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Old 12-16-2011, 07:26 PM   #211
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Just finished up mine, with Walkers suggestions. 100k pot, 22uf cap, also added the .01uf cap and a diode

Seems to be working fine. Will probably try it Sunday, once I get it installed it my control box.

Thanks again for the help!!




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Old 12-16-2011, 07:30 PM   #212
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do you only have two wires connected to the pot?


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Old 12-16-2011, 07:35 PM   #213
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do you only have two wires connected to the pot?
lol no the center one is white can't see it against that white table.
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Old 12-17-2011, 07:39 AM   #214
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Wow, Walker. A lot of helpful information in posts 180 and 182. Good job, and thank you.
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Old 12-17-2011, 06:52 PM   #215
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This is how I build them:


I'm looking to build one of these. However, I couldn't find any 1N5818 diodes at Radio Shack. What specs on the diodes am I looking for? I picked up 1N4742A Zener diodes because they said 12-volt and I figured this circuit was 12-volt with the wall wart.
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Old 12-18-2011, 02:23 PM   #216
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A diode, in general, is a one-way valve. Current can only flow in one direction though it. If you try to run current through it in the backward direction, nothing will happen. However, if you apply a large enough voltage to it while trying to get current to run through it backward, the diode will eventually breakdown and let current flow backward. It will probably also overheat and burn up. This breakdown voltage us usually pretty large.

Zener diodes are a special kind of diode. They operate like a diode as described above, but they have a greatly reduced breakdown voltage for the reverse direction, and will not overheat and die when you run them backward. In the case of the zener diodes you bought, the breakdown voltage is 12v.

Sorry for being long winded, but the "one-way" property of the diode is something the PWM circuits rely on. In the case of my schematic, the two diodes ensure that one side of the pot us used to charge the capacitor, and the opposite side of the pot is used to discharge the capacitor.

Your zener diodes will work, but only if you stay below 12v. If you power this with something more than 12v, then the diodes are going to be allowed to let current flow backward, and the thing just won't work right.
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Old 12-18-2011, 02:27 PM   #217
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A diode, in general, is a one-way valve. Current can only flow in one direction though it. If you try to run current through it in the backward direction, nothing will happen. However, if you apply a large enough voltage to it while trying to get current to run through it backward, the diode will eventually breakdown and let current flow backward. It will probably also overheat and burn up. This breakdown voltage us usually pretty large.

Zener diodes are a special kind of diode. They operare like diode as described above, but they have a greatly reduced breakdown voltage for the reverse direction, and will not overheat and die when you run then backward. In the case of the zener diodes you bought, the breakdown voltage is 12v.

Sorry for being long winded, but the "one-way" property of the diode is something the PWM circuits rely on. In the case of my schematic, the two diodes ensure that one side of the pot us used to charge the capacitor, and the opposite side of the pot is used to discharge the capacitor.

Your zener diodes will work, but only if you stay below 12v. If you power this with something more than 12v, then the diodes are going to be allowed to let current flow backward, and the thing just won't work right.
No apologies needed for being long-winded, much appreciated. Since my circuit is being fed with a wall wart that (upon review) has an output of 5V, these diodes should work right? Any other diodes you can suggest from these diodes?
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Old 12-18-2011, 02:46 PM   #218
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Pretty much any of those diodes will work. They have different ratings total amps thay can handle and things like that, but in a PWM, you are working with low current and low voltage, so all is well.
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Old 12-18-2011, 05:54 PM   #219
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Pretty much any of those diodes will work. They have different ratings total amps thay can handle and things like that, but in a PWM, you are working with low current and low voltage, so all is well.
Much appreciated, Walker. I went with one of the 1N4000 series rated for 1A.
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Old 12-19-2011, 12:03 PM   #220
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If the pot you have available is 1/5th the size as the one in the schematic, then you can probably just use a capacitor that is 5 times the size of the one in the schematic and maintain the same timing behavior.

So... 11uF in this case.

But, you could even go larger and still get a reasonable frequency for the PWM. If I am reading/understanding it right, the circuit with the 2.2uF cap and the 500kOhm pot is actually cycling a little too fast (about 1.3Hz).

If I am correct on my calculations, I'd probably throw a 22uF cap in that with the 100KOhm pot and get a frequency of 0.65Hz.
I went with the 22uf on this board. I used it yesterday and it worked fine. But, The pulse width is a just a bit to long. Which way would I go to get it to cycle just a bit faster.

The way it is now I can get a decent boil but it in extremly sensitive between no boil and to agressive with a 4500w element.


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