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Repurpose PID with 240v output to control 12V fan for BBQ / Smoker

Repurpose PID with 240v output to control 12V fan for BBQ / Smoker

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  #1  
Old 05-01-19, 06:44 PM
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Repurpose PID with 240v output to control 12V fan for BBQ / Smoker

Hey Electricians, I'd like to automate the temperature control on my Weber Smokey Mountain using an Inkbird ITC-100VH Digital PID Temperature Controller and a 12V Brushless DC Cooling Blower Fan to control the air hitting the charcoal.

I've previously used the PID with a 240V heater setup like this. Is there an easy way to wire the PID to control the 12V fan directly, or will I need to put a 12V power supply between the SSR and the fan?

Thanks
 
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Old 05-01-19, 07:24 PM
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Would need to know the characteristics of the 12V blower motor such as current draw.

The grill appears to be designed for natural convection venting, so even with a small amount forced air you could risk overheating it. Unless you're intending to build a forge and don't care if it melts down ;-) I guess it could be done but you would need to restrict the air flow to no more than it would get with all vents open.
 
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Old 05-01-19, 07:38 PM
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Thanks very much for the reply:

This is the DC fan I have: Brushless DC Cooling Blower Fan 12V 4020s 40x40x20mm 0.13A Sleeve-Bearing/A5

Attaching a small air blower to this BBQ is a common modification so should be safe heat-wise. The process involves blocking off all the vents except the PID controlled blower vent giving oxygen control to the PID controller. Typically the commercial offerings are USD300+ so to be able to make one from spare parts would be neat.
 
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Old 05-01-19, 11:00 PM
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From what I saw on the manufacturer's website your model of the controler supplies 12V at 30 mA max on the port that normally drives a SSR (solid state relay). If you want to drive the motor directly on 12V I suggest using a FET such as the common FQP30N06L. Tie drain terminal of the FET to black lead of the blower, connect source terminal to both terminal 6 of controller and to 12V supply ground , and connect the gate terminal to the controller's output signal at terminal 8. Also connect the plus lead of the blower to the 12V supply.
The FET I mentioned is overkill but it's cheap, and the starting current of the blower can be quite high especially if they have a significant capacitance across the supply leads.
 
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Old 05-02-19, 07:45 AM
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By the way, your SSR very likely uses a Triac which is only suitable for switching AC power not DC like your fan requires. A Triac can be triggered to latch ON, but it requires the voltage it's swiching to go to zero before it can turn OFF (which AC does every half cycle).
 
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Old 05-02-19, 07:57 PM
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Indeed, the SSR was supplied for 240V switching.

Is there any risk in not using a FET?
 
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Old 05-02-19, 08:27 PM
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Is there any risk in not using a FET?
You don't have to use a FET but you need some way to convert the 30ma output to several amps for the blower. The MOSFET that engr picked is a good choice.

I install commercial alarm systems and carry sensitive relay boards with me for special applications. The link below illustrates this type of relay where you purchased your controller. Although this would work for your application..... unlike the MOSFET...... the relay has a limited lifetime.

Sensitive relay board
 
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Old 05-02-19, 09:09 PM
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Alternatives to using a FET would be to:

1. turn on and off the 220V AC input to a 12v supply through the SSR. This would perform the required function, but it would be relatively slow in responding and the frequent on/off of AC input is not likely to have been considered in the design of most 12V supplies. That being said, I think it would probably work if the controller output does not turn on and off too quickly. A PWM (pulse width modulated) proportional control would not likely work very well. I don't know enough about the controller you have to make further comments about using an SSR to power on/off the AC input to a 12V supply.

2. Using the 12V output of the controller that's intended for driving a SSR to control the 12V motor directly. This will not work because even the running current of the blower exceeds the 30mA maximum.

3. Controlling a mechanical relay with the controller SSR port, where the relay contacts connect the12V to the blower motor. This would probably be more responsive than option 1., but not as much as a FET which could switch as fast and as frequently as you wish (no constraints on the type of control that can be used). Also a more limited lifetime as I just noticed that Pete has already mentioned in his post.

4. Other semiconductor switching devices such as BJTs or IGBTs. They could be used but don't have any clear advantage over FETS in this application.
 
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Old 05-02-19, 10:39 PM
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You don't have to use a FET but you need some way to convert the 30ma output to several amps for the blower. The MOSFET that engr picked is a good choice.
Ah right sorry I missed the discrepancy, blower needs 0.13A, SSR output supplies 0.03A

I suggest using a FET such as the common FQP30N06L. Tie drain terminal of the FET to black lead of the blower, connect source terminal to both terminal 6 of controller and to 12V supply ground , and connect the gate terminal to the controller's output signal at terminal 8. Also connect the plus lead of the blower to the 12V supply.
Thanks very much for the suggestion I've ordered one. A small clarification; you mentioned "connect source terminal to both terminal 6 of controller and to 12V supply ground". In this image, which is the 12V supply ground?

Should I understand that the FET is converting the 0.03A output to a potential 32A? Sorry, I'm learning here.
https://cdn.sparkfun.com/datasheets/.../FQP30N06L.pdf
 
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Old 05-03-19, 12:08 AM
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You need a separate 12V supply powered from 220V (or whatever source you have available) and it is not on your schematic of the controller. This is to supply the power needed for the motor, and it needs to support the motor's starting current which is often several times its running current. By the ground of the 12V supply I mean the negative terminal of this power supply.

The FET is a voltage controlled device so it will take essentially no current from the output terminal of the controller, just what is necessary to drive the capacitance at the FET gate terminal as the signal switches between 0 and 12V.
The 32A is just the maximum allowable current that the FET can tolerate. The maximum current in your case will be determined by the load that the motor presents when it is driven at 12V
 
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Old 05-05-19, 09:22 PM
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Thanks for the help, I successfully wired everything up as suggested and came to the conclusion that it'd make a lot more sense to just plug a standard 240 -> 12V 1.5A adapter into the existing PID controlled 240V output. As suggested by engr3000 above as 'option 1', there's a small delay between on/off cycles however the PID controller learns quite quickly what level of power is required to get a response in heat. I also dug up the specs on the SSR, should be fine I believe.

Here are the finished front/back images.



 
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Old 05-05-19, 10:21 PM
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Very nice craftsmanship in putting it together.
Let us know how the BBQ'ing works out under electronic control. :-)
 
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Old 05-06-19, 01:15 PM
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I like that wood case. Very appealing. Nice job.
 
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Old 05-13-19, 02:16 AM
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Yeah it turned out well, my one regret is not adding a plug on the back for the sensor rather than wiring it in directly. The t-slot screws make it really easy to pop the top off and make changes inside the case. I used this tool to output the case plans: https://www.makercase.com/ . I then laser cut the case from 6mm ply. I coloured everything black to match the smoker.

I attached the blower to a cylinder to mount it to the bottom of the smoker, unfortunately I haven't had a chance to mount it to the smoker and test it yet, but I've already bought a bunch of ribs to smoke when I do.





 
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