DIY Safe Peltier Dehumidifier

This is more a demonstration.  It will have to be modified to fit whatever components you might use as they are easily available.  It's simply a box and simple ducting, hot air up, cold down.  Purpose of the box is so there is no air leakage into the door cavity which isn't sealed well.  I reused the heat sink and fan housing but that could be printed to.

Before this I thought it wasn't working well but I had a dud cheap temperature humidity sensor and its actually working quite well for the last year.  I use Home assistant to control it.  It keeps humidity generally down to around 35%.   I've reset controls so it has about a 50% duty cycle while the solar is up.  The residential type sensors aren't terribly good.  I'm using Ecowitt sensor which is good enough, could be it can't show below 35%.

It seems the the peltier performance is limited to getting down to ~10g/m3 absolute humidity .  You can see this as the Relative Humidity in the safe will go up if the temperature drops.

It's about same wattage (20w) as the heat rod I was using before so win win.  The original dehumidifier's life was short as it was crushed, not much room in there.  Took it apart and fitted it into the door.  It does also heat the safe by about 4deg.

If I open the door on a humid day it takes a couple of hours to drop it from say 75% to 40%.  

Apr '25

Proving to be very reliable and not had any issues with Dehumidifier.  Everything in safe is remaining in good condition with no sign of rust, even where bluing is very thin and had signs of rust previously.

Sept '25

The AliExpress power board capacitors blew leading to an interesting exercise finding a replacement power supply.  Had old 12v switch mode power supply that I thought would be more efficient and power,  no it overstressed the heat sink.  Added a pwm to reduce the wattage and it kind of worked but not as well.  Turns out these Peltier's are extremely dependent on the hot side heat sink and a constant voltage, pwm's half the efficiency.

Repaired the AliExpress PS and found it is supplying 9v at 24w to the 30x30 17004 Peltier and fan.  So I tried to boost it up with a 40x40 17006 and found it quickly overpowers the heat sink and needs to be kept to less than 40w by restricting voltage 7-8v.

So it's gone back to the stock 17004 and 9v which is just adequate to keep humidity down to 40% and temp 25degC.  While I think about the difficulties of ducting and cooling to use the 17006 to supercharge it, interestingly I've got an old cpu water-cooled block with a rattly fan bearing.  Trouble is upping to 60w is giving temps over 30degC.

When you look into the design of these things it turns out it's really complicated balancing all the specs and performances.  Lots of formulas, charts and science.  So final advice is there are lots of cheap kits on AliExpress and they're a pretty good place to start and cheaper than buying parts individually.  Turns out the unit I have is a pretty close match for my needs.  It should be easy to print a mounting and allow for airflow to suite your application.