power supplysafety
I'm helping a friend make a 48 inch hot-wire foam cutter. He would like it to be as economical as possible, so I'm giving him two 15V 1.1A wall wart power supplies that I have sitting around.
I don't think the voltage will be enough to heat such a long wire.
Is there any problem with wiring the output of the two wall warts in series to obtain a 30V output?
I started this as a comment but the diagram was needed and you asked for critique as part of the answer
Place a ban on Olin coming within 10 miles of your diagram or he will kill you/it.
Your circuit diagram has much unnecessary non-functional mess on it that makes it much harder to read than is necessary. There are little marks and annotations everywhere that have no meaning to most readers and no place on a diagram that is meant for others to inspect for circuit design analysis purposes.
Consider this extract. Most messy stuff is marked. R25 value is very hard to read/ Large circles at component ends are very detrimental to appearance. Dot where vertical line from C3 meets grounds is OK and others should be like that or disabled for presentation purpises. It is fine for a drawing to look like this when you are working with it as long as it is not allowed to escape from captivity.
Depletion mode FETs have their place but it is very uncommon to need them and you should try hard not to do so. They are relatively old tech in most cases and have poor specs (as you found). As you note, a P Channel FET will do what you want about as well in most cases and biasing an N Channel on at all times as its default setting via a whisper of gate drive is acceptable if needed.
A boost converter may never have Vin > Vout + 1 diode drop unless you are happy for Vin to flow to Vout via the output diode. You may be.
If you rely on a processor to terminate a LiPo charge cycle, when the uC crashes the battery will too in due course.
Using a regulator's inbuilt current limit as a max chg regulator for a LiPo is OK only if allowable max I is >> Ireg max as reg max I out varies widely.
1A charge - what size cells?
More can be said.
Olin may choose to say some of it.
Flame suits on :-).
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.
Best Answer
To answer your question: if the wall warts are not grounded and are isolated from the mains then there should be no problem connecting them in series. I even connected a couple of PC power supplies in series, just made sure to connect them to ungrounded outlets.
Now, will ~30W be enough to heat the wire? Also, you should measure the resistance of the wire and find out what voltage and current it needs, because it may be that it has lower resistance so you would need to connect the wall warts in parallel to get 15V and 2A, but you shouldn't do it, since connecting two power supplies in parallel can lead to problems.