That is what I was thinking of doing, but for an oven that may have 4 elements, $4 SSR's each would be $16 which is 1.5 times the price of the SSR I posted above.
Still researching on how to connect USB directly to the board without a USB to serial cable.... Not finding much except for explanations. Was hoping for a schematic. If I can get the complete parts list finished tonight, I will try to get this ordered tonight or tomorrow.
I have been gathering the parts to build a toaster oven.
FWIW I bought SSR's, 'K' type thermocouples (ones with a stainless barrel and threaded at the end - so that I can fix the tip in the centre of the oven, one above and one below the pcb) and thermocouple meters on eBay quite cheaply.
I can't seem to find and cheaper thermocouples on DigiKey, but here is one that I think will work for me : http://www.digikey.com/product-detail/en/GK11M/290-1911-ND/415777 Is there an alternative to where I can get a thermocouple from or is the one that I already have good enough to use?
Here is the schematic of the Prop Plug I am using : http://www.rayslogic.com/propeller/DemoSched.gif Does this look correct? If this is the case, I could order some boards with this design so I can use it on a few of my other computer controlled projects
That is what I was thinking of doing, but for an oven that may have 4 elements, $4 SSR's each would be $16 which is 1.5 times the price of the SSR I posted above.
Still would require only two SSRs, one for the upper elements, one for the lower.
Do not use an ssr that barely meets the current requirement (12.5A for 1500W). Choose one that will handle at least 15A, and preferably 20A. The peak instantaneous current when the heating element is cold will be higher initially so the ssr will not last very long if it is under rated.
I figure that 16A rating would be good enough for what I am needing since if the oven is rated at 1500W, it should not exceed this even when first turning on. The 1500W is the maximum absolute rating which means the unit will / should not ever draw more than 1500W correct? So the most the SSR should ever see is 12.5A. Once the elements have been heated, the amperage would go down slightly.
On the USB FT232 circuit, the NPN transistor is just a standard switching NPN? I don't see any specs on what to use for this transistor. I am looking for a surface mount transistor that would work for this, but I do have some MSP transistors somewhere that are through hole style.
I figure that 16A rating would be good enough for what I am needing since if the oven is rated at 1500W, it should not exceed this even when first turning on. The 1500W is the maximum absolute rating which means the unit will / should not ever draw more than 1500W correct?
Well, that is interesting. If that is the case, I should be looking for at least double the amperage capabilities then? I have read that SSR deteriorate after awhile and this is sped up when the SSR is run at maximum capabilities. Digikey has many SSR's available, but they are pretty pricy. Since I am purchasing everything else though them, I feel it would be more cost effective to purchase an SSR through them instead of paying additional shipping from another site which would make up the difference in price difference.
On page 5 of this Prop Plug schematic, could someone tell me what NPN specs I need? I can't seem to locate any specs on the output specs of the FT232's DTR pin nor can I find any specs on the NPN needed for the DTR.
Any general purpose npn will work. Typically P2N2222 are used - be careful of the different pinouts (particularly the P2N2222 plastic is different from the 2N2222). The 2N3904 and the MMBT3904 smt can also be used. The rc+transistor circuit from DTR is used to generate a reset pulse when DTR goes invalid (not as you would expect going valid).
I bought my SSR on eBay (25A IIRC) way cheaper inc postage than I could buy locally. Just make sure you use a reliable eBay seller.
I have been looking at the datasheet on the MMBT3904 and trying to understand it. I have a hard time understanding how to use the specs in a datasheet to know what parts to use. This is a page I found that explains a little, but I am still having difficulties : http://www.onsemi.com/pub_link/Collateral/AND9129-D.PDF
If I use the MMBT3904, are there any changes that would need to be made to the displayed circuit in the above post? It shows a 10nF cap and a 33k resistor. Nothing else. What I am trying to learn is how these components are determined. The FT232 has a typical output on a pin of 2.7V and can deliver up to 24mA per pin according to the datasheet here : http://www.ftdichip.com/Support/Documents/DataSheets/ICs/DS_FT232R.pdf How is the 10nF cap and the 33K resistor determined?
It says 3 - 32V input, but what about current requirements? Am I missing something here? The Prop can only supply 40mA per pin.....
Unlike mechanical relays the SSRs have high impedance input. The SSR shouldn't draw much current. You could always add a 100 ohm resistor just to make sure it doesn't draw too much current.
1) This is the thermal curve from a similar solid state relay (Crydom H12WD)
The left hand side of the graph shows that when the relay is supplying say 10 A, itself it burns 10 W. Heat sink highly recommended. At that power dissipation, a heat sink rated at 3°C per watt would end up at +30°C above ambient.
2) I don't like the reset circuit mentioned. I like this better, because it provides a solid reset pulse without putting a sudden load on the FTDI 3.3 regulator:
3) What you have is almost certainly a type K thermocouple. One indicator would be how hard it is to solder to the wires. Type K wires (chromel and alumel) are very difficult to solder without special equipment and are usually welded at the tip. If you are preparing a circuit board, plan to use a connector of some sort. The other possibilities, copper-constantan (T) or iron-constantan (J) can be identified by looking for the unmistakable copper or iron wires. The constituents are easier to solder, but have other disadvantages, so type K is what is normally found with multimeter kits.
Wow, I really appreciate the information! I never could figure out those graphs, but you explained it in a way I could understand it
I think I will sway away from a surface mount transistor for the time being since I have hundreds of through hole switching transistors on hand. The most common one I have is an MSPA06 which I use on a circuit for the SX28's. I have used them in many applications and kind of know their limits.
