With modern wall warts (switchmode converters), the power switch is on the DC side at the equipment, not the AC side. How safe are these things when left plugged in indefinitely?
kk6gm
Switchmode wall wart safety?
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Kartman
One would hope that if they die, they die passively - ie not go up like a Kiss concert. The general intention of the various certification agencies is to ensure that these things don't burn the house down.
gchapman
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kk6gm wrote:
IIRC for isolated AC-to-DC, not with most products from On Semi and Power Integrations.With modern wall warts (switchmode converters), the power switch is on the DC side at the equipment, not the AC side.
kk6gm wrote:
If its label has a logo from a safety standards testing organization then it should be safe for that. But safe for what it's supplying is another question. Some "expensive" boards (like the TI-sponsored Beagle series) have an over-voltage IC to cut power when the wall wart fails or becomes out-of-spec.
How safe are these things when left plugged in indefinitely?
gchapman
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Kartman wrote:
Most semiconductor failures are shorts so depending on what shorts the load could get cooked.
One would hope that if they die, they die passively ...
Kartman
There are normally internal fuses that will hopefully pop before flames erupt.
Brian Fairchild
Doesn't exactly fill one with confidence though...
"general intention...it should be safe...There are normally internal fuses...hopefully pop before flames erupt"
...the reality is that I'm not personally aware of any dangerous failures of a wall-wart.
If I have the choice I always buy one of the name brands though.
DocJC
The other issue to be aware of is that the small switching wall warts generally are not "isolated", and the output is not floating with respect to the Mains supply.
You probably know that, and it might not be an issue, but if you grew up with the older transformer style wall warts then it is worth noting.
Nard, (Plons), has a thread on wall wart isolation.
JC
LDEVRIES
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Quote:
With modern wall warts (switchmode converters), the power switch is on the DC side at the equipment, not the AC side. How safe are these things when left plugged in indefinitely?
I don't think I have ever seen a transformer wall wart with a switch in the primary! I have seen cooked transformers!
I personally have not come across switch mode wall warts that have gone seriously awry. I have never had a wall wart go high voltage on me( and potentially causing damage), but lots that go low voltage (Ie. 0 volts). Cracking the latter apart in a vice & replacing the fast recovery diode usually gets them going again. IMHO I reckon they are probably safer than transformer based wall warts
Quote:
The other issue to be aware of is that the small switching wall warts generally are not "isolated", and the output is not floating with respect to the Mains supply.
Can you explain that a little further DocJC. I have personally not come across any switch mode wall warts that are not isolated from the mains (???), but I am willing to learn!
Brian Fairchild
LDEVRIES wrote:
Can you explain that a little further DocJC. I have personally not come across any switch mode wall warts that are not isolated from the mains (???), but I am willing to learn!
To meet EMC/EMI rules some, but not all, SMPSU wall warts have a capacitor between the 0v LV output rail and a rail somewhere on the HV side. Sometimes this point is the N input, sometimes it's somewhere in the HV DC section. The idea being that any RF on the output side has a low impedance path back to the mains section and then via the input filtering back to the low impedance mains earth.
This capacitor needs to be a safety rated part.
DocJC
Hi Lee,
I think Brian explained it sooner, better, and more concisely than I could!
This Thread discussed wall warts and isolation.
JC
LDEVRIES
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Sure, I know where you are coming from now.
Thanks.