12v DC Thermostat Burns Out 12v DC 90mm Cooling Fan

[Omargosh]

And here's a look into one of these Cheapos R Us Matell fans.

[Stray Electron]

Oh, and is there no device online that has the same function as those 3 diodes? I can soder, I'm just lazy 😂

   Why not just use a 7812 12 volt regulator? It's just two wires in and two wires out (one wire, the negative, is shared so it only has three pins) and if you attach it to some metal bit to use as a heat sink the real 7812s (not the fakes) are good for up to an amp.

   I agree that 12.9 volts should not burn out a fan but given the very poor quality of many imported electronic devices today, I can't say that I'm surprised. 

   "  is there no device online that has the same function as those 3 diodes?"

    Three diodes operate differently then a single 7812.  A 7812 regulates (limits) the output voltage to 12 volts regardless of the input voltage (within certain limits, see the spec sheet). But a silicon diode is simply a 0.6 (nominal) volt drop so three diodes would give you a 1.8 volt drop regardless of the input voltage (they will operate up to 400 volts or so depending on the diode). That's fine if your PSU outputs 13 VDC but if it happens to output say 16VDC then the output after the diodes will still be 14.2 VDC. A 7812 regulates the voltage but diodes are simply a fixed voltage drop. I've used both in circuits but for a poorly regulated input and for a fraction of an amp I prefer to use a 7812.  The 7812 and similar regulators will also shutdown if the current is too high and they over heat so they do add a bit of protection.  The 7812s (and similar) operate by switching the power off and on at high frequency (pulse modulation) so they do generate some electrical noise on the output, and usually, you need to add input and output filter capacitors to them to reduce the electrical noise but for a mechanical device such as a fan, you shouldn't need them.

  PS 7812s are very common and are used in many devices so if you're operating on the cheap, you can easily find 7812s in things like wall warts. electric clocks, transistor radios, timer boards for washing machines etc and they're also very widely available as surplus. In the US it's not hard to find partial rolls (end of the reel) of brand new high quality ones for a few cents per part. 

[themadhippy]

Why not just use a 7812 12 volt regulator?

Dont they have a  2v minimum drop out voltage?

[Stray Electron]

Why not just use a 7812 12 volt regulator?

Dont they have a  2v minimum drop out voltage?

   I think that all of the ones made for a long time now have a drop out voltage of about .6 volts and I think that some of the newest have a drop out that is less than that.  They are made by many companies and in a wide range of specs. I would look up the data sheet for anything that I was considering using and see what all of the specs (current, heat sinking etc) are before using it. And also a fan would probably operate just fine even if the output dropped below 12 VDC.

[Zero999]

Oh! I really like the idea of using this fan! The biggest that will fit in there is 90mm. I looked in DigiKey and is it just me or do they not sell a 90mm fan?

How about filing it down to size? You should be able to get away with taking 1mm either side off.

90mm doesn't appear to be a standard size. 92mm is very common. Perhaps it's imperial. 92mm = 3 ⁵/₈ inches.

Why not just use a 7812 12 volt regulator?

Dont they have a  2v minimum drop out voltage?

So what?

The output voltage might be a little below 12V, but it will still work.

Heck, the fan draws only 0.2A whilst the 7812 is rated to over 1A, so it'll probably still output 12V. The graph on the data sheet shows a dropout voltage of under 1.75V, with an output current of 500mA, so at 200mA, it'll be even better.

https://www.ti.com/lit/ds/symlink/lm340.pdf

[TimFox]

Actual TI 7812 regulators are rated for typical dropout voltage of 2 V, and recommended minimum operating input voltage of 12 + 2.5 V.
https://www.ti.com/lit/ds/symlink/ua78.pdf?ts=1741961745062&ref_url=https%253A%252F%252Fwww.google.com%252F
There are other regulator ICs, with very different insides, that have lower dropout voltage.

[Gyro]

Hence the LM2940 suggestion.

