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 Popper Build Airflow Problem
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| CharcoalRoaster |
Posted on 11/11/2018 3:35 PM
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 1 1/2 Pounder  Posts: 634 Joined: April 13, 2012 |
I've got a buddy who I've been supplying homeroasted beans to for a while now. I introduced him to my 500g FB roaster I built a couple years ago. For Christmas, I'm building him a little air roaster from a Presto Poplite to get him on the homeroast bandwagon. I'm an enabler, what can I say?  Honestly, I'm not trying to get too fancy with this thing. I wired everything up utilizing the thread here https://www.engad...e-roaster/ I didn't have a transformer on hand so I cannibalized this little guy (seen below) since it seemed to meet the specs I was seeing online for a build like this. The switch successfully fires up the heating element and the dimmer (Home Depot Leviton) successfully fires up the fan. However, I know there's not enough fan power to loft any greens. Is there an easy workaround to this issue? I've never done a popcorn mod build before. Only utilized a vacuum motor and heating coil so I'm not sure if/how I can boost the fan to loft the beans. I loaded it with 175g of beans? Maybe I exceeded max capacity for one of these units... thanks for the help!  https://www.dropb...5.JPG?dl=0 |
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| JackH |
Posted on 11/11/2018 4:38 PM
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 Administrator  Posts: 1809 Joined: May 10, 2011 |
I believe you need 25V AC for the fan motor. The fan has a bridge rectifier (4 diodes) soldered to it's pins which converts the 25 volts AC to DC for the fan. Too bad Radio Shack is gone. It would have been easy to get a transformer there.
JackH attached the following image:
Edited by JackH on 11/11/2018 4:52 PM ---Jack
KKTO Roaster. |
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| CharcoalRoaster |
Posted on 11/11/2018 6:13 PM
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1 1/2 Pounder Posts: 634 Joined: April 13, 2012 |
Ok thanks Jack -- I'll see what I can rustle up. Can I go above 25vAC or will that fry the fan? |
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| renatoa |
Posted on 11/12/2018 1:48 AM
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 Administrator Posts: 2507 Joined: September 30, 2016 |
Usually these are Johnson or Mabuki Rs-385 24v motors, so 25 is a bit over. These motors are consumables, $5 or so, we change them every 20-30 kg. |
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| ChicagoJohn |
Posted on 11/12/2018 6:21 AM
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 Pounder  Posts: 513 Joined: June 15, 2015 |
Quote CharcoalRoaster wrote: Ok thanks Jack -- I'll see what I can rustle up. Can I go above 25vAC or will that fry the fan? I used 24VAC and it always worked well. However, you have to watch the VA rating. I initially tried a cheaper one made by Packard rated at 40VA that I got from Home Depot. That over heated. I finally settled on a 50VA one made by Honeywell. They can go up to 80C in operation but that's about it. I did two popper mods using these Honeywell transformers and never had a problem with them. Also, after hundreds of 90 gm batches, I never had a problem with the DC motors. In their design, the AC voltage drop is accomplished with a secondary heating coil forming a voltage divider to drop 120VAC down to (as I recall) around 22VAC, close enough to 24VAC. When you rectify with a full wave bridge, you will get a little over 30VDC after the voltage drop across the diodes. If you use a household rotary dimmer switch on the input side of your transformer, you'll be able to vary the output voltage and fan speed. But if you do that, it's a good idea to put a physical stop on the dimmer switch so that it's impossible to shut it off; the design relies on continuous air cooling and without it heated coils will instantly overheat, melt the thermal fuse or worse if you have removed the thermal fuse. I learned that the hard way -- twice. Edited by ChicagoJohn on 11/12/2018 6:41 AM So many beans; so little time.... |
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| CharcoalRoaster |
Posted on 11/12/2018 6:43 AM
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1 1/2 Pounder Posts: 634 Joined: April 13, 2012 |
Something like this John? https://www.amazo...+honeywell |
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| renatoa |
Posted on 11/12/2018 6:51 AM
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Administrator Posts: 2507 Joined: September 30, 2016 |
By rectifying AC, the diode bridge output is not DC, but a single polarity pulsed shape signal whose average voltage is same as AC voltage, minus the typical 1,5V diodes drop. Is the peak voltage that is 1.4x times the average, i,e. 24*1.4 = 33.6 - 1.5 = 32V, for 24AC input. But this peak has not the same "feeling" for a load as a 32V DC source, unless you add a filter capacitor after the rectifier bridge. |
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| CharcoalRoaster |
Posted on 11/12/2018 7:43 AM
