Not likely as the peltier is quite robust and it is run at a current that is within the specs. of the power transistor.Mind you the peltier is rated at about 3.9Amps,8 Watts, 3.8 Volts ( this is max @ 0 degree centigrade). In actuality when set at the blue range it's not even at maximum of peltier.Of course this peltier is a small one, larger drycabs may have bigger ones and higher voltage supply.There is extra footprint for another power transistor on the pcb.The bad component is a 400V capacitor of the switching power supply.It had zero capacitance (22uF.) and rated for 2000 hours at 105C only.Can you guess how many hours it is when it has been working for about 7 years X 365 days? FYI the power supply switches at a frequency of 25Khz. or more. Normally 105 degree C rated caps are used as it gets hot internally inside the cap.The electrolyte in the cap would have dried up and hence reduced capacitance or not function at all.The cap costs me $2.50. The experience gained is priceless.Cool tinkering! I wonder if this is also the 'repair' that is done by them when we send it in... replacing the peltier module?
I have almost the same SMPS in my PCB. It is a much older unit (i think from 2010). The 22uF capacitor had bulged and the MOSFET shorted causing the fuse to blow up. I replaced the Capacitor but before I could order the MOSFET, I misplaced the MOSFET. Now I don't know which MOSFET to order. Is it possible to let me know the MOSFET which is there in this circuit? Right now I have done some drastic surgery on the unit and feeding it 3.8 volts from an external power supply. I have edited the original image and put a label - "MOSFET" for the part I need to know. If anyone can let me know the markings on the MOSFET, I will be grateful. Thanking you in advanceI have not seen a tear down of a dry cabinet dehumidifier unit on the internet not even in electronics website so I will do it here.Do note you are dealing with 230 Vac.from the wall so it can kill so be forewarned! Do not try if you have no knowledge of electricity or soldering/desoldering tools. I found this digi-cabi DB-036 (36 liters)left for dead at the void deck. Since I'm also an electronics hobbyist I thought I might try to repair it.I suspect this post will be highly viewed from now on.:bsmilie:
The diagnosis: DOA (dead on arrival) On power-up the red led lit up but is dim after I replaced the switchmode power supply capacitor (22uf 400V 105 degree C) which was open circuit ( no capacitance reading).I then replace red led with a 8mm super bright transparent bulb led. 8mm is non existent in sim lim tower.I suggest you use 5mm. anyway it's just a light indicator.Of course you can buy 8mm. at element14 at taiseng but subject to minimum $10 order.So far I have not tested if it still work as I measured the voltage to peltier device at -2V supply.That's right negative sign as I followed same colour probes of multimeter to the correspong wires.FYI if check on the net if you supply 12V to peltier device ice will form almost in seconds! Don't know how big is the device but assuming like for like a 2V. supply make sense as it will slowly do it's job? I think the inside of the cabinet is the cold side as there is a curved recepticle at the bottom of the peltier draining to the outside of the cabinet.
A picture is worth a thousand words: The dehumidifier unit removed from cab.
The pcb close-up: If you can't get the exact size capacitor height-wise then remove one of the plastic panels give space not so professional but it solves the problem of obstruction.
This will expose it to the elements/dust or cut a hole?
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Hi, Your shorted device is not a MOSFET but a bipolar transistor npn type Japanese fully insulated device 2SC3866. You can try Sim Lim tower or order from Lazada but what I saw was lots of 10pcs.I have almost the same SMPS in my PCB. It is a much older unit (i think from 2010). The 22uF capacitor had bulged and the MOSFET shorted causing the fuse to blow up. I replaced the Capacitor but before I could order the MOSFET, I misplaced the MOSFET. Now I don't know which MOSFET to order. Is it possible to let me know the MOSFET which is there in this circuit? Right now I have done some drastic surgery on the unit and feeding it 3.8 volts from an external power supply. I have edited the original image and put a label - "MOSFET" for the part I need to know. If anyone can let me know the markings on the MOSFET, I will be grateful. Thanking you in advance
Hi, one eye jack. Thanks for sharing your thoughts trying to fix the dry cabinet. Let just share my thoughts on what you did.UPDATE: Good news and irony!
