Question about reverse polarizing using solid state - Page 1
Forum section: electronics / projects

author: EasyGoing1
author_rank: Regular Contributor
timestamp: October 11, 2024, 04:51:22 am
content: I am looking for feedback on whether or not this circuit will work.I am considering purchasing a peltier based cooling module (https://www.amazon.com/gp/product/B07HCBMJM4) that will be used to keep a cabinet at relatively consistent temp (86 degrees F). I live in a desert and sometimes the temps can vary widely on either side of the target and the cabinet will be in a garage. The idea here is that if the temp needs to be brought down to target, I would run the cooler so that the heat sinks inside the cabinet run cool to bring the cabinet to the target temp. However, if the temp inside the cabinet is too cool, then I would reverse polarity on the cooling chips so that heat would be used to bring the inside temp up to the target temp. There will be a larger fan inside the cabinet to circulate air throughout the cabinet and it will be blowing directly over the smaller heat sinks from this device.Here is the schematic I came up with to allow me to switch polarity on the peltier devices.I wil use a microcontroller to handle the switching.All MOSFETs will have a 1k resistor at the gate (removed from schematic for simplicity)The arrows indicate the direction of current flow.P21 will be brought low, followed by a delay, then P20 will go high. This will activate cooling.P20 will be brought low, followed by a delay, then P21 will go high. This will activate heating.If either P20 or P21 is high, P22 will also be high, which keeps the fans running on the heat sinks and they always run in the same direction (moving air onto the heat sinks).The MOSFETs are all logic level MOSFETs and are rated to handle more than 4 times the amount of current that the peltiers will use.Will this circuit work?

author: IanB
author_rank: Super Contributor
timestamp: October 11, 2024, 05:05:00 am
content: It may or may not work, electrically. But the headline says "12 V, 12 A" which is 144 watts power consumption. On the other hand, the description says "8 watts". This might mean 8 watts of cooling, which is not very much.You might like to watch this video about the performance (or lack thereof) of Peltier coolers, before you go too far down your intended road:

author: moffy
author_rank: Super Contributor
timestamp: October 11, 2024, 06:13:30 am
content: All the MOSFETs are P-Channel and configured so that the body diodes will provide short circuits. What you need is a full H bridge and drivers, though IanB's comments are valid, peltier effect devices are horrendously inefficient, but in your case might be a simple choice.

author: EasyGoing1
author_rank: Regular Contributor
timestamp: October 11, 2024, 04:34:14 pm
content: Quote from: moffy on October 11, 2024, 06:13:30 amI cannot think of any other way to get what I need other than using peltier. The cabinet will be used to cultivate home grown mushrooms and I simply need to keep the inside of the cabinet around a certain temp. I understand all of the downsides to using peltier but I'm still stuck on ... what other choice do I have?I'll look at an H-bridge config ... I had not thought of that but it makes perfect sense.I updated the drawing to N-type MOSFETsMike

author: BrokenYugo
author_rank: Super Contributor
timestamp: October 11, 2024, 04:43:19 pm
content: Put a heater in a compressor type mini fridge? Replace thermostat with a proper controller, maybe add a circulation fan, should do well and not drive up the power bill like a peltier device running around the clock will.

author: EasyGoing1
author_rank: Regular Contributor
timestamp: October 11, 2024, 05:14:21 pm
content: Quote from: moffy on October 11, 2024, 06:13:30 amSomething like this?

author: EasyGoing1
author_rank: Regular Contributor
timestamp: October 11, 2024, 05:15:46 pm
content: Quote from: BrokenYugo on October 11, 2024, 04:43:19 pmBut the issue is, I will need to use both heat and cooling to maintain temp inside the cabinet. When the months start getting colder, the cooling side of the unit won't be used at all. The peltier will not be running around the clock, I know that for sure.

author: tom66
author_rank: Super Contributor
timestamp: October 11, 2024, 05:27:12 pm
content: If you put an ordinary strip heater (100-200W, aquarium heaters come to mind) inside a mini fridge it will heat up pretty well. It will less efficient than when refrigerating, but the net efficiency would likely be better than a Peltier. You would simply use a dual output thermostat to switch between the fridge function and the heater function with enough hysteresis between the two to avoid oscillating between heat and cold.

