Flintlock Flanger: sub 1N5818 for 1N5817?

[brucer]

Hi All.

I'm looking to build a Flintlock Flanger (http://lectric-fx.com/wp-content/uploads/2014/11/FlintlockFlanger_V1.2.pdf), but couldn't find the 1N5817 diodes at Small Bear.  I asked and they wrote back "If you are using the 1N5817 for reverse-polarity protection, use the 1N5818, my SKU 2215A. Will work identically for that purpose."  So ... sorry for the newbie question  ... but: am I just using the 1N5817 diodes for reverse-polarity protection?  I'd appreciate receiving some wisdom on this point.

Thanks, Bruce.

[Scruffie]

IIRC the 5818 has a higher fV than the 17 and as they're in series connection for the charge pump may not be as suitable depending on the exact output voltage of your PS... But I can't check atm. They might work fine but are not ideal if i'm right, also assuming none of the other specs of importance are different for the 18... Hopefully someone able to pull up the datasheets can weigh in and confirm.

[TGP39]

Here is the data sheet comparison.....

http://www.diodes.com/datasheets/ds23001.pdf

Steve

[Haberdasher]

PM me your address, I'll mail you some no problem

[midwayfair]

5718 is the same diode as the 5817, but with a higher reverse breakdown voltage. It will have the same Fv. Potentially lower, actually.. They're used in the flintlock to rectify the switching in the charge pump.

In other words, yes you can use 5817, 5818, or 5819 with the same results in this design.

[Haberdasher]

thanks jon!

[brucer]

5718 is the same diode as the 5817, but with a higher reverse breakdown voltage. It will have the same Fv. Potentially lower, actually.. They're used in the flintlock to rectify the switching in the charge pump.

In other words, yes you can use 5817, 5818, or 5819 with the same results in this design.

Woohoo!  Thanks guys for the quick offers of help and the knowledgeable replies.  Very much appreciated. 

[TGP39]

5718 is the same diode as the 5817, but with a higher reverse breakdown voltage. It will have the same Fv. Potentially lower, actually.. They're used in the flintlock to rectify the switching in the charge pump.

In other words, yes you can use 5817, 5818, or 5819 with the same results in this design.

Hey Jon. The data sheet I looked at in the link above shows that the Forward voltage of these three diodes is a little different. Am I reading the data wrong?

Thanks for any clarification.

Steve.

[RobA]

5718 is the same diode as the 5817, but with a higher reverse breakdown voltage. It will have the same Fv. Potentially lower, actually.. They're used in the flintlock to rectify the switching in the charge pump.

In other words, yes you can use 5817, 5818, or 5819 with the same results in this design.

Hey Jon. The data sheet I looked at in the link above shows that the Forward voltage of these three diodes is a little different. Am I reading the data wrong?

Thanks for any clarification.

Steve.

The data sheet I have (Fairchild) shows them as different too. Looking at the plot of forward current versus forward voltage, they aren't very different at lower current draw, maybe 300mV for the 1n5817 and 350mV for the 1n5818.

I think it would probably still work, but it might be close. The LT1054 gives the output voltage (voltage doubler configuration) as V_out = 2 * V_in - (V_loss + 2 * V_diode) where V_loss is the voltage loss in the LT1054. This figure depends on the caps used and the output current, but it can range up to about 1.6V at 100mA. So, if you were right at 9V then V_out could be as low as 15.7V and that would be too low for the dropout voltage of the LM7815 (speced at 2.0V).  That would be an absolute worst case kinda thing. The output voltage from the charge pump section on my Flintlock reads high than that. But, a combination of using high ESR caps and the higher voltage diodes might possibly be enough to push you in to the danger zone on the drop out voltage for the regulator.

The difference in the forward voltages of the two diodes is pretty small, but there is some chance it could make a difference in marginal cases.   

Edited: Fixed the formula for the output voltage from the LT1054.

[midwayfair]

5718 is the same diode as the 5817, but with a higher reverse breakdown voltage. It will have the same Fv. Potentially lower, actually.. They're used in the flintlock to rectify the switching in the charge pump.

In other words, yes you can use 5817, 5818, or 5819 with the same results in this design.

Hey Jon. The data sheet I looked at in the link above shows that the Forward voltage of these three diodes is a little different. Am I reading the data wrong?

Thanks for any clarification.

Steve.

There was a discussion about them on DIYSB recently in the Thunderbird thread about subbing the 5817 for the 5819 in the Thunderbird.

The best thing to do would obviously be to measure the diodes on hand ...

[RobA]

The formula I typed above has a typo in it. It should be V_out = 2 * V_in - (V_loss + 2.0 * V_diode).

I took some measurements on my Flintlock using a variable power supply. If I set the input voltage to something like 9.1V, then the voltage from the charge pump will oscillate a little bit around 16.85 to around 17.25 and the output from the regulator is a dead stable 15.1V. But, if I drop the input voltage down to 8.9V, then the regulator can't hold the voltage steady and the oscillations come through at the test point, going down to about 14.8V or so. I've used good, low ESR power supply caps on the charge pump too, so that will lower the losses in the charge pump. The 1n5817's I have in there measure at 167mV for the forward voltage .