As reported in my previous post that even after 5 days of power up the RH stayed at 80% so obviously something still is not working and I checked the resistance of the thermoelectric module or peltier unit's postive and negative terminal after unplugging from connector showed zero ohm which mean a dead short. Yesterday I ordered a suitable peltier unit from Element14 at Taiseng rd. order no: 2466914 and paid $30.20 gst.Since stock is in uk it will arrive between 3 - 5 business days.
Later at night I removed the peltier unit from heatsink assembly, cleaned up the heatsink compound from both sides of the peltier unit and measured the resistance to confirm...it had a resistance of 1.5 ohm and 3.9 ohm depending on the polarity of the meter probes! What the...so to really test if peltier is functional or not I plug it back into the control circuit and power it up at the same time holding it between my fingers,the adjustment pot is at 12 o'clock position in the middle of the blue range (recommended).I also noticed not only the red power Led is lit but the green Led too faintly.(I also check the components
of the control circuit and they all test functional from what I can guess)
Sensing no heat at my fingers I cranked clockwise towards the red range and
wow it got hot fast to burn my fingers.Turn knob back to 12 o'clock and placed the hot side on heatsink and power on again but now my finger is pressing down on the cold side with hot side on heat sink and now I feel feel peltier is getting cold! That means peltier is working. Note that when you turn knob towards red range (clockwise) the green Led gradually brightens.
I reapplied heatsink compound to both sides of the peltier unit and placed the hot side on the big heatsink assembly and a small heatsink on the cold side facing the inside of the cabinet.Do not over tighten the screws with the round plastic clamps to heatsink, just enough that the heatsink does not move or slide easily with a push of the finger or else you night damage peltier or worse crack the ceramic body.
Then I reassemble whole unit back into drycab and power up.Within 30 minutes the RH dropped 1%. It's encouraging...this morning after 7.5 hours the reading is 62% down from the starting 80%. Woohoo.. it's working although it's still far from the recommended 45% - 55%.After about 45 min. it dropped another 1% to 61%. As I looked at the hygrometer reading just now it's still at 61% so the RH is not coming down in a linear manner vs time but calculating by 7.5 hrs. 80% - 62% is a difference of 18%.If you divide 18 by 7.5 you get 2.4%. so ASSUMING linearly it will take another 16 hours to get to target range but I don't think it's going to happen but longer at current 12 o'clock setting or cold temperature.It may mean that to get to target range Rh (relative humidity) I need to increase the coldness but still remaing in the blue range.
A little about how a peltier works in a drycab, A peltier is a semiconductor device made up of P and N junctions.Please google for more detailed discription. It has a hot and cold side. Technically it is a heatpump. Depending on the polarity of the power supply at it's positive and minus terminals which side get hot or cold.The cold side is used in the drycab to attract moisture to form on the small heatsink that becomes cold instead of hot.Condensation (water) forms on it and if alot drips into the "cup" and overflows into the sponge "outside" where there is an open slot so that
water can evaporate with the help of heat dissipated by the big heatsink or naturally.Mostly like thsi will not occur as mositure is little as can be seen in photo.
Generally if set in blue range not much heat is produced and this means not much electrical power is consumed but if you want RH to change faster you can also crank up more meaning get peltier colder but at the same time more heat is dissipated.Since I'm a miser..haha I will monitor tommorow if RH falls or not to target range and adjust accordingly.
Photo below to show proof that drycab is working. To be continued...
Note: I did not attach the peltier cover as I want to see and monitor the peltier.
My standalone hygrometer is for comparison, reading is not far off (63% vs 61%) I don't care about temperature but I think drycab meter can be adjusted but I'm going to leave it as is.
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