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1 1/2 Pounder Posts: 634 Joined: April 13, 2012 |
Thanks for the info Renatoa -- none of which makes any sense to me lol I'm sorry! Can you give me the dumbed down version... |
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| renatoa |
Posted on 11/12/2018 7:51 AM
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Administrator Posts: 2507 Joined: September 30, 2016 |
Short story: if having just a diode rectifier bridge, and nothing else added between AC and motor, the output value is slightly lower than AC input. 24V AC in = 22 averaged Volts out, volts which are NOT DC ! https://upload.wi...ier.en.svg |
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| CharcoalRoaster |
Posted on 11/12/2018 8:05 AM
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1 1/2 Pounder Posts: 634 Joined: April 13, 2012 |
So then a filter capacitor is necessary to achieve the desired output in DCv? I haven't seen filter capacitors on build threads before. How are others achieving the necessary voltage to successfully roast on poppers? |
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| renatoa |
Posted on 11/12/2018 10:51 AM
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Administrator Posts: 2507 Joined: September 30, 2016 |
If desired output is +30V, yes, but I doubt the 24V motor will appreciate this boost... Beware, the capacitor will be larger than expected, something in the thousands microFarads ballpark, for 1V ripple and 1A consumption. Such capacitor dimensions are of a Coke can... Check here for more details: https://www.changpuak.ch/electronics/power_supply_design.php No need to power the motor by pure DC, is simpler to dim the turation in AC, and power the motor with resulting chopped pulses, instead rectifying to DC where you need a PWM dimmer, more difficult to find. |
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| JackH |
Posted on 11/12/2018 12:46 PM
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Administrator Posts: 1809 Joined: May 10, 2011 |
Quote renatoa wrote: Short story: if having just a diode rectifier bridge, and nothing else added between AC and motor, the output value is slightly lower than AC input. 24V AC in = 22 averaged Volts out, volts which are NOT DC ! https://upload.wi...ier.en.svg Looks like a DC output to me. Has a + and - polarity. I don't think the motor cares. ---Jack
KKTO Roaster. |
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| renatoa |
Posted on 11/12/2018 1:22 PM
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Administrator Posts: 2507 Joined: September 30, 2016 |
For a motor, yes, and a bonus, can be controlled with a simple A/C dimmer. But you can't power a laptop for example, from this "DC"... |
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| ChicagoJohn |
Posted on 11/12/2018 4:31 PM
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Pounder Posts: 513 Joined: June 15, 2015 |
Quote That's 40VA and the ones I've used are 50 VA. I found the 40VA ones got excessively hot. You might be able to get one roast out, but usually with a popper, you will be doing several in a row. I'd recommend looking for a 50VA. I didn't measure it, but I think at full power these motors draw over 2A and 2 X 24 = 48VA. So many beans; so little time.... |
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| ChicagoJohn |
Posted on 11/12/2018 4:43 PM
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Pounder Posts: 513 Joined: June 15, 2015 |
Quote renatoa wrote: Short story: if having just a diode rectifier bridge, and nothing else added between AC and motor, the output value is slightly lower than AC input. 24V AC in = 22 averaged Volts out, volts which are NOT DC ! https://upload.wi...ier.en.svg If I understand what you're saying, it's not the case. When you use a full wave bridge, as is used in these poppers, the AC is converted to unregulated DC. The voltage out is not RMS voltage (e.g., 24VAC) but rather peak voltage which is RMS X 2^0.5 open circuit. So if you connect the bridge output to a capacitor for smoothing purposes and you measure the voltage across the capacitor, it will be 24VAC X 1.414 = 33.9 VDC less the approximately 1.4VDC drop across the silicon rectifier diodes, two in each direction, So you wind up at around 32VDC. (For purposes of calculating VA of the AC transformer, however, you would use 24VAC X AC current.) The DC motors they use in the poppers run just fine on unregulated DC, and you don't need worry about ripple. So many beans; so little time.... |
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| ChicagoJohn |
Posted on 11/12/2018 4:45 PM
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Pounder Posts: 513 Joined: June 15, 2015 |
Quote CharcoalRoaster wrote: So then a filter capacitor is necessary to achieve the desired output in DCv? I haven't seen filter capacitors on build threads before. How are others achieving the necessary voltage to successfully roast on poppers? You don't need a filter capacitor. The motors run fine on unregulated DC. Note there is no filter capacitor in the popper design off the shelf. So many beans; so little time.... |
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