About WallWarts and Bricks...

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Joined: Thu Jul 27, 2017 12:55 pm
Location: Canada

About WallWarts and Bricks...

Post by johng »

There was a recent post about problems with a 9V adapter ("Let the smoke out", Dec24/18) for the DSO Shell. I was going to reply there but thought this might serve better as a generalized post about using plug-in power supplies (as opposed to batteries).

First: *always* measure the output voltage of a new/unknown power supply before you connect it to the DSO Shell. As noted below, some actual supply voltages are not what they're rated to be.

I'm in Canada, so am speaking from North American experience. For other countries, there may be differences.

a. WallWart
Think of a wallwart like a wart on the hand -- the wart is a "bump" that sticks out above the skin's surface. The wallwart is a bump that sticks out from the wall's surface. The shape may be close to a cube, or somewhat oblong. It plugs directly into the wall and has a low-voltage DC lead coming out of it, usually terminating in a plug of some kind.

b. Brick
Named for it's shape -- an oblong flat rectangle, like a building brick. A brick has 2 power cords -- one for AC-in, the other for DC-out, each ending in a plug of some kind.

a. WallWarts...

a.1 Unregulated
The original wallwart (it's been around for many years) has a transformer, along with rectifier and filter capacitor built in. It's generally cube-shaped and, because of the transformer, feels heavy for its size.
The rectifier is a single diode, and there is no regulator. So, the output is half-wave, with lots of ripple. The actual output voltage will be considerably higher than the nominal rating. Some examples (based on 120VAC source, and no load on the output)...
- 9V measures 15.9V actual
- 12V measures 19.3 actual
- 15V measures 21.6 actual
Google will find you websites that explain half-wave and full-wave rectification, and the math behind the actual output values.

This is an unregulated unit:
a.2 Regulated
This is a "newer" variety. It may be somewhat oblong in shape, and will be relatively light-weight. These have a switching supply built in (no transformer, thus the light weight), so put out a full-wave, regulated voltage. Because the output is regulated, what you see is what you get, ie: A nominal 9V rated wallwart will actually measure 9V out.

This is a regulated unit:

b. Bricks...
I haven't opened any of these, but I have seen "heavy" and "light" units, which leads me to think the heavier ones have a transformer (not sure what else), and the lighter ones use a switching supply. The few brick outputs I've measured have been 12V units, and they've all measured just over 12V (12.3 - 12.6) out. This suggests they're all regulated.

This is a Brick:
Brick - heavy.jpg

Be aware that some power supplies can have an output voltage that's different from the rated/nominal value. If in doubt about a power supply, *test* the unit before using.

Hopefully this info will help save someone grief from unnecessary burnout.
Posts: 1
Joined: Wed Jun 10, 2020 9:31 pm

Re: About WallWarts and Bricks...

Post by bondsan »

Please pin this topic for new builders;

I used an unregulated power supply that exceeded 10V DC.
When checking (the 'Wall wart') directly with a multi-meter it measured roughly 13V.
( I incorrectly assumed the loading of the device would pull the voltage down. )

When checking the PCB test point voltages ( using a regulated power supply ) against the manuals nominal test point voltages,
it pointed to U5 ( Bad V- (7660) ).

U5 - ICL7660 - is a positive to negative voltage convertor with a maximum input voltage of 10V

I replaced this with a;

Maxim ICL7662EBA+, that has a maximum input voltage of 20V ( £5.06 ( inc. VAT ) from RS Components )

* The input voltage directly connects to this IC.
Perhaps a Zener diode ( with a series resistor ) might provide extra input over-voltage protection?

I also purchased a 9V regulated power supply;

Output connector type: P1J , 5.5 x 2.1mm , centre positive

Mean Well 9V DC Power Supply, 1.66A
Manufacturer part number = GS15A-2P1J
( £9.89 ( inc. VAT ) from RS components )

U5 full part number = Intersil ICL7660AIBZ
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