author: TimFox
author_rank: Super Contributor
timestamp: October 11, 2024, 05:34:14 pm
content: Historical note: before the “energy crisis” of 1974, it was common in large buildings to run the cooling at full output and control the temperature by regulating the heating.You can reverse this approach for your chamber.Before I retired, we stabilized the temperature for x-ray detectors with Peltier coolers to slightly below room temperature (in lab environments). The main engineering problem was cooling the heat sink (hot end), often requiring a fan, since the heat load into the sink was the sum of the heat removed plus the power applied to the Peltier, the latter being higher than the former.

author: IanB
author_rank: Super Contributor
timestamp: October 11, 2024, 06:16:05 pm
content: Quote from: BrokenYugo on October 11, 2024, 04:43:19 pmQuote from: tom66 on October 11, 2024, 05:27:12 pmThis seems to me like a good approach.

author: moffy
author_rank: Super Contributor
timestamp: October 11, 2024, 09:43:38 pm
content: Quote from: EasyGoing1 on October 11, 2024, 05:14:21 pmYes with a suitable driver IC.

author: EasyGoing1
author_rank: Regular Contributor
timestamp: October 13, 2024, 09:59:49 pm
content: Quote from: moffy on October 11, 2024, 09:43:38 pmWhen you say "Driver IC", I'm assuming you're referring to the IC that will drive the MOSFETs. In this case, I will be using a Raspberry Pi Pico microcontroller and writing the code myself.Mike

author: moffy
author_rank: Super Contributor
timestamp: October 13, 2024, 11:26:58 pm
content: Quote from: EasyGoing1 on October 13, 2024, 09:59:49 pmThe high side N-FETs require a voltage above the supply to drive the gates, so you would need a driver IC that can generate that extra voltage unless you generate it with an external supply. The other thing the high and low side drivers have to do is translate the logic signal into a suitable gate drive signal. A slight problem is that most MOSFET applications are repetitive fast switching and that is used by the high side drivers to generate the extra voltage, but in your case since it is static you would need to provide that voltage via a small supply. If you change the high side N-FETs to P-FETs then you won't need that extra supply and can use a simple transistor to switch the gate.P.S. Missed the fact that the high side MOSFETs are P type, thanks to PCB.Wiz

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 13, 2024, 11:28:53 pm
content: Quote from: EasyGoing1 on October 11, 2024, 04:34:14 pmMushrooms like humidity, have you considered a small evaporative cooler? Have a servo control the amount of recirculated vs. fresh air as the first stage of temperature regulation, second stage would be a small heater of some sort.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 14, 2024, 01:52:17 am
content: Quote from: EasyGoing1 on October 11, 2024, 05:14:21 pmThat shows PMOS and NMOS parts, so is relatively easy to drive at 100%, but you will need extreme care to avoid turning same-side FETs on at the same time, and crowbar the 12V.Gate drivers for PMOS and NMOS do exist but are somewhat rare, eg FAN3278, and that does not take care of deadband.eg 2 x FAN3278 and a 74HCS238 or HCS237 could manage HW deadtime, and enable ( & even over current), giving low risk SW control of Direction and On/off.NMOS gate drivers are plentiful and cheap, and many half-bridge drivers include deadband, but they do not naturally drive 100%.You might decide 98% is near enough and set a PWM from your PI, or add more parts and create an upper side charge pump.You would also need a fan control for the Peltier.I see some use water jackets, which can help keep the hot part well clear of the chamber.

author: Marco
author_rank: Super Contributor
timestamp: October 14, 2024, 03:27:05 am
content: Quote from: BrokenYugo on October 11, 2024, 04:43:19 pmI'd just run an insulated coolant loop from a fridge or freezer, you can build the cabinet any way you want and use the fridge/freezer.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: October 14, 2024, 09:11:21 am
content: The bridge will switch so infrequently that I would consider mechanical relays and drive the coils with BJTs. Actually, reversing current flow can be done with a single two pole form-C relay.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 14, 2024, 07:13:14 pm
content: Quote from: Ground_Loop on October 14, 2024, 09:11:21 amYes, or even a decent switch, while this is mocked up and the heat flows confirmed, before spending too much time and effort on fancy electronic control.If this is a fixed location, the OP may be better using nature to do most of the work : have two ducting air flows, a cool one from underfloor, or on a concrete slab and one from a roof space.Some modest heating may be needed for coldest nights, but thermal mass takes care of most and it changes to a thermistor and fan timing problem.