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Schottky 12v rectifier

scootsity5

scootsity5

AK Subscriber
Subscriber
Hello!

I'm working on a preamp (in name only), by which I mean a bunch of relays on a switch. The power supply I have for it is a 7.5VAC (1VA) transformer. With no load, it's at something like 9.4VAC.

My relays are 12v relays, drawing something like 11mA. My regulator (a 12vdc-12vdc DC-DC switcher) is 83% efficient.

So, the draw should be in the order of 13-14mA. I'm hopeful that at that draw, the transformer should (hopefully) still be over 8.5VAC, rectified to about 11.8V.

The 12v regulator can operate from 10.8-13.2V.

All of that is to say I need diodes that don't drop a ton. Is there any reason not to use schottkys instead of rectifier diodes?
 
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gadget73 said:
try a voltage doubler config. That should easily get you what you need
Click to expand...
That's an interesting idea. The transformer also has an 18vac tap, but I planned to use that for something else. I could probably put a voltage divider on that, I guess. But the voltage multiplier is a good idea, too.
 
Is there any practical reason not to use a schottky for a full wave rectifier? Something like a 1n5818?
 
scootsity5 said:
Hello!

I'm working on a preamp (in name only), by which I mean a bunch of relays on a switch. The power supply I have for it is a 7.5VAC (1VA) transformer. With no load, it's at something like 9.4VAC.

My relays are 12v relays, drawing something like 11mA. My regulator (a 12vdc-12vdc DC-DC switcher) is 83% efficient.

So, the draw should be in the order of 13-14mA. I'm hopeful that at that draw, the transformer should (hopefully) still be over 8.5VAC, rectified to about 11.8V.

The 12v regulator can operate from 10.8-13.2V.

All of that is to say I need diodes that don't drop a ton. Is there any reason not to use schottkys instead of rectifier diodes?
Click to expand...
Schottky diodes would work fine if going from about 0.7Vdc forward drop to about 0.35Vdc or so of drop is enough to provide the additional voltage you need. For 60Hz operation, there's no other significant advantage in using Schottky diodes besides the lower V-forward.
 
egellings said:
Schottky diodes would work fine if going from about 0.7Vdc forward drop to about 0.35Vdc or so of drop is enough to provide the additional voltage you need. For 60Hz operation, there's no other significant advantage in using Schottky diodes besides the lower V-forward.
Click to expand...
That's what I'm trying to accomplish: lower voltage drop. As I'm building a full wave rectifier, it's the difference of about 1-1.5v drop (2 diodes, 2x the drop). OK, so it sounds like it should be fine with appropriately sized schottkys.

Thanks!
 
scootsity5 said:
That's what I'm trying to accomplish: lower voltage drop. As I'm building a full wave rectifier, it's the difference of about 1-1.5v drop (2 diodes, 2x the drop). OK, so it sounds like it should be fine with appropriately sized schottkys.

Thanks!
Click to expand...
Another approach would be to go with synchronous rectification. Look up that term on the electronics supplier sites such as Digikey and Mouser, as examples. Linear Technology and On-Semi make several different models of them. Instead of a diode forward drop, the drop across the on-resistance of a MOSFET is the only loss, and that can be much lower than that of the diodes.
 
If the drop is below 1v, I think it should be fine. It seems like the open voltage is about 9.5. 1VA/9.5 is .105A the worst case scenario is the circuit will draw .014A. 1VA/.119A = 8.4V. That should rectify to about 11.85. By my math, the rectifier will only drop about 0.6 or so and the DC-DC converter can regulate as low as 10.8vdc. So in theory with schottkys or a bridge rectifier, I should have about a .5V margin.

Thanks all. I'll post some pictures when I get it further along.
 
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