boylesg
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Sounds surprizing. Up to now, I thought that you either need an independent voltage source or an active switching circuit to "boost" a voltage. A schottky diode on it's own hardly can.I happened to find a forum post about boosting the voltage slightly and a suggestion to use a schottky diode to acheive this.
Sounds surprizing. Up to now, I thought that you either need an independent voltage source or an active switching circuit to "boost" a voltage. A schottky diode on it's own hardly can.
I actually tried it with a schottky diode that I salvaged from a tv circuit board and it seems to work quite well as long as I bias it correctly on the -12V line. It takes the output voltage to about 11.88V
I am used to the idea that any load reduces the voltage rather than increase it and find it hard to get my head around how a schottky increases it.
If this diode is used as a rectifier, resp. in series with the output voltage, the lower forward voltage drop of a Schottky diode (compared to a standard Si junction diode) is responsible for the larger output voltage.
If the diode is in series somewhere between the input and the output:
output-voltage = input-voltage - (Vf = forward voltage drop of the series diode)
... the lower voltage drop of a Schottky diode allows for a higher output-voltage.
Silicon junction diode: Vf ≈ 0.7V
Silicon Schottky diode: Vf ≈ 0.4V
The Schottky Diode has lower voltage drop, resulting on higher voltage output if the diode is in series.
You didn' tell about the measurement condition. The descriptions sounds like you have connected a large capacitor but about no load current. In this case, the capacitor will store possible overshoot during switch on and also charge up to the peak value if a ripple voltage exists.If I measure at the diode the voltage seems to shoot up to about 13.6V and then slowly settles back to 11.8V
You didn' tell about the measurement condition. The descriptions sounds like you have connected a large capacitor but about no load current. In this case, the capacitor will store possible overshoot during switch on and also charge up to the peak value if a ripple voltage exists.
If you apply reasonable load current, you'll hardly observe a voltage increment. Or you invented a new kind of perpetual motion :smile:
You're thinking of tunnel diodes not Schottky diodes.
I am sure that I read that Schottky diodes can also have negative resistance under some circumstances.
Try to find that reference in your browser history, I am curious to see it.
12-14-2003, 07:09 PM #7
Jhhnn
Lifer
Join Date: Nov 1999
Location: Denver Co
Posts: 16,049 RE: My -12V voltage rail on my power supply reads -13.01V?
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Don't worry about the -5v, either, it also does nothing. The newest atx specs at www.formfactor.org allow for elimination of the -5v, the -12v will be the next to go...
The reason that they'll often read strangely is that the psu has a very difficult time regulating a near zero current draw...
Jhhnn
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12-14-2003, 07:32 PM #8
Soulkeeper
Diamond Member
Join Date: Nov 2001
Posts: 5,300 RE:My -12V voltage rail on my power supply reads -13.01V?
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the negative voltage readings are hardly ever in spec
probably because they have no or little load on them
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