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Old 08-28-2010, 09:15 PM   #1
lustrum
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Default PWM Troubleshooting - Need Input

I wired a PWM according to CD's schematics like many brewers here have done. I tested it on a light bulb and can not get it to dim the light. The SSR will have the 115v line OFF when the pot is cranked to 0%, and then passes current to the bulb when the pot reaches around 50%. It does not dim.

I'm wondering if this might be an issue with the timer chip (lm555)? Baring the possibility that I wired this circuit wrong (wrong pins on the chip, i.e.) I wanted to see if anyone has experienced this and if there was an obvious error I made.

Though, I guess this begs the question: Is the PWM supposed to dim the light? Or only switch it on/off?

When the power is left on (50%+) the light does not switch on/off, just stays on.

Here are some pics:

Set up to test... a little cluttered.


Pot turned to ~50%. Led lights on circuit and SSR passes current to bulb.

lustrum

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Last edited by lustrum; 08-28-2010 at 09:22 PM. Reason: forgot the pics
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Old 08-29-2010, 02:05 AM   #2
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The light should not dim. It should go on/off as per "duty cycle" of your circuit.

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Old 08-29-2010, 02:39 AM   #3
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Without seeing the schematic it sounds like the cap that sets your timing is not functioning correctly. It sounds like it isn't doing it's charge/discharge cycle to turn the output to the light off and on. You should see the led on the SSR pulse as you adjust the pot as well as the light go on and off with the pulse. Disconnect the SSR and just watch the output pin 3 with the multimeter and see what it does to rule out anything on the SSR's end. You should see the voltage cycle full on and off to what your input is.

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Old 08-29-2010, 05:32 AM   #4
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As mentioned the light shouldn't dim, just switch on and off. For example, if you have a 2 second cycle and set the pot to 50%, it should switch on for 1 sec, and then off for 1 sec, and if you turn it down to 25% it should switch on for 0.5 sec, and then off for 1.5 sec. What size capacitor did you use? A 4.7uF cap gives a cycle of about 1.5 seconds, and the larger the cap the longer the cycle. Could it be that you have built a circuit with a long cycle time and just aren't waiting long enough to see it work properly?

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Old 08-29-2010, 12:56 PM   #5
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Quote:
Originally Posted by interplexr View Post
Disconnect the SSR and just watch the output pin 3 with the multimeter and see what it does to rule out anything on the SSR's end.
Thanks I'll try that again and record what it does.

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Originally Posted by JuanMoore View Post
What size capacitor did you use? A 4.7uF cap gives a cycle of about 1.5 seconds, and the larger the cap the longer the cycle. Could it be that you have built a circuit with a long cycle time and just aren't waiting long enough to see it work properly?
I am using a 4.7uF bipolar electrolytic cap. My understanding is that these can be soldered in any direction. Perhaps I will swap it out with a standard electrolytic cap and see if this makes a difference.

Originally I had wondered if my timer chip was shot, since it was not pulsing.

Thanks for the help so far.
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Old 08-29-2010, 03:04 PM   #6
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Put a radial lead electrolytic cap (4.7uF) in place of the axial cap and still have the same issue. Also put a meter to the power output and it had no fluctuation, only a solid 5.75VDC once the pot was turned to ~50% and higher.

I'm thinking it is the chip now... I'll swap it out when I get home and see.

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Old 08-30-2010, 01:38 AM   #7
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What voltage are you sourcing the circuit with if you're only getting 5.75V out?

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Old 08-30-2010, 12:30 PM   #8
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Cell phone wall wart... 9vdc. The SSR only needs 3vdc to operate.

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Old 08-30-2010, 02:01 PM   #9
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What resistors do you have in the circuit? Here is a calculator to determine how fast it should be cycling. http://www.electronicdesignworks.com/utilities/555_frequency_calculator/555_frequency_calculator.htm If you are cycling too fast it will give you wierd results like this, since SSRs cannot be used to dim AC current.

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Old 08-30-2010, 07:45 PM   #10
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Quote:
Originally Posted by lustrum View Post
Put a radial lead electrolytic cap (4.7uF) in place of the axial cap and still have the same issue. Also put a meter to the power output and it had no fluctuation, only a solid 5.75VDC once the pot was turned to ~50% and higher.

I'm thinking it is the chip now... I'll swap it out when I get home and see.
Don't trust your meter to measure the output of a PWM. You may need a scope to see the changes.

The reason is, my meter reads the voltage for a moment before it displays on the screen. When I put it on my PWM, it says 0v until I get upwards of 40%. Then it says 9v the whole time - It's simply because the response time of the meter is higher than the off cycle of the PWM at that setting. - IOW, it's not off long enough for my digital meter to display 0v before the PWM fires high again.

Hook a 9v lamp, or LED (With the proper voltage divider) - They should be able to show you that current is flowing, but still have the response time to let you see the "off" portion of the pulse.
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What does the primary pressure gauge on the tank tell us? That's right, the temperature. Put it on a scale if you want to know how much is in it...
Put some duct tape over the gauge - Or better yet - Replace the high pressure gauge with a plug - High pressure gauges are useless!

Last edited by SweetSounds; 08-30-2010 at 07:48 PM.
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