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Steff Shield


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#21 Magpie

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Posted 30 May 2012 - 10:02 AM

Congratulations on that!


Did you manage to the reflow at home, how did you do that?


I used and electric frying pan that I bought at a junk shop for 7 dollars.
I haven't tried this on FRP4.

Just start low at approx 150C for 90 seconds to warm things up. This apparently is an important step.
Then turn up to about 300 till everything melts properly then take out of the frying pan, becuase the frying pan takes a too long to cool. Dont stress too much about the high temperature as the chips can normally take it.
Use a eutectic solder paste, not 60/40 pb/sn as this will give a greater cooling window for bad joints. eg If it gets bumped while the joint is in the quasi state between liquid and solid it can form a bad joint. This could happen while you take it out. 60/40 is in this state for 3 degrees but 63/37 is eutectic and is only in this state for 0.1 degrees apparently. solder paste is generally in more complex combinations.

I tried making my own toaster oven (still incomplete) but the frypan works particularly well with the mcpcbs so i didn't need it.

Edited by Magpie, 30 May 2012 - 01:18 PM.

STEFF Shield High Powered Led Driver shield.

#22 hanzibal

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Posted 30 May 2012 - 10:41 AM

That's really cool - you mean like one of these?

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Do you put the PCBs directly onto the tray?

When you say "...take out the frying pan..." - what do you mean?

#23 Magpie

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Posted 30 May 2012 - 01:27 PM

Basically yes. Similar but a really old one from the 1980s I think. Very heavy but I dont think this is an advantage. It does need enough grunt to get up to 300 degrees or so. But I wouldn't cook sausages at the same time. Try first on some small sacrificial boards and chips first to get the temperatures and process right. Especially watch your double sided boards. The best way for double sided FRP might be to go a bit lower and use a heat gun from the top to push it over the edge. I think the main thing is to watch what is happening to the solder. Large boards might be difficult. Then your friends will really think of you as the mad professor.
STEFF Shield High Powered Led Driver shield.

#24 Magpie

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Posted 03 June 2012 - 01:52 PM

Here are the design files. Schematic and Brd. As sent to Olimex. They are Kicad files, it probably wont load in other Designer packages. There were a few mistakes, the main on was R1, R2, R3 needed to go to ground instead of +5. These probably are best done as 2 or 3 resistors in parallel for each channel to give some room to change make odd resistances. Also very important was VBias should be left floating, so I had to cut the track to pin 6 of the A8447. Check the datasheet. I am only using standard designs. I also added a TVS diode to cut in at about 30-35 volts, right across the input terminals. I needed to make more space for D4 as It came in a bigger package than I expected. Also I was fairly lazy with the silkscreen, I could have done more work here. Especially marked +- on the terminal blocks. The main problem is the flickering at low duty cycles, when it is dimmed. I am fairly sure it is due to EMI crosstalk between the channels. I don't see an easy fix for this, so I am going to revert to using Attiny's. The good thing is that it runs nice and cool, the inductors are only slightly warm. The chips and diodes are cool. The leds themselves are only minimally heatsinked and are sitting at about 55 degrees Celsius (just too hot to touch) as I write this. The light is blinding, I have to turn it away as it dazzles my eyes. Jarrod: I guess you will need a lot of light for a green house. Full sun is 1kw per metre^2, my system, bright though it is is only delivering say 24watts @ about 100 lumens per watt.

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#25 hanzibal

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Posted 19 June 2012 - 11:20 AM

Hi Magpie, have you got the boards yet - any pictures?

#26 Magpie

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Posted 19 June 2012 - 12:56 PM

Hanzibal, Yes yes I got the boards back and posted a photo on the 6th of May. It has been working happily every since, well except when I turn the lights off at bedtime. As I mentioned before the dimming wasn't working properly probably because of some sort of emi. It runs cool and efficiently though. I had to make about 4 ugly soldering hacks due to my mistakes. Right now I am workng on an Attiny solution, without the Netduino smarts. The links are in this thread on about the 30th of May.
STEFF Shield High Powered Led Driver shield.

#27 hanzibal

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Posted 20 June 2012 - 05:14 AM

Sorry, I seem to suffer from alzheimer, I even commented when you got the boards from Olimex. Good luck with the attiny project!

#28 mcinnes01

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Posted 26 June 2012 - 11:34 PM

Hi Magpie, Hows your attiny solution going? Have you had any luck with the low end flickering on your STEFF shield? Cheers, Andy

#29 Magpie

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Posted 27 June 2012 - 12:05 AM

It is going ok.
I have posted a schematic, and now I am choosing parts footprints.

steff2

I am wondering whether to use the super torroids or to go with lower profile.
Also I am wondering what connectors to use.

But I have decided on the transistors and fets.

AHellene gave me some good tips for the Attiny.
STEFF Shield High Powered Led Driver shield.

#30 mcinnes01

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Posted 27 June 2012 - 10:29 PM

Hi Magpie, I have been looking over your schematic, as you know I am planing on driving 1 LED per channel. I was wondering what is the lowest voltage I could go to and still be able to power the LED, for example could I run the entire circuit on 5 volts? Regardless of the minimum voltage. which part of the circuit can I omit and which components will need changing for my lower voltage? I remember you said I won't need the black regulator, will I need the dimmer and the input protection? I will definately need the ISP break out. I was wondering if you have a list of manufacturers part numbers or a saved shopping basket? Many thanks, Andy

#31 Magpie

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Posted 27 June 2012 - 11:36 PM

I will reply on yiour Aquarium thread.
STEFF Shield High Powered Led Driver shield.




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