Re: 2 19VDC laptop chargers in series
 

There may be more, but the first thing that comes to mind is the electrolytic capacitors. They will fail even under small reverse voltages, and if they aren't properly vented, they can fail quite catastrophically: http://www.youtube.com/watch?v=D-RZ5RTAdSg.

Re: 2 19VDC laptop chargers in series
 

In addition to what Jason said, there is a voltage feedback to the switching controller that may not like a voltage on it's feedback input being lower than it's ground.

Mostly right. When current is pulled from 2 supplies with outputs connected by 'OR'ing (matched) diodes, the current will only be pulled from the supply with higher voltage until that supply cannot keep up. At that point its output voltage will start to droop. When it droops to the level of the second supply that supply will then take the rest of the load.

It's not the best way to parallel supplies, but it does work when 1 supply wont source enough current.

Re: 2 19VDC laptop chargers in series
 

Guys,
I would really like to get a handle on this discussion. It is OK to add power supplies in series. It is not OK to add battery chargers in series. Luckily I believe we are talking power supplies. Current flow will will be supplied by both power modules and since they are in series there is no need to add anything to smooth or share current flow. It will occur naturally. Ether, I am going to guess you did not measure low resistance from the third pin to both outputs. Likely what you measured were output protection devices like MOVs or some other device meant to keep ESD spikes to a minimum. As long as the resistance was a couple of kohms you are fine. Good practice would suggest that the three pin supply provide ground to the circuit you are powering and the two pin supply provide the higher voltage. Since the two pin supply has no continuity to primary wiring, connecting it as the higher supply should be just fine. As additional protection, a fuse in one leg of each supply might be a good idea in case something should happen with one supply, it would isolate the fault from damaging the other supply.

Re: 2 19VDC laptop chargers in series
 

I'm not comfortable with those blanket statements. We really don't know what is inside those converters.

Turning on one before the other applies a reversed polarity voltage across the slacker. That could become a problem. The classic and cheap transformer rectifier linear regulator setup has two components that possibly could have a problem. The output capacitor could be an electrolytic (as jason said) or a tantalum. A majority of linear regulators can't handle reversed polarity. As for switchers, depending on the topology the commutation cell could become an unintended reverse path and pop like the linear.

My comment about output filtering assumed a pair of identical supplies. Stacking switchers that operate at the same frequency isn't a good idea, as their control loops can become confused. The quick'n'dirty fix is to pi filter the accidental control connection away.

Re: 2 19VDC laptop chargers in series
 

Here are the two "power supplies" (laptop chargers) in question.