I’m converting a mini-fridge into a fermentation chamber to brew my own beer at home. My project consists of two PCBs connected via I2C. This question concerns the second PCB located inside the fridge.
It contains a TC74 temp sensor and a PCF8574 which is used to control a relay for the refrigerator compressor and a circulation fan which runs on a timing interval.
The relay coil draws 65ma and the fan draws about 70ma.
This is the first time I’ve used a PCF8574 and it’s set up to sink current from the base of a high side PNP transistor which applies power to the relay coil and circulation fan.
I’ve always used a flyback diode in parallel with an inductive load when low side switching.
Do I still need a flyback diode across the relay and fan coils when high side switching??
Seems to me that as the coil field collapsed and induced voltage, it would just pass straight to ground and there is no need for the diode.
I'd include the diodes, cheap insurance. The back-EMF is likely to be a relatively high voltage, so not something I'd want running around. Would hate to see hand-crafted beer ruined
I guess it's better safe than sorry, but I'm thinking they are not necessary when high side switching. I'm going to do some testing this weekend to validate this...after I brew another batch of course.
The problem, to my thinking, is that the back-EMF might produce a high negative voltage at the collector of the pnp. If the voltage is high enough it might exceed the max reverse voltage on the base-collector junction.
My vote is to err on the side of caution and add the diode.
Whether you switch high or low the back Emf pulse is still present and the pulse should be dealt with, It should never be ignored. A diode works well across the coil and If I was really concerned about noise I would use a zener diode.. a 15 V 1 W zener works well to clamp small to medium sized relay coils for both excessive forward and reverse coil voltages.
michinyon:
Unless your climate is very bizarre, it seems unlikely that you actually want to try and ferment beer in a fridge.
Maybe he lives in a cold place and has connected the pump backwards.
(Which is also why he needs high-side switching...)
Maybe he's doing the unthinkable and using the well insulated fridge with a small heating element to maintain a nice fermenting temperature with little or no heat loss through the side walls / door etc....
Thanks for the interest and the replies. Sorry I took forever to get back on here myself. Life calls.
I kept the diodes in the circuit just in case and it is working just fine.
Zapro:
Why even use high-side switching ? Why just not use a N-mosfet or NPN transistor - the outputs on the PCF8574 isn't Open-collector...
Easy answer there is that I sidnt have any N-MOSFETs in my box. As for the NPN BJT, when I noticed from the datasheet that the PCF8574 was designed to sink FAR more current than source, I just decided to play along with that concept. I guess it could work either way.
michinyon:
Have you made beer before ?
Don't most people use a 23 liter container, will that fit in a "mini fridge"
Unless your climate is very bizarre, it seems unlikely that you actually want to try and ferment beer in a fridge.
Yes, I'm an avid homebrewer.
I have a 6 gallon (~23L) carboy for primary fermentation and a 5 gallon for secondary (if I even use a secondary). I modified the fridge a bit to get it to fit. The cooling plate was lowered and I swapped the low grade insulation in the door for some thin high grade stuff. Adding a thin whiteboard on top of that still left just enough room for my largest carboy.
I just moved to south Texas and no longer have a basement. For most of the year it is far to warm in the garage, so I've been designing this fermentation chamber. Also, now I'll be able to try lagering!
majenko:
Maybe he's doing the unthinkable and using the well insulated fridge with a small heating element to maintain a nice fermenting temperature with little or no heat loss through the side walls / door etc....
I've got spare pins available on the PCF8574. When it cools off again I just may add another relay and a heating element of some kind
