The peltier cooler I'm looking at calls for a max voltage input of 29.8 VDC and a max current of 6A (4.2A recommended).
A way to control the peltier cooler's temperature so condensation occurs without frosting over.
A component to turn the peltier cooler on and off so my sketch can maintain humidity at desired level.
A component to turn the ventilation fan on and off according to need.
Who recommended that? They should be fired.
Current is pulled, not pushed. The cooler will pull whatever current it needs. Your power supply needs to be able to supply that amount of current.
First thing you need is a way to measure the temperature, and humidity, in the cabinet.
The cooler may be PWM controllable, or it may not. If not, you would just need to turn it on and off at some interval, experimentally determined, that prevents the device from freezing.
Perhaps it means something other than what I thought it meant?
I want to build a dehumidification cabinet for my guitars and need advice on what parts to get.
What is the ideal humidity level / temperature for your guitars?
Maybe just need one or two packs of Zorb-it for your cabinet.
There are two different and conflicting issues here. The first is that a traditional PWM scheme is a bad idea because it will be inefficient. As you increase the voltage, or current through a Peltier the cooling output will increase reach a peak and then decrease due to the additional power dissipated in the Peltier. In a PWM controll scheme you are emulating a lower supply voltage / current by pulsing a higher one on and off. But in this case the higher your input voltage is the less efficient the device will be, so you are much better off having a lower steady input voltage than a higher pulsed input voltage from an efficiency standpoint. That said, if your Peltier is sufficiently oversized you might be able to just write off the efficiency loss. The other issue with PWM is thermal cycling. Anything heated and cooled enough times will be mechanically harmed by the effects of differential expansion and contraction of materials. What you want to avoid is a rapid thermal cycling of the Peltier device. If you PWM quickly (faster than 10 Hz) no appreciable temperature change will occur in the device during the cycle and you should be fine. If you can capacitivly filter your PWM signal so that it is essentially a DC signal you will solve both of these issues at one time, so that is the best solution, but a quick PWM with a maximum voltage that is still in the good efficiency range would probably work.
Which are you trying to control - temperature or humidity?
Have you researched humidors? An oversize humidor seems like what you are trying to create.
There's no PWM needed here, or putting it another way there's no reason why you'd need to switch the peltier on/off more than a couple times a second to keep a reasonably stable temperature.
"Significant precautions must be employed with PWM, however. First of all, the PWM should be at a high enough frequency to minimize thermal stresses to the TE devices. While we like to keep the frequency in the low killihertz (Hz) range, in many applications these days we must compromise at around 120 Hz for the sake of electromagnetic compatibility. Another important issue is the potential for generating electro-magnetic interference (EMI) in the wiring to the TE device. If you are using PWM, you may need to shield your power wiring or keep it away from any sensitive electrical signals."