Maker Pro
Maker Pro

Making a cap fed supply safe!

cjdelphi

Oct 26, 2011
1,166
Joined
Oct 26, 2011
Messages
1,166
I've thought about this for a while now...

On the low side, a tvs diode (eg if over 12v it clamps) and a fuse which should protect a user from death?

Or what about simply a very low current blow fuse on the 110/240ac .. would these methods not make a capacitor fed power supply safe?

Never built one, but i think i shall for a simple 1w led which will never be touched... question is could one be made safe?
 

chopnhack

Apr 28, 2014
1,576
Joined
Apr 28, 2014
Messages
1,576
I could argue for an isolation transformer that would have intrinsic fusing to make things safer, but it really depends on the design of the schematic. Specifically the goals needed to be achieved, i.e. - balancing cost with size, etc.

Do you have a schematic?

One of the safety issues that comes to mind with cap fed supplies is that there is no isolation from mains, so if a component fails, which they all will do over time, parts of the circuit will come to mains potential. If the cap (which would need to be rated for this type of function - X/Y) fails, mains then runs to the next component.
 

cjdelphi

Oct 26, 2011
1,166
Joined
Oct 26, 2011
Messages
1,166
The point of the tvs diode is to clamp causing the fuse to blow, i know how they work and the dangers hence the question...

Once the cap fails and 240v appears across the cap, the tvs diode will clamp blowing the fuse.... Maybe even a zener diode?

Can't supply a circuit i've not yet built lol... i think a tvs should do the job if the cap fails blowing a fuse 240v side, i'll design a circuit in a few!
 

cjdelphi

Oct 26, 2011
1,166
Joined
Oct 26, 2011
Messages
1,166
The sim does not have tvs diodes or fuses! Lol

But a fuse onto 240vac input side say 100ma or something and a tvs after the cap to ground ...

When the cap fails the diode blows taking the fuse with it... should work (and i'm aware this is not driving a 1w, simple enough)
 

Attachments

  • Screenshot_2015-11-19-14-13-06.png
    Screenshot_2015-11-19-14-13-06.png
    255.8 KB · Views: 73

Alec_t

Jul 7, 2015
3,587
Joined
Jul 7, 2015
Messages
3,587
Unfortunately, the time it takes the fuse to blow may well be longer than the time it takes the mains voltage to kill you :(.
 

Bluejets

Oct 5, 2014
6,901
Joined
Oct 5, 2014
Messages
6,901
In addition, as a guide, in my testing, the time for safety switches to trip are from 10 to 30 milliseconds. (that's from 0.5 to 1.5 cycles on 50 Hz at between 10 and 30mA leakage)
Better way to travel one would think, approved and proven, along with isolation approach.
 
Last edited:

cjdelphi

Oct 26, 2011
1,166
Joined
Oct 26, 2011
Messages
1,166
Unfortunately, the time it takes the fuse to blow may well be longer than the time it takes the mains voltage to kill you :(.


It does not matter, nobody would be touching it anyway, it would simply be on or off in an isolated box away from people... the fuse should be fine as long as it blows (the tvs diode should blow it fast...)
 
Top