I'm really stuck on this circuit I'm trying to build to heat and cool a block. The thermal blocks heats and cool based on whether I apply a +12/-12V at each of its inputs and I have to swap them manually to invert the heating/cooling function.
I was thinking of using an Arduino that sends a Digital signal to a multiplexer to convert the voltage whenever I reach a certain temperature.
So when I am at 60c, I want to heat, and at 95C I want to cool and cycle back and forth. I want to swap the voltages but I'm not sure if I can do this with a multiplexer. Can someone help me out since I'm not great at this?
Thanks!
You'd end up with a much simpler design by simply using a pair of relays.
Since the thermal blocks likely use a decent amount of power you're running them with a decent set of Transistors or Relays in the first place.
>>1010541
And I noticed you already got the exact same answer on /g/.
Good on them.
>>1010537
>I'm really stuck on this circuit I'm trying to build to heat and cool a block. The thermal blocks heats and cool based on whether I apply a +12/-12V at each of its inputs and I have to swap them manually to invert the heating/cooling function.
I was going to say to use an H-bridge, but somebody in /g/ knew enough to say that already....
An H-bridge is a type of circuit that does exactly what you want here: it swaps polarity between two wires.
You can buy H-bridge ICs that are big enough to run small motors, but Peltier coolers tend to draw rather a lot of current. You may have to build the circuit out of separate parts.
And there is at least a thousand pages online showing that, I'm not explaining it here.
>I was thinking of using an Arduino that sends a Digital signal to a multiplexer to convert the voltage whenever I reach a certain temperature.
That is not what a multiplexer is for.
If you were thinking that you would use some method to apply variable-DC to the Peltier cooler, then what you would want is a ADC (analog-to-digital converter) operating a MOSFET. But you don't need that.
You don't need to supply variable-voltage to this thing, you just run it for ~30 seconds and then check the temp.
>So when I am at 60c, I want to heat, and at 95C I want to cool and cycle back and forth. ...
You do want some 'distance' between the heat and cool thresholds, or else it will constantly ping-pong back and forth between heating and cooling.
Also Peltiers heat better than they cool, so the heating cycles should be shorter than the cooling cycles, probably.... like mebbe 30 seconds for heat, and 45 seconds for cooling.
What are you trying to heat and cool exactly, anyway? Are you using a Peltier?
Personally I would use a configuration with relays instead of mosfets, their current capacity and power supply isolation would make them useful for things that might draw large amounts of power like a heater/cooler (I'm assuming peltier module, those can draw a lot of power if they are properly heat sink'd)
Even now I am imagining how this could be done with analog parts like an instrumentation amp and comparitor with hysteresis.
>>1010585
>>1010602
Yup! I'm using a Peltier Module from Adafruit:
https://www.adafruit.com/product/1335
It draws 4A of current so I was looking at a lot of H-bridges and they seem to max out at 1A which is a problem.
>You do want some 'distance' between the heat and cool thresholds, or else it will constantly ping-pong back and forth between heating and cooling.
Also Peltiers heat better than they cool, so the heating cycles should be shorter than the cooling cycles, probably.... like mebbe 30 seconds for heat, and 45 seconds for cooling.
I'm going to have a feedback loop from my Arduino. In the actual design, I'm going to have a thermocouple on the peltier that monitors the temperature. When the temp hits 95C it will send a 0 to cool down, and when it cools to 60C it will send a 1 to heat up. So this won't be time dependent but based on a feedback of temperature from the peltier module temp.
>What are you trying to heat and cool exactly, anyway? Are you using a Peltier?
Using a Peltier to build a thermocycler for PCR. I want to keep this as simple as possible and inexpensive if possible. I am trying to create an inexpensive thermocycler for PCR where I can monitor temperature and control it fairly precisely with the thermocouple.
The main thing is that there are two leads going into the Peltier and I'm not too experienced so I've been reading a bunch of docs on building these cyclers but can't seem to figure out the right way to switch polarity.
Looks like H Bridge with some modifications to allow higher current is the right way to do it.
>>1010607
>H-bridges and they seem to max out at 1A which is a problem
You could just use a beefier set of transistors. 1A is not an inherent limit of an H-Bridge circuit.
Aight, I threw this together in like a minute. If the switching part is the only thing you are having trouble with then this will help you out, I got lazy and just stuck with abstractions when it came to measurement and arduino stuff. So basically you have a double pull double throw (DPDT) relay (these are cheap and common) which is used to switch the polarity of the power going to the peltier module. The relay itself is controlled by a mosfet which applies whatever voltage the relay is designed to run at (which is usually 5 or twelve volts). Don't forget that inductive spike protection diode across the relay, the relay will likely fry the mosfet if it is missing. Then you got a pulldown resistor on the gate of the mosfet, this is more just good design whenever using a mosfet since the gate can actually gather a small capacitive charge and cause it to go on when it isn't supposed to.
>>1010624
Also holy shit, forgot to turn down the res on my phone.
>>1010624
The only problem I see with this circuit is that the control can't turn off the peltier. It's always heating or cooling, unless you cut the voltage from it somwhow.
Thats why I'd rather have control over each individual relay
