Advice on schematic design for dummies

Community PCB Design
1

Hi everyone, first of all thanks for helping, I’ve been reading and learning with this forum for a while, but this is the first time I ask on it (and I hope soon I can help people too).

I’m a software developer and hobbyist electronics, currently I’m working on a project that involves arduino and raspberry pi, it is just another greenhouse controller.

I’m trying to design a simple board that allows me to power the system with just one power entry point (12v wall adapter) the board is supposed offer 12v, 9v and 5v. I’ve designed a schematcis with two regulators and some extra components to do the work, I have been reading forums, googleing and reading datasheets for a while and I end up with the schematics below, but because of my lack in electronics knowledge I have some doubts I would like to share with you and try to fix :slight_smile:

So… there we go:

I have some capacitors for filtering on the regulators, I extract that from the regulators datasheets, but I’m not sure if the caps fit well on the design, on the datasheets the “schematics” is just for one regulator and I have two so:

Is the placement and values of C1, C2, C3 and C4 right?

Should I place instead of C1 and C3 some filter at the input and remove those caps?

I’m using tantalum capacitor for C1,C2 and C3 because the datasheet says so, but I have been reading about tantalum vs electrolytic (aluminium foil) and the information I got is not very clear, some sites says it is better tantalum, some say not and I don’t really understand the reasons.

What is better to use, tantalum or aluminium foil?

Let’s talk about diodes now :slight_smile:

I placed tree diodes on the circuit, D1 is a 3A protection diode, is it really necessary and well placed?

D2 and D3 are protection diodes too, I’ve placed them because I read in some websites it is good to protect the regulator, but again I have not very clear its function and i’m not sure if they are necesary.

I know the circuit is very basic and maybe my questions are stupid, but it is important to ask the doubts for learning (sometimes internet gives you too much information and it is overwhelming and impossible to make it clear…)

Just to clarify SW1 is a socket to connect the main power switch and ISENSE is a socket to connect a current sensing module.

And my last question, what do you think about the circuit?

Would you add or remove any component? Is it fine?

Sorry if the post is too long or not very clear and thank you very much for any help or advice you can give me.

Best regards :slight_smile:

[Old schematics V1

Schemtics V2

[](ImageShack - sch.ffaa.v2.jpg)](ImageShack - sch.ffaa.v2.jpg)

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Good enough…-ish. It’ll work, no doubt about that.

If it was me…

I’d get rid of that LM317 and swap it for an 7809 or equivalent.

If you plan on pulling 1.25a off the 5v side, you’re going to need a fairly huge heatsink on that LT1086.

12v - 5v = 7v dropped at the regulator. 7v x 1.25a = 10 watts of power dumped off as heat. Without a fan (and/or huge heatsink), pulling that kind of load thru that regulator will likely cause it to overheat and shut down in short order.

More cap’s of various sizes on the input after the diode but before the fuse, maybe a 10uf, 1uf, and .1uf, all in parallel.

If you’re using a typical cheapo wall wart, the output of the thing is going to be ugly.

And you’ll likely have to vary the resistor values gonig to the LEDs themselves. Each LED is getting about 5mA. Might be good enough for your LEDs, might not. You can calculate that all day to figure out exactly what value resistor you need.

In the end, for LED current limit resistors, I’ve always started high and worked down in resistor value until the LED is bright enough for me to see…unless of course I’ve got a good reason to calculate it ahead of time.

3

Thank you for your answer skimask :slight_smile:

My first schematic/prototype was using an LM7809, but reading over internet the people and papers discuss the benefits of using the LM317, like the better noise rejection. Because of that I change the regulator.

Why would you get ride of the LM317 and use a LM7809?

I’ve followed the datasheet for both regulator to calculate the heatsink needs, the LT1086 is supposed (over the paper) to work fine with 1.25A and a 5.5ºC/W heatsink, if the ambient temperature is not higher than 60ºC.

Regarding the caps, if I add the chain of parallel caps you say at the input, should I remove C1 and C3? Is better to use tantalum or aluminum foil electrolytic caps?

The current limiting resistors for the leds has been calculated over the paper and after tried on a breadboard, with this values the leds bright the same for the different voltage sources.

4

I’ll let the others comment on the circuit - I’ll comment on the schematic so that you can only get better :slight_smile:

Its not good practice to have 4 connections into a junction point- when printed out a fold can make the junction point become harder to see and it may look like the connection just passes over - best to stagger them.

C4 is missing the junction point above it.

Try to avoid dot notation - I.E. 100n is better than 0.1uf as the dot can become faded. Ditto the 1.25a .

R1/R2 - LED2/3 +5v/+9v etc are reading bottom to top where everything else is left to right. You have the room - make it easy for a reader to read.

Best of luck with the pi.

5

Mattylad thank you for the comments about the schematics :slight_smile:

I don’t see what points do you mean when you say:

“have 4 connections into a junction point”

Could you please provide me with an example of where i am doing that?

I’ve already correct the rest of the things you point out, it is great to have best practices feedback.

I’ve been doing some tests with the LT1086 and skimask was right, even with the calculations done the temperature rise of the regulator is too high.

With an ambient temperature of 26ºC and a 5.5ºC/W heatskin I’m having the following temperature readings (with an NTC at the back of the regulator)

1A → 98ºC

500mA → 70ºC

I guess that this is too hot and with a Ta of only 26ºC :frowning: not very good…

So keeping that in mind a start searching again for a new design :slight_smile: I found some DC/DC modules, like this http://www.ebay.com/itm/1pcs-LM2596S-Po … 0883602068.

The DC/DC is rated 2A so it will be enough to power the PI and the Arduino.

What do you think about connecting the 12v wall wart to the input of the DC/DC converter and output 7v from the DC/DC, this 7v will be the input of the LT1086 to power the PI and will serve for the arduino (using the integrated LDO) as well?

My only concern (so far) is about the noise of the DC/DC, Could it be a problem for the system?

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A 4-way junction is

   |
   |
---*---
   |
   |

It is better to draw it as

   |
   |
---*--*-
      |
      |

That way, it’s clear that anywhere two lines just cross, there’s no connection. Otherwise a small tear or a little schmutz on the paper can change a connection to none or worse add a connection where there is none.

/mike

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Thank you Mike for the clarification, I’ve updated the schematics with all the comments so far :slight_smile:

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Fusein should be before the D1, in case the capacitors break. I would suggest to put TVS diode, and a ferrite on the input. The other two fuses seems to be pointless to me. If one power supply goes bad, it doesn’t matter where the fuse broke. You could consider to put extra protection for other errors:

http://nandblog.com/power-input-protection/

If I were you, I wouldn’t use tantalum also.You can buy ceramics in the same size, and they don’t make fire for you. Make sure, that the regulators like the low ESR in that case.

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Thanks for your reply NANDBlog :slight_smile:

I’ve readed your post abuout input protection it was very usefull, i will implement some of them on my design :slight_smile:

What is so bad with tantalum? i don’t really understand :frowning: the datasheet of the LT1086 says that should use them.

10

Tantalums, especially early ones, had an annoying habit of igniting. We had one go off inside a customer’s clean room; they weren’t too happy about that, and they almost had my previous employer banned from doing business in Japan… You also need to derate them for voltage at least 50%. Now that ceramics are available in quite high values, all I tend to use is either ceramics or ceramics in parallel with aluminum electrolytics. You still need to watch the ESR on ceramics; they may be low enough to turn certain regulators into oscillators.

/mike