The thermocouple I have is welded at the tip and I have never tried to solder to the wires. They have the same plug on them that connect to my meter's ports. Since I use this meter a lot, I think it would be best if I purchased another thermocouple so I don't mess with the one I already have. I found one on Digikey for ~$15 which is ok for me now.
I am still waiting for the circuit boards I ordered from OSHPark a little over a week ago. These are the boards that will need to be used in this reflow project. From most of the parts I am selected, I will probably need another circuit board for this project but I don't want to wait 2 weeks to get in 3 boards.... Are there any other US based PCB manufacturers that are quicker (Besides ExpressPCB)?
I am not sure if you should ever solder thermocouple wire, maybe only crimp connectors.
If you do want to wire it up to a board or chip, there is going to be a voltage created by the dissimilar metal that requires recalibration. If you can get a thermocouple socket with the same junction type wires (for a Type K, you would use all Type K wire), you can keep your dissimilar metals as short as possible (as close as possible to the IC inputs).
If you add in a few inches of copper wire, you may have lost any accurate readings.
Comments
No, but if you are driving the elements separately it would be.
You could just break up the elements into separate circuits and give each one a SSR and just parallel the inputs.
Still researching on how to connect USB directly to the board without a USB to serial cable.... Not finding much except for explanations. Was hoping for a schematic. If I can get the complete parts list finished tonight, I will try to get this ordered tonight or tomorrow.
http://forums.parallax.com/showthread.php/108385-Propeller-Controlled-Reflow-Oven?highlight=Timothy+Swieter+toaster+oven\\
FWIW I bought SSR's, 'K' type thermocouples (ones with a stainless barrel and threaded at the end - so that I can fix the tip in the centre of the oven, one above and one below the pcb) and thermocouple meters on eBay quite cheaply.
Here is the schematic of the Prop Plug I am using : http://www.rayslogic.com/propeller/DemoSched.gif Does this look correct? If this is the case, I could order some boards with this design so I can use it on a few of my other computer controlled projects
Still would require only two SSRs, one for the upper elements, one for the lower.
On the USB FT232 circuit, the NPN transistor is just a standard switching NPN? I don't see any specs on what to use for this transistor. I am looking for a surface mount transistor that would work for this, but I do have some MSP transistors somewhere that are through hole style.
http://en.wikipedia.org/wiki/Inrush_current
http://www.parallax.com/Portals/0/Downloads/docs/prod/prop/32200-32201-PropClipPlug-v1.3.pdf
I bought my SSR on eBay (25A IIRC) way cheaper inc postage than I could buy locally. Just make sure you use a reliable eBay seller.
If I use the MMBT3904, are there any changes that would need to be made to the displayed circuit in the above post? It shows a 10nF cap and a 33k resistor. Nothing else. What I am trying to learn is how these components are determined. The FT232 has a typical output on a pin of 2.7V and can deliver up to 24mA per pin according to the datasheet here : http://www.ftdichip.com/Support/Documents/DataSheets/ICs/DS_FT232R.pdf How is the 10nF cap and the 33K resistor determined?
Link : http://www.ebay.com/itm/Solid-State-Relay-SSR-25DA-25A-3-32VDC-Output-24-380V-AC-Solid-State-Module-/130905531225?pt=LH_DefaultDomain_0&hash=item1e7a93e359
It says 3 - 32V input, but what about current requirements? Am I missing something here? The Prop can only supply 40mA per pin.....
Unlike mechanical relays the SSRs have high impedance input. The SSR shouldn't draw much current. You could always add a 100 ohm resistor just to make sure it doesn't draw too much current.
EDIT : Also, would a snubber circuit be necessary in this type of application?
The left hand side of the graph shows that when the relay is supplying say 10 A, itself it burns 10 W. Heat sink highly recommended. At that power dissipation, a heat sink rated at 3°C per watt would end up at +30°C above ambient.
2) I don't like the reset circuit mentioned. I like this better, because it provides a solid reset pulse without putting a sudden load on the FTDI 3.3 regulator:
3) What you have is almost certainly a type K thermocouple. One indicator would be how hard it is to solder to the wires. Type K wires (chromel and alumel) are very difficult to solder without special equipment and are usually welded at the tip. If you are preparing a circuit board, plan to use a connector of some sort. The other possibilities, copper-constantan (T) or iron-constantan (J) can be identified by looking for the unmistakable copper or iron wires. The constituents are easier to solder, but have other disadvantages, so type K is what is normally found with multimeter kits.
I think I will sway away from a surface mount transistor for the time being since I have hundreds of through hole switching transistors on hand. The most common one I have is an MSPA06 which I use on a circuit for the SX28's. I have used them in many applications and kind of know their limits.
The thermocouple I have is welded at the tip and I have never tried to solder to the wires. They have the same plug on them that connect to my meter's ports. Since I use this meter a lot, I think it would be best if I purchased another thermocouple so I don't mess with the one I already have. I found one on Digikey for ~$15 which is ok for me now.
I am still waiting for the circuit boards I ordered from OSHPark a little over a week ago. These are the boards that will need to be used in this reflow project. From most of the parts I am selected, I will probably need another circuit board for this project but I don't want to wait 2 weeks to get in 3 boards.... Are there any other US based PCB manufacturers that are quicker (Besides ExpressPCB)?
If you do want to wire it up to a board or chip, there is going to be a voltage created by the dissimilar metal that requires recalibration. If you can get a thermocouple socket with the same junction type wires (for a Type K, you would use all Type K wire), you can keep your dissimilar metals as short as possible (as close as possible to the IC inputs).
If you add in a few inches of copper wire, you may have lost any accurate readings.
http://blog.lesman.com/2012/05/30/wiring-industrial-thermocouples-basic-tips/
http://www.maximintegrated.com/app-notes/index.mvp/id/4026