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Old 03-10-2011, 07:07 PM   #1
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Default PID + PWM = good idea or crazy?

I am putting together a single keggle eBIAB system as the first phase of going all grain with bigger batches. However, what I have is a cheap PID that doesn't do manual control, which means it won't work for the boil. (Obviously, buying a new, correct PID would be the smart solution, but I'm trying to not spend any more money) I ordered the cheap PWM circuit that was mentioned on another thread to be able to switch to manual control of the element for the boil. I've been trying to figure out what new relays and switches I would need (which actually could have added up to teh coast of a new PID anyway) when a light bulb went on...

Do I really NEED extra relays and switches? Can't I just wire up the PWM control between the PID and relay input? If the PWM is set to 100% output, the PID should be happy as a clam for controlling mash temperatures. When I'm ready to boil, I can dial down the PWM knob. The PID would still be sending its signal, but that signal would get intercepted by the PWM and scaled back.

Best idea since sliced bread? Or would this not work and possibly end up damaging the PID, PWM or both? It's been a LONG time since I took an electronics course, and it was never my best subject. Thanks for teh sanity check and any other advice.

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Old 03-10-2011, 07:42 PM   #2
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You probably don't want to do this. The two devices (PID and PWM) are both outputting a DC voltage signal to power the PID, but they are not going to be the same voltage signals.

No guarantees that anything bad would happen, but you really just need a low rated DPDT switch to select which device is in the control of the SSR. You'd only save a couple of dollars by omitting the switch and would risk potential harm to both the PWM and PID without it.

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Old 03-10-2011, 07:44 PM   #3
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i misread the first post.

you're talking about using the PID's SSR control as the power source for your PWM circuit?

*might* work. But I'd still spend the extra $2 and remove the potential risk by making the PID and PWM separate control devices and putting a selector switch in there.

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Old 03-10-2011, 07:58 PM   #4
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OK, yeah. Thanks. Brain fart. I kept thinking that I needed a high-current switch or relays to switch back and forth. But I'm just switching the control signal leading into the SSR, NOT the high voltage/current circuit coming out of it. Yeah, I can spring for a switch. Might even get a perty one that lights up 'n junk.

Still, now I'm curious how my original idea might work and part of me is itching to try it anyway.

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Old 03-10-2011, 08:03 PM   #5
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the idea is interesting for sure. I'd check the amp requirement of the pwm vs the amp limit of the PID's ssr control output. If the PID can handle the PWMs current needs, then this is a pretty clever way of adding a throttle to the PID output. You save a selector switch AND the separate dc voltage source required for a PWM circuit.

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Old 03-10-2011, 08:06 PM   #6
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I dunno I think it is just going to confuse the PID.

You might be better off just wiring in a 3-way switch so that you can switch between PID, off, and PWM.

I understand you don't want to spend any money, but switches are cheap (at least compared to a new PID) and it will definitely solve your problem.

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Old 03-10-2011, 08:30 PM   #7
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Quote:
Originally Posted by weirdboy View Post
I dunno I think it is just going to confuse the PID.

You might be better off just wiring in a 3-way switch so that you can switch between PID, off, and PWM.

I understand you don't want to spend any money, but switches are cheap (at least compared to a new PID) and it will definitely solve your problem.
Yes, the switch I need is cheap and that's the way I'm going to go. But the switch I convinced myself I needed wouldn't have been. As far as confusing the PID, that's not the part I would be worried about. If anything, the PID would try to fire the element more often which would just mean the PWM would get a more steady input signal. IF the probe returned a value under 100*C, the PID would just up the output. But since I'd be maintaining a boil, even that shouldn't be an issue. Worst case scenario, the PID would be all "Screw this, I'm turning this sucker on full blast." And the PWM would be like "Good show, old bean. But might I suggest a more rational heat output?" Hey... maybe I should write a comic strip about it.
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Old 03-10-2011, 10:53 PM   #8
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PID won't get confused. When he wants the PID really in control, the PWM will be set at 100%, so the PID will get what it wants. The PWM is totally tranparent in this case and passes SSR on with no changes.

When boiling, the PID just gets set to something over boiling temps and will just end up trying to run the element at full blast. The PWM then has a constant power source and can be used to dial down boil strength.

It's a good idea... If the PID can support the PWM's power needs.

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Old 03-11-2011, 01:47 AM   #9
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After thinking really hard about this What the op is describing is really an off-board manual mode addition to the PID. Manual mode in a PID is pulse width modulation; its actually quite genius!

But.... By the time you add a PWM circuit to the "inexpensive" PID you have already spent enough to buy an auber with manual mode. For the OP this is a viable option, but I would'nt suggest this method for a start-up.

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Old 03-11-2011, 02:02 AM   #10
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The PWM circuit will have a startup delay, so it would funk up the timing. For a low cycle speed it wouldn't be that big a deal but at anything over 10hz I think you would lose a lot of control.
Of course this is just an engineering assumoption based on experience with similar circuits, you'd have to do timing analysis to actually know what you would end up with.

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