Topic: Embedded power
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Re: Embedded power
Jul 25, 2012, 05:01 am
Cool build Constantin. Where did you get the insulation from?
In drawing up the schematic, if I'm getting 5v out from the encapsulated power supply, would adding a 7805 regulator create a stable enough analog reference? I'm thinking that if 3.3 is better for the analog reference, I'll go with a LM1117T. Anytime someone says "jittery", I think of my first car and that was a nightmare.
What I've got so far is PSU>10 uf electrolytic>7805>10 uf electrolytic>10 uf tantalum>LM1117>10 uf tantalum
Does that look correct? For the extra $1.50 or so, is the 7805 setup going to do me anything?
If anybody knows of a better 5v psu that might provide me with a better bang for the buck and less mess, I'm all ears.
Location: Anaheim CA.
Re: Embedded power
Jul 25, 2012, 07:45 am
Depending on the actual load current there are several "Better" methods for an Accurate 5V reference. I think this is where your Education will truly begin. This is NOT a low power design so I might recommend a TIL 431 which is an "Adjustable zener diode, it takes a pot and a resistor to make a Stable
reference from 1.25 to 30+ volts and is better as a reference than a 7805. On the subject of 7805's I personally remove them any time I can use a switcher instead, If noise is an issue several caps and a resistor or small coil w\ill deal with that Easily. (I do own an analog scope a Tektronics 2213) and finding and dealing with noise isn't a big deal for me. The TIL 431 is the simplest answer to your reference issues, simple because it is a variable shunt regulator and although there is always the pink noise issues with Zeners this device has it's Zener buried well enough that any generated noise can be dealt with with a .1uF and a 10uF cap.
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Location: Peoples Republic of Cantabrigia
Re: Embedded power
Jul 25, 2012, 02:58 pm
Not sure a stable 5V supply is actually needed for this project.
Looking over the screaming rocket shield, I see a dedicated chip for the K-thermocouple, whose use makes sense because the thermocouple head has very little mass and is very temperature resistant. You can get pre-configured K-style thermocouples with fiberglass insulation @ $9 for 2 at a time on e-bay. Makes sense to me.
As for controlling the SSR, I suppose just about any external transistor will do, some may even be able to be driven off the Arduino itself. Depends on the current draw, see the spec sheet. My personal preference for this task would have been an opto-isolated darlington transistor array with a isolated power supply. Just in case the SSR-control wires ever rub / melt / whatever and could get shorted on the high side. Adds cost, but also adds a little bit of fool-proofness. A nice touch would be a fuse and removable darlington chip (think ULN2003A in a DIP form factor)
The rocket scream display is functional though I may modify the messages a bit. A larger display (like the Nokia 5110 series, for example) could be nifty from the point of view of being able to show more relevant data. For example, besides announcing the state of the process (i.e. soak, reflow, etc.) show the temperature, rate of change, how well the oven is doing vs. the manufacturer spec sheet, etc. This can be achieved on the 8x2 display to a limited extent but it requires frequent display wipes.
As for the insulation, McMaster Carr is your friend for all types. I only use insulation that is rated well above the intended temperatures. Today, I hope to glue the semi-rigid stuff to the roof of the oven and the door (leaving a slit for observation) with furnace cement / adhesive to minimize the heat transfer and maximize the homogeneity of the temperatures inside the oven. Yesterdays trial board came out great, though I had to move it closer to the center of the oven during reflow as the edge by the oven door was not reflowing in tune with the rest of the board.
This also marks my transition to lead-free solder. The Kester 96/3.5/0.5 lead-free paste I have works much better than the leaded stuff I had been using. Even tricky chips like the ADE7753 with its 0.5 or 0.65 mm pitch came out clean with no bridges. Also ordered lead-free solder wire...