Try powering the thermostat and fan via an LM2940CT-12 linear regulator (use the typical application circuit in the datasheet). It has a dropout voltage of around 150mV @ 200mA and very predictable dropout behavior. It also has automotive grade input transient and reverse polarity protection... https://www.ti.com/lit/ds/symlink/lm2940c.pdf.

If the controller really is putting out 13.9V then it won't need a heatsink. If there are transients (of less than 45V), it should mop them up.

[TimFox]

I was referring to the post above (by StrayElectron) about 7812s having 0.6 V dropout.

[Zero999]

Actual TI 7812 regulators are rated for typical dropout voltage of 2 V, and recommended minimum operating input voltage of 12 + 2.5 V.
https://www.ti.com/lit/ds/symlink/ua78.pdf?ts=1741961745062&ref_url=https%253A%252F%252Fwww.google.com%252F
There are other regulator ICs, with very different insides, that have lower dropout voltage.

Did you not read my other post?

1) The dropout voltage will be much lower at only 250mA out. 2V is specified at 1A.

2) It doesn't matter if it goes into dropout, because a regulated power supply is not required for a fan, which will still work at 11V.

3) Just use any plain old µA7812 or LM7812.

[Stray Electron]

[
   Why not just use a 7812 12 volt regulator?

   What I had in mind, and should have said more clearly, was a 7812 type 3-terminal voltage regulator . They are made by a large number of companies today and most newer parts are a great improvement over the original 7812 and I'd sure that the OP can find one that will suit his purposes.

[floobydust]

From the disassembly pic, fan controller IC OCS477H (same as OCH?) rated for 24V use BUT it must have an input capacitor. There is none on the PCB.
"In the 3~18V application in the right figure, C2 is an optional capacitor to improve IC reliability."
I would say the fan is not overvoltaged but is getting killed from the turn on spike/line inductance, the lack of input capacitor.
PS -> The axle C-clip is behind the sticker usually, no need to karate the blades off.

[TimFox]

[
   Why not just use a 7812 12 volt regulator?

   What I had in mind, and should have said more clearly, was a 7812 type 3-terminal voltage regulator . They are made by a large number of companies today and most newer parts are a great improvement over the original 7812 and I'd sure that the OP can find one that will suit his purposes.

Don't say "7812-type" if you mean a low-dropout linear regulator, which are not 7812-type and should not have 7812 part numbers.
Both 7812s and low-dropout regulators have their uses, but should not be confused.

[Omargosh]

Thanks for the reply! Don't forget these are the cheapo Barbie fans. No clip under the sticker on either side. It's like they were molded like that. So I drilled away just enough so I could break away in mostly one piece. But either way, where's the fun in no karate? Thanks anyway, though!

[Omargosh]

Ok, big reveal. Drumroll please .......IT WORKS!! Well, hopefully I didn't jinx it just now. Lol! But I connected while load out from thermostat was switched off. Triggered it on with the sensor, left it on for 5 min. It's a bright and sunny day and voltage to the fan (past the diodes) was very steady at 11.27. Triggered on/off 3 more times and still works.
So I'll clean it up some by shortening the diode contacts, resoldering and insulating it.
If there are any further developments I'll let you know. Otherwise....
THANK YOU SO MUCH!!!! ALL of you! Sure, some of it sounded like Huttese to me due to my lack of knowledge in this field. Lol! But I genuinely appreciate all of you for taking the time to read, discuss, and guide me!! I was completely lost. So again, THANK YOU!!!! 🙏🏻
May you all have a wonderful day!

[floobydust]

I'm not sold on it, I think the fan needs the input capacitor- but time will tell. Because it's a mosfet H-bridge drive and the back-EMF has no where to go when the relay contacts open. Hopefully I'm wrong. Or ignore me and load up on champagne lol
Always have a fuse just for this thermostat/fan circuit anyhow because battery power and skinny wires does not go well.