5V cell charger transformer for step up from 12V?

[Earthscum]

I swear I saw a thread on this awhile ago, but couldn't find it... anyways, I'm wondering if I could use a 5V, 1A cell phone charger as a step up from 12V safely, like the Real McTube power supply? I'm not sure if it's safe to run the 12V through the secondary side, since it's only rated for 5V output. If it's safe, it will give me more than the 100V I have now.

It's a Kyocera charger. It's just a transformer, doesn't even have rectification if I remember (it's buried right now... tried to find it, and I know it's there, but it's hiding).

I'm sure SOMEONE has checked into this... too many DIY geeks with old chargers for long gone gadgets   

[R.G.]

I'm not sure if it's safe to run the 12V through the secondary side, since it's only rated for 5V output. If it's safe, it will give me more than the 100V I have now.
It's a Kyocera charger. It's just a transformer, doesn't even have rectification if I remember

Unfortunately, there isn't any good way to tell whether the transformer puts out 5V (probably not, probably more) or whether running 12VAc into it will damage the transformer, or whether that would be dangerous to people and not just the transformer, at least not without some electronic testing.

Important things to look at:
1. is it a line-frequency transformer only, meaning AC power line in, lower voltage AC out, no rectification, no filters, and no regulators or switching power supply? Any deviation from pure AC line in, low voltage AC out immediately disqualifies it.
2. If 1 is true, then what is the open circuit voltage? It's theoretically non-damaging to a transformer to put the same voltage on its secondary that would be there at no load with the primary driven from the nominal AC line voltage. That is, if you put in 120Vac 60Hz, and it puts out 13Vac on the secondary, then it will not damage the transformer to drive it with up to 13Vac on the secondary. More than that **might** be OK, depending on a whole lot of details about what the  designer thought, what compromises he/she made in the design, what they had for lunch, etc. You'd have to test it for saturation at any higher voltage. And for really small transformers, saturation may not be tragic because winding resistances *might* limit saturation current. Might.
3. Given that you decide you can drive it with whatever voltage you get out from whatever you're driving the secondary with (?!?), power it up, see what voltage you get out after the transformation/rectification/filtering and whether you like that. Small transformers are notorious for having bad voltage losses.
4. If you like the voltages, run it on a test bench under full load for a long time, checking to be sure it does not overheat under actual use conditions and doesn't suddenly quit because the thermal cutout that's possibly buried in it opens. And that it does not somehow go up in smoke and flames. Make sure the bench is non-burnable, that you have fused the incoming power supply, that no pets, children, family heirlooms, etc. can get destroyed if it goes horribly wrong. Make sure your test setup does not electrocute YOU accidentally, and that your AC power wiring is safe even for the limited time of the test. Bad test setups are one way to get dead too.