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12V power supply puzzle

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I have two Maplins regulated AC to DC 12V power supply devices.

One of them produces a curious problem.

The L25AY is shown on this link

12V 5A Cigarette Socket Power Supply : 12V Car Accessory Power Supplies : Maplin

It works fine with my telescope and is also OK powering a 12V hair dryer.

The other power supply is the XM21X shown here:

13.8V Regulated Mains Power Units : Bench Power Supplies : Maplin

This works UK with the telescope, but will not make the hair dryer work. This is the puzzle. It is quite happy with another 12V device (a dashboard fan).

I have connected it with a triple socket adaptor wired direct to the posts on the regulated power supply. An electrical test meter shows the full voltage being delivered at the three sockets. When the telescope, or the dashboard fan, is connected to one of the three sockets, the other sockets still show the full voltage, whether the devices are on or off - which I would expect. When the hair dryer is connected and switched off, the other sockets have the full voltage, but when it is switched on, the power to the other sockets disappears.

I am no electrician, and cannot understand why the hair dryer works with a couple of different power tanks and the L25AY power adaptor but not with the XM21X.

Any ideas?


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Hello Allan,

Because the hair dryer is essentially a resistance heater (the thin wire inside gets hot as electricity flows through it) - it is necessary to limit the total voltage that can go through the wire to protect it from melting and possibly causing a fire.

I'm assuming that your 12V dryer has such overload protection and will not function if the incoming voltage is over 12V. Electronics such as your telescope's electronics also have sophisticated circuitry to help them regulate the power that goes to the computer chips (almost always less than 12V!), so a bit of overvoltage there is less problematical.

All this said, it is always best to stick to the manufacturer's original specifications on voltage/amperage inputs to avoid burning anything out. Warranty coverage typically will not apply to such situations, especially if you are using a 3rd party power supply, etc.


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Hi Alan,

I know this may not be the answer but have a look at this...W-10AM Watson 10A PSU.

The variable voltage output makes it particularly useful for a wide range of applications and the front panel control offers a smooth transition from 0 - 15V DC

I got one of these the other day and it solved all my regulated power issues. At a price mind you..


Edited by acme
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Thanks for the replies, Dan and Acme. I suspected some sort of voltage limitation, but since the hair dryer works fine from a jump starter socket, etc, delivering more than 12v; and according to some posts on other sites I have looked at, the 12v versus 13v should not be a problem. Still confused, butI guess I'll just use a power pack if it's dewy.


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The two different supplies use different technology...

The first one is a swich mode and the second is a linear - the linear is only rated at 50% of stated current continous use so my guess is that the unit is going into shutdown as the current requirement for the Hair drier is too high for the unit...

The linears are "designed" for use with radio transcievers where the high current is drawn intermittently during transmissions which for "CB" or "amateur" use are usually pretty short...


Edited by Psychobilly
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With those switch modes the output is floastign with repect to earth and theres a chian of caps across the transformer - this can sometimes give rise to a slight "tingle" form connected equipment... the current that flows is way to low to cause you any probs but its something to be aware of... I make sure I earth the negative terminal with mine...

IIRC I looked into this in detail a few years back...

Paste it was in a thread started by Kevin...



Edited by Psychobilly
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