enough with one, this helps
 

CDRIVE

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The drawing for polarity looks something like this (again, electric illeterate): (-)-C●-(+) (where the dot is inside the C)[/B]

You lost me with that. :confused:
 

(*steve*)

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I think he meant this:

220px-Centre-positive.svg.png
 

(*steve*)

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Indeed, I couldn't find an image, though... I've read that whatever side the "C" opens on is the polarity, meaning this one is positive, is this correct?

what it means in this case is that negative is connected to the outside, and positive to the inside (or centre).
 
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