Schottky barrier Diode. help.

[ivanmarch]

Hi!
I need some help with a diode.
I would like to substitute one BAT74 schottky barrier diode.
This uses SMD technology.
This is a double diode so I would like to use 2 "normal" (no smd) diodes...

here the datasheet:

http://pdf1.alldatasheet.net/datasheet-pdf/view/16004/PHILIPS/BAT74.html

As you can see  (Vr) is 30V.  (If) is 200mA.

What I can use?

This is for an octave pedal.

Thanks.

ivan.

[Hides-His-Eyes]

Practically anything. I'd use 5817.

Germanium diodes have a similar V_r and might sound different.

[ivanmarch]

Thanks Hides-His-Eyes !

I'll get a couple of 2N5817 and I'll try with 2 germanium too.

Then I'll post back the result.

Thanks again!
Have a good weekend!
ivan.

[Earthscum]

If you want a smaller package, check out the 1N60P's that Tayda carries. They are glass case like 1N914's. These are what I've been using for small signal stuff, and comparing them with the 5817's, they sound the same (or highly negligible difference).

[km-r]

if you can provide us with a schematic it would help a lot. i think the schottky are used to "rectify" the signal from a transformer typical in most analog octave circuits.
schottkies have a higher forward voltage drop of about 0.45v IIRC.

any schottky MAY work, just compare their data sheets. you would want a Vr and If greater than that.

[Earthscum]

schottkies have a higher forward voltage drop of about 0.45v IIRC.

The kewlest thing about schottky's are that the Vf goes down as impedance goes up. Just last night I tried this out with a 4-string of schottky's, a 10k to ground, and a 1M pot. At 1M+10k, the Vf total was less than 80mV. That's down to 20mV Vf per diode. At 10k, they were all sitting around 180mV-200mV.

[km-r]

the impedance or the forward current? thats weird, right? 

[Earthscum]

A Schottky's Vf drops as anode current drops. This is why you can use them as passive overdrives. As a matter of fact, I have a 5818 installed in my bass with a switch. I switch it on, and I get the smoothest overdrive I can find. It's just installed anode to ground, cathode to signal out. The higher the impedance of the effect I'm driving, the more it grinds. When I drive my amp directly, it gets really smooth sounding. Works wonders with a Wah, and I love kicking it on so I don't have to turn down the sensitivity on my Nurse Quacky for certain parts, since it "mutes" down the envelope (cuts off part of the driving half of the signal).

If you use a 42TM019 transformer and drive the primary, and use schottky's on the recti side, you get GREAT response. Much better than with a 1k:8R like in Gus Smalley's design. I actually have just swapped it in a couple times, but found that ya gotta rebias transistor drivers. The 386 designs worked great just swapping transformers. And, all of them had better response with Schottky's, but the 42TM019 really made the difference noticeable due to the impedance. I have also tried increasing resistance in a phase-split rectifier circuit and got better response from schottky's. They REALLY get sensitive when you use them in high impedance CMOS circuits.

[ivanmarch]

if you can provide us with a schematic it would help a lot. 

Hi!!

I don't have the schematic yet...
I'm reverse engineering a commercial analog octaver.....
There is no such schematic on-line, no layout or photos...

and the problem is that is SMD......!!!!!!!!!


bloody H***!

Such a headache!

Didn't think was so difficult.....

I didn't finish yet, but I will follows your advice and try 5817 and germanium..


thanks to everyone!


ivan.

[Steben]

schottkies have a higher forward voltage drop of about 0.45v IIRC.

The kewlest thing about schottky's are that the Vf goes down as impedance goes up. Just last night I tried this out with a 4-string of schottky's, a 10k to ground, and a 1M pot. At 1M+10k, the Vf total was less than 80mV. That's down to 20mV Vf per diode. At 10k, they were all sitting around 180mV-200mV.

that's exactly what the forward drop curves stand for ...

[Steben]

Schottkies are great in tueb screamer circuits, they are a bit sharper than germs, but softer than silicons.