Help with PSU (Temp control fan, load bank, & PWM circuit)

[jocanon]

Do you think I will be able to run both fans off the one FET?

 

[ronv]

Yes, I think so.

 

[jocanon]

found these two:

https://www.mouser.com/ProductDetai...=sGAEpiMZZMttgyDkZ5WiupfPEynZ6cj0Fo1A2rC58og=

https://www.digikey.com/product-detail/en/WA-DT2-101E/WA-DT2-101E-ND/2347901

Would I need the bigger one? Looks like it's for two FETs, but it is the only one that is larger than 12 square inches. per the data sheet the first one is only 8.4 square inches and the second one is 15.1 square inches of surface area.

 

[jocanon]

It's official...now I just need to get selling these units!

 

[ronv]

Try this one. It will still get pretty hot but if you can get it in some air it will make a big difference.

Looks like we need to do cost reduction next so you can make some $$$$.

 

[()blivion]

It's official...now I just need to get selling these units!

Ehhh..... What? You went through the trouble of registering for a legit business? I'da just sold em on Ebay.

 

[jocanon]

Yeah, it was cheap...I set it up myself. Figured it would be good to have some liabilty protection. It was only about $200, I would pay more than that for insurance.

 

[()blivion]

Hummm. Well, you know tax law and I don't. So do what you feel you have to. It's prolly the right choice.

Back on target, if you want a cheep and dirty heatsink for your FET's you can just bolt them to the case. Unless they are the metal tab type, then you *MAY* have to insulate them from shorting out. Of course, if the tab is grounded already as is with the dummy load then you don't have to worry about it either way.

 

[jocanon]

Hummm. Well, you know tax law and I don't. So do what you feel you have to. It's prolly the right choice.

The entity will be disregarded for tax purposes and the net income will flow through to my individual tax return on schedule C, meaning I will be taxed exactly the same as if I did not set up the LLC, the only thing is I will have liability protection in case someone burns their house down or electrocutes themselves and tries to come after me, they will only be able to sue me for the assets in the business not my house, my car, my bank account, and get a judgment against me for the rest...I think a couple hundred spent now to prevent a catastrophe down the road makes sense. Now if I was selling Barbie dolls or toy trucks or something, maybe I would not be as concerned about liability protection. There are a lot of people out there that want to charge a lot to set up LLC and what not. For instance, one company charges $50 to register for an EIN which you can do in about 10 minutes for FREE on the IRS website if you know where to go (just do a Google search and you will find it)...it's as easy as setting up a new email account! I deal with these types of things in my profession, so this part of the business is easy for me, just a minor fee for much added peace of mind.

 

[jocanon]

Try this one. It will still get pretty hot but if you can get it in some air it will make a big difference.

ronv, I had a link to two of them, do you think I should try the larger one, or will the smaller one be good enough?

 

[ronv]

I guess the link would help.

https://www.aavid.com/products/standard/504222b00000g

 

[jocanon]

Thanks ronv

 

[jocanon]

Alright, I took a crack at putting togather a BOM for the temperature control circuit for the large 24 volt fans. Are all these parts OK, did I miss any, or do any of them need to be changed? (note: I added another pot because I want to be able to also have manual control to slow the fans down when desired in addition to the temperature control):

Pot for manual adjust (the one I added): https://www.mouser.com/Search/Produ...1-B5K-3virtualkey14860000virtualkey31VA305-F3
Trim pot for temp adjust: https://www.mouser.com/Search/Produ...5KLFvirtualkey57700000virtualkey858-67WR25KLF
33K resistor: https://www.mouser.com/Search/Produ...tualkey66000000virtualkey660-MF1/4DCT52R3302F
heat sink: https://www.mouser.com/Search/Produ...000Gvirtualkey53210000virtualkey532-504222B00
mosfet: https://www.mouser.com/Search/Produ...Fvirtualkey57370000virtualkey942-IRF7468TRPBF
10K Thermistor (already have): **broken link removed**

 

[ronv]

Try this for the FET.

https://www.electro-tech-online.com/custompdfs/2012/09/NTP5864N-DPDF.pdf

I'm not sure I understand what you want to be able to do with the second pot.

 

[jocanon]

I'm not sure I understand what you want to be able to do with the second pot.

Well, I want the fan to be able to run on auto pilot and be temperature controlled 99% of the time, but the other 1% of the time I want to be able to manually decrease the speed. So I was thinking I would set the trimpot on the inside of the dummy load to the right level of resistance so that when the pot that I will put on the outside of the case for manual adjustment is turned all the way to the setting that creates no resistence everything will operate normally, i.e. fan will speed up to max speed as the temperature rises, then when I want to manually slow the fan down temporarily I can turn the dial on the pot on the outside to increase it's resistance and thereby lower the voltage being delivered to the fans...does that make sense, or is there a more effecient way to accomplish the same thing? I think if I just had one pot, I would not be able to get it so the setting is in the "normal" operating mode when it was turned all the way to one side or the other...not sure I am explaining what I am thinking very well.

Edit:
Maybe the way you designed the circuit was to have the 25K pot do the manual adjustements and no need for a trimpot in on the inside of the dummyload to tune the fan settings (i.e. when it comes on at a what temperature...maybe you already calculated that to work properly with the fix resistor values)?

 

[ronv]

The way it is now at one extream of the pot the fan voltage is about 20 volts hot and 18 volts at room temperature. At the other extream (low speed) the fan voltage would be about 11 volts hot and 10 volts at room temperature. I'm guessing the fans will not start or may stall if the voltage is much lower than this. As a matter of fact we should make sure they will start at 12 volts. If that is okay you could just replace the trimpot with a 25K potentiometer with a knob on it.

Is what you want to do is override the temperature sensor and slow the fans regardless of the temperature?

 

[jocanon]

As a matter of fact we should make sure they will start at 12 volts. ...
Is what you want to do is override the temperature sensor and slow the fans regardless of the temperature?

They do run at 12 volts, and yes, you got it exactly...I want to be able to override the temperature sensor. Also, why would we not want the fans to stall out completely if things are running cool? I am wondering because I want to know if that same idea applies to the temperature controll circuit I build for the PSUs that I am selling, i.e. should I not allow the fan to turn off completely even if it's not needed. For instance, I have it set up right now where when they first plug it in, the fan does not even come on, then it begins to rev up as the temperature rises...is this a bad idea?

 

[ronv]

DC magnet motors also act as generators. So when they are spinning they generate what is called back emf. This has the effect of lowering the voltage that the motor sees when it is running. (It's also why your power supply can't start your fans from a stop) So when the motor isn't turning the current is quite a bit higher than the running current. Chances are the fans will tolerate this but will heat up a bit. It's probably ok to let them stall so let me recalculate some values tonight. We can also probably optimize the speed range if we don't worry about a stall. Another thing to think about is to get a pot with a switch on it that would allow you to shut them off completely.

 

[jocanon]

I like the idea of a pot with an on/off switch.

 

[ronv]

You can put the switch between the thermistor and the top of the pot.

https://www.mouser.com/catalog/specsheets/TW-700166.pdf