power supply
Can I connect in series two identical switching wall wart supplies (12 V each) that have no measurable resistance between either DC output pin and either ac pin (measure open and have no ground plug) to make a regulated power supply (±9 V) like this? If so, any hints on how to improve the circuit? I want to power a headphone amp with 300 mA and need a quiet supply.
Thanks
The only places i've seen wall wart supplies with an earth ground terminal are on supplies built for the alarm system industry and normally only on the 16.5/24V AC supplies.
As to why you don't see them. The ground terminal on mains power is intended to be used to earth ground the enclosures of devices operating at mains, or high voltage. For example a blender with a 120V motor and a metal case. This prevents the chassis from ever being at dangerous voltages due to wiring faults or shorts within the product.
DC supplies normally have a floating ground that can be at a much different potential level than earth ground. There normally isn't a use for a earth ground in such systems. You'd have to be very careful with it to not cause a ground loop and grounding the chassis to the floating ground is normally done but more for emissions issues as unsafe chassis voltages aren't a concern in low voltage DC systems.
It should not cause any extra hazards or problems that I can think of, beyond the output being 'live'. However there are potentially serious consequences to the output being live- see below.
Typically the output (-) is AC-connected to the neutral anyway, but galvanic isolation provides safety and the capacitor is extremely high reliability high-voltage Y-type capacitor because a direct connection (what you want to do) is potentially fatal in general purpose applications.
Keep in mind that the cord will not typically be rated for mains voltage and that will have to be dealt with. If it's all in a box and the cord is sleeved with approved sleeving, and the connector cut off, it may be okay.
If you're planning on powering a device connected to the mains with a wall wart actually plugged into a wall (rather than locked in a box) or with the original barrel connector and/or cord still attached no, don't do it.
Here, for example,
is a cord connected to a 9V "wall wart" that has sustained damage from a desk chair running over the cord. As you can see, the "live" wire is exposed and ready to shock someone were it not to be isolated. Same thing applies to the barrel connectors- they are not designed to be shock-proof.
Best Answer
Yes you can. No problems at all. In order to use them for low noise schematics, provide good filtering of the linear regulators. Use low inductance ceramics capacitors in parallel with the electrolytic capacitors in the input and output filters. Keep the linear and switching regulators on some distance.