Question on PWM

I built my PWM using the information in this post. Instead of the 33μF cap, I'm using a 22 (because that was what I found).

When I connect everything up and I put my voltmeter on the output, I get 0v when the pot is turned all the way counterclockwise. Great, power is out. However, when I turn it all the way clockwise, I see my voltmeter cycle between 5v and 0v. I thought I should get 5v the entire time. Am I wrong?

I would suggest posting this question in the thread you referenced, and if that doesn't work PM Walker.

My understanding of the PWM is that it basically cycles the power to the element on and off. The more you turn up the potentiometer, the more time the element spends "on". I can't speak to specific voltages, but the fact that it's cycling doesn't seem odd.

GregKelley said:

I built my PWM using the information in this post. Instead of the 33μF cap, I'm using a 22 (because that was what I found).

When I connect everything up and I put my voltmeter on the output, I get 0v when the pot is turned all the way counterclockwise. Great, power is out. However, when I turn it all the way clockwise, I see my voltmeter cycle between 5v and 0v. I thought I should get 5v the entire time. Am I wrong?

Click to expand...

Cycling between 0V and 5V would be correct and the percent time it's at 5V represents the PWM duration.

But I would expect to see 5 V all the time when full on and varying PWM depending on where the pot is set.

Thanks guys. I know it is supposed to cycle but from what I read last night on some other sites, it should be 5v all the time when full on. I'm going to add this question to the PWM thread.

I think even at full-on it still cycles but it's like 99% on vs. 1% off. I don't think a typical multimeter will really give you a good sense of the on/off time unless your cycle time is really slow, like >2-3s. I think I have my cycle time set around 0.5s and the multimeter doesn't give me anything indicative.

JKoravos said:

I think even at full-on it still cycles but it's like 99% on vs. 1% off. I don't think a typical multimeter will really give you a good sense of the on/off time unless your cycle time is really slow, like >2-3s. I think I have my cycle time set around 0.5s and the multimeter doesn't give me anything indicative.

Click to expand...

My cycle time is around 1.52 seconds. The "off" time is also enough to blink the LED on the SSR.

The 555 IC is a neat little device. I did a presentation on it in high school and wrote a long paper on it. When wired as a PWM it is an astable circuit; has no stable state.
The Pot will control your duty cycle; 100% will not be reachable with this circuit (0% will not be reachable either).
The frequency will be set by the capacitor that you choose. The smaller the Cap the higher the frequency. You can replace the cap with a variable capacitor to help dial in your system if you wish.
Another tip is to check the resistance of your Pot in the clockwise position where you expect the PWM to be at 100%; should be near 0 ohms.

This is a good site:
http://www.kpsec.freeuk.com/555timer.htm

Like mredge73 mentioned, you'll never get a full off or full on with a 555 timer based PWM unless your trim pot has an "off" setting. The circuit that I used is very similar. When all the way "off", I get a very faint flicker on an LED that I wired into the output; all the way on results in a very brief dimming of the LED. Hooking up an LED on the output (or use the one on the SSR) will give you a better idea as to what your duty cycle is.

Thanks guys. I'm thinking about getting a little toggle switch and adding it to the PWM circuit. Flip it one way and ground is fed to the PWM and the PWM controls the circuit. Flip it the other way and ground is fed directly to the SSR bypassing the PWM. That way I can get 100% on until the BK reaches boiling.

Sounds like a plan.
I would use a 3 position switch so you can have ON, OFF, PWM.

Not a bad idea. My control panel currently only handles the BK. I don't do all grain and do not have pumps. So my "off" is facilitated by a double pole single throw switch.

I like the idea of having a three way switch for on, pwm, full! I wonder how that'd be wired though

Here you go; here is a SP3T switch:

mredge73 said:

Here you go; here is a SP3T switch:

Click to expand...

Wow you rock mredge!

Good idea! I also used a 22uF cap and it gives me about a 6 second period which seems like a good speed to me?

GregKelley said:

Not a bad idea. My control panel currently only handles the BK. I don't do all grain and do not have pumps. So my "off" is facilitated by a double pole single throw switch.

Click to expand...

I think what is needed is a SINGLE pole, DOUBLE throw, center off (not momentary but maintained) switch...

So this is slightly related, but since we're talking PWMs, I figured I'd ask.

I'm looking at grabbing a PWM from eBay (got a stack of eBay Bucks sitting around), but want to make sure I can adapt it to suit my brewing needs.

I'm looking at this (many of them available ~$10 on eBay):

http://www.ebay.com/itm/PWM-DC-Conv...ltDomain_0&hash=item4cff0aefe3#ht_5438wt_1037

Can this be adapted with a a 2.2uF cap to hit our target frequencies? I may just got with the Bakatronic kit, and put the eBay bucks towards an SSR...

You definitely want to do something as it runs at 25 khz. I would suspect that would burn out the SSR. You'd have to trace the wires to determine how the circuit flows and then replace the appropriate capacitor with a 2.2uF capacitor.

I used two adjustable pots. 1Mohlm and roughly 200k, gave me adjustment over off time and on time. I did add 1k resistors in series with the pots.

Gave me the ability to set the off period between 0-2 seconds and the on time to 0-19 seconds. With a bigger cap you can go to periods up to 20minutes.

In practice off times longer than .5-.7 seconds and you could see the boil kill and then start. you would only set it longer if you were playing maintaining at temp hot liquor or something like that.

With two pots you can use just about any size cap, within reason.