Components

[pgorelik]

Hello,

I am building an Open Spritzer for someone in my department, and I have a few minor suggestions to improve the PCB:

I think in your design D3 should be a Schottky diode, not Zener. Many of the top layer traces are very tiny (10 mil?) increasing their width would reduce the likelihood of manufacturing "oopsies" *It's good practice to include a decoupling capacitor (.1 uF) physically close to the VCC pin of the Mcu

Best, Pavel

[amchagas]

Hi Pavel,

thanks for reaching out and for your suggestions!

Although we can certainly incorporate these changes to the design, we do not have anyone actively working on it at the moment, so these updates could take a bit of time...

At the same time, if you are already building one of these and could/would test the suggested changes and then add them as a pull request, we would be more than happy to integrate them!

[AnxogaelM]

Hello,

I am building an Open Spritzer for someone in my department, and I have a few minor suggestions to improve the PCB:

I think in your design D3 should be a Schottky diode, not Zener. Many of the top layer traces are very tiny (10 mil?) increasing their width would reduce the likelihood of manufacturing "oopsies" *It's good practice to include a decoupling capacitor (.1 uF) physically close to the VCC pin of the Mcu

Best, Pavel

Curious to know whether you tried the Shottky alternative, Pavel? And if so, which Shottky did you use?

[AnxogaelM]

A couple of bits I noticed when looking at the PCB annotation - the NPN pin allocation seems to be switched around the wrong way. Also, the voltage step-down regulator recommended on the BOM (and shown in the pictures by the looks of it) is the through-hole component with 5V output (https://uk.rs-online.com/web/p/buck-converters/5333743/), but the annotation lists the surface-mounted 12V component (https://uk.rs-online.com/web/p/buck-converters/1977226/).

https://uk.rs-online.com/web/p/buck-converters/1977226

[AnxogaelM]

Curious also about potential alternatives for the D4 (1n5824) with very long lead times by the looks if it. In case anyone has any suggestions...

Thanks

Anxo

[pgorelik]

Hi Anxo,

I think any Schottky diode with at least 5A current rating and ~40V voltage rating should work. Make sure you pick one that's physically the right size to fit on the PCB.

Here's a search query on DigiKey with the above parameters

And here's the datasheet for the DC/DC Buck regulator that describes what kind of diode to use: https://rocelec.widen.net/view/pdf/jrhzyqh0e0/NATLS12926-1.pdf?t.download=true&u=5oefqw

Pavel

[pgorelik]

"the NPN pin allocation seems to be switched around the wrong way."

Thanks for pointing this out! The labels on the PCB indeed appear to be wrong! Collector and Emitter are swapped on the silkscreen.

-Pavel

A couple of bits I noticed when looking at the PCB annotation - the NPN pin allocation seems to be switched around the wrong way. Also, the voltage step-down regulator recommended on the BOM (and shown in the pictures by the looks of it) is the through-hole component with 5V output (https://uk.rs-online.com/web/p/buck-converters/5333743/), but the annotation lists the surface-mounted 12V component (https://uk.rs-online.com/web/p/buck-converters/1977226/).

https://uk.rs-online.com/web/p/buck-converters/1977226

[AnxogaelM]

Hi Anxo,

I think any Schottky diode with at least 5A current rating and ~40V voltage rating should work. Make sure you pick one that's physically the right size to fit on the PCB.

Here's a search query on DigiKey with the above parameters

And here's the datasheet for the DC/DC Buck regulator that describes what kind of diode to use: https://rocelec.widen.net/view/pdf/jrhzyqh0e0/NATLS12926-1.pdf?t.download=true&u=5oefqw

Pavel

Thanks Pavel,

I reckon the SB540 from Vishay should do the job as a Shottky in the D3 position...https://www.vishay.com/docs/88721/sb520.pdf - but would it not be important to have the voltage around 24V as well, like the Zener on the BOM?

I'm using the same Buck converter as you. Seems to work okay, although I only get 3.9V out of the 5V pin on the Arduino

Anxo

[pgorelik]

Hi Anxo,

Yes, SB540 should work just fine. The purpose of this diode is to absorb the inductive voltage spike that occurs when the solenoid switches off. Normally this diode would see the power supply voltage, so its rating should exceed the maximum that it'll ever see in the circuit. So if we power the circuit with a 24V supply, 40V, 50V, or 100V diodes are all fine.

Schottkys and Zeners are very different diodes. A Zener is designed to operate in breakdown regime. This means that when you apply a reverse voltage across it, it starts conducting at a very specific voltage -- this is useful for making a voltage reference. But in this application a Zener is not what we want. A Schottky diode, on the other hand, is a normal diode that's very fast and has low capacitance. So it can turn on very quickly, which is what we need in order to dissipate the voltage spike that will occur across the solenoid when it's switched off.

Which version of the Buck converter are you using? If you installed the one with a 5V output, then the Arduino will not have enough voltage headroom to produce 5V. In fact, if you power the arduino with 5V on its Vin pin, it might not be stable because it requires 7-12 V on Vin. I think you should be using a 12V Buck converter and the arduino will regulate that down to 5V.

Take care, Pavel

[AnxogaelM]

Hi Anxo,

Yes, SB540 should work just fine. The purpose of this diode is to absorb the inductive voltage spike that occurs when the solenoid switches off. Normally this diode would see the power supply voltage, so its rating should exceed the maximum that it'll ever see in the circuit. So if we power the circuit with a 24V supply, 40V, 50V, or 100V diodes are all fine.

Schottkys and Zeners are very different diodes. A Zener is designed to operate in breakdown regime. This means that when you apply a reverse voltage across it, it starts conducting at a very specific voltage -- this is useful for making a voltage reference. But in this application a Zener is not what we want. A Schottky diode, on the other hand, is a normal diode that's very fast and has low capacitance. So it can turn on very quickly, which is what we need in order to dissipate the voltage spike that will occur across the solenoid when it's switched off.

Which version of the Buck converter are you using? If you installed the one with a 5V output, then the Arduino will not have enough voltage headroom to produce 5V. In fact, if you power the arduino with 5V on its Vin pin, it might not be stable because it requires 7-12 V on Vin. I think you should be using a 12V Buck converter and the arduino will regulate that down to 5V.

Take care, Pavel

Great! Thanks for the explanation. Much appreciated. Will give the higher voltage Buck converter a go as well. Currently have a 5V out on there.