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Old 03-01-2011, 05:47 PM   #1
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Default Pwm circuit trouble

I built this circuit. 9V wall wart supply. Tried an extra filter cap.



It tested fine originally. Now with it hooked up to the SSR, It gets hung up on the extremes of the pot settings - not getting triggered. It works fine in the mid range and nearly to the extremes. F=0.64. I was thinking of just upping the 1k's to 4.7k. Anyone see an obvious problem here?

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Old 03-01-2011, 06:53 PM   #2
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When you say "not getting triggered" do you mean that the SSR is not being driven when you set the duty cycle very low or very high?

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Old 03-01-2011, 06:59 PM   #3
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One other question: how hard is it to disassemble this? Did you solder it all down, or are you using a solderless breadboard.

The reason I ask is because you can re-wire this and get the PWM built with out any resistors (other than the pot) and drop one of the diodes.

Having the fixed 1k resistors in there means you don't have a full range of control. There is ALWAYS some resistance on the charge and discharge path, so you won't be able to get to 0% or 100%, but it might be close enough that you can't notice.

With the resistor-less, 2-diode circuit that I use, I can get a full 0%-100% adjustment.

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Old 03-01-2011, 07:00 PM   #4
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Quote:
Originally Posted by Walker
When you say "not getting triggered" do you mean that the SSR is not being driven when you set the duty cycle very low or very high?
No. The 555 circuit sticks off. I was just looking at it. It seems to be only on the Max side. If I crank it up it will time out one cycle and just stay off.
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Old 03-01-2011, 07:02 PM   #5
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Quote:
Originally Posted by Walker
One other question: how hard is it to disassemble this? Did you solder it all down, or are you using a solderless breadboard.

The reason I ask is because you can re-wire this and get the PWM built with out any resistors (other than the pot) and drop one of the diodes.

Having the fixed 1k resistors in there means you don't have a full range of control. There is ALWAYS some resistance on the charge and discharge path, so you won't be able to get to 0% or 100%, but it might be close enough that you can't notice.

With the resistor-less, 2-diode circuit that I use, I can get a full 0%-100% adjustment.
Full on/off would be cool. Do you have a schematic? I will build another. Max pot I have is 100k though.
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Old 03-01-2011, 07:05 PM   #6
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I am not saying that your citcuit is responsible for the behavior you are getting, so keep that in mind. something else is going on at the high end.

anyway, here is the schematic for what I use:

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Old 03-01-2011, 07:15 PM   #7
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Quote:
Originally Posted by Walker
I am not saying that your citcuit is responsible for the behavior you are getting, so keep that in mind. something else is going on at the high end.

anyway, here is the schematic for what I use:
Something is funny with my circuit. I am going to try to figure it out for curiosities sake.

I like your circuit much better. I will just build a new one. It will be worth it for full on/off. Thanks for the help.
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Old 03-02-2011, 10:46 PM   #8
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It was a bad pot. I built your circuit anyway. It did the same thing with the old pot. I get the slightest flash when set to off. Is this normal? Maybe all my pots are just old.

Thanks again for the help Walker.

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Old 03-02-2011, 10:48 PM   #9
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Quote:
Originally Posted by lschiavo View Post
It was a bad pot. I built your circuit anyway. It did the same thing with the old pot. I get the slightest flash when set to off. Is this normal? Maybe all my pots are just old.

Thanks again for the help Walker.
I *sometimes* see the light illuminate in the 0% setting, but not even enough to really light up. It has to be really dark in the garage for me to even notice it.

And it doesn't last long enough for the SSR to react to it. The switching time of the SSR is longer than that faint pulse, so no heating happens.
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Old 03-02-2011, 11:18 PM   #10
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Okay. I will call it a success then. The controller is done!

This is actually for Yooper. Now I need to work on my panel because hers is better.

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