Help repairing fan in fiber optic christmas tree

Thread Starter

nailleb

Joined Jan 24, 2019
8
Hi,
I have one of the fiber optic Christmas trees that has 7 halogen bulbs and a couple of wheels that change the color/light pattern. The fan died on my unit, so it would keep tripping the heat sensor and shut off the bulbs. I bought a replacement fan (60mm x 60mm x 25mm). The power source for this unit appears to be 12v AC. However, the original fan was 12v DC and had a 470 microfarad capacitor (25v) wired in parallel with the fan.

I got a new capacitor to be safe (although the old one looks fine). I wired everything up and turned it on and it worked beautifully for about 10 seconds before the capacitor became a firecracker. I believe the new fan draws closer to 0.2A while the original was rated at 0.1A.

I'm trying to understand the purpose of the capacitor and if maybe I need to change the size since the new motor draws a little more current? I'm a little confused as to whether the capacitor is there to help the DC fan run on AC or whether it's for RM interference? There are no diodes or rectifiers or anything else that would convert the current to DC.

Can someone help? I can add pictures if needed. Thanks!
Brian
 

djsfantasi

Joined Apr 11, 2010
5,617
Depends on the type of capacitor you purchased. An electrolytic cap has a definite polarity requirement. If you connect one to AC, it won’t like being reversed and explode. As you described.

You need a ceramic capacitor of sufficient rating. But ceramic caps typically have a smaller capacitance There is a way of connecting two electrolytic caps that may be used on AC. Like this +—+. But you may want another solution because of the current draw if the fan.
 

AlbertHall

Joined Jun 4, 2014
8,381
A DC fan (with or without a capacitor) is not going to run on AC so something else is going on here.
Can you give us some clear pictures of the works?
 

Thread Starter

nailleb

Joined Jan 24, 2019
8
Thanks for the quick responses!
The replacement capacitor did have polarity and it appeared to be the exact same rating as the original. The replacement fan is also DC. When I get home tonight I will add pictures so you can all see what is going on. I too am confused of how the power supply says AC yet its a DC fan.

Hold tight and I will add pictures tonight.
 

djsfantasi

Joined Apr 11, 2010
5,617
Did you have the capacitor connected with the correct polarity ?

Les.
The power supply is AC.
There is no polarity.
The fan is DC.
The capacitor exploded.

We got a lot going on here.

We don’t know the rating of the new fan. We could put in one diode and run the fan on half wave DC. But it may not be enough voltage to keep the fan running. We could add a full wave rectifier, but the 12V might become higher than the fan is rated for. We don’t know the polarity of the fan. There’s a lot we don’t know.
 

Thread Starter

nailleb

Joined Jan 24, 2019
8
Agreed. I will add pictures. I'm also going to put my meter on the power and see what the power supply is putting out. Maybe there is a DC conversion inside the unit somewhere (you'll understand when I include the pics) because I just don't get how a DC fan was being powered that way. The wires are red and black, at least implying polarity which would also make me think it is DC current. I thought I had the polarity correct on the capacitor but admittedly it was after 11pm, lol.

All of your input and desire to help is much appreciated. Let me get you all some more info so that I'm not wasting your time. Hopefully it will become more clear when I show you more details.
 

Thread Starter

nailleb

Joined Jan 24, 2019
8
OK my next post will be the pics. This is not making sense to me because the power supply is AC and I checked with a meter to be sure. As you can see from the pics, the old fan was DC. 20190124_200142.jpg 20190124_200643.jpg 20190124_200815.jpg 20190124_200840.jpg 20190124_201104.jpg 20190124_201246.jpg 20190124_201254.jpg 20190124_201318.jpg 20190124_201308.jpg
 

Thread Starter

nailleb

Joined Jan 24, 2019
8
The original capacitor was sitting right on the side of the fan (where I cut the leads). It was in parallel and the negative terminal was connected to black.

The new fan also has a blue wire which I ignored in my test - I did red and black.

I have verified that the red and black leads coming from the housing unit are putting out just shy of 12v AC. So, that's kind of a wtf moment with the fan being DC. But, it did spin during my test.

I'm wondering if this is a half-assed design to save money and why the original fan seized up?

Let me know what other thoughts and questions you have.
 

Thread Starter

nailleb

Joined Jan 24, 2019
8
Not that I'm aware of. As far as I know everything is original. However, I did get this from a friend. My friend, though, is not electronically inclined so I seriously doubt he made any modifications. The fan does spin with ac power - that seems odd to me.

I guess my question is what should I do? Should I replace the capacitor with something that will convert to DC? I want it to be safe and it definitely needs the cooling fan else the temp sensor trips.
 

LesJones

Joined Jan 8, 2017
2,325
In your second picture in post #10 it looks like there is a diode (Probably a 1N400x series.) covered with heat shrink sleeving connected to the positive of the capacitor. Am I correct in that assumption ? If so did you connect the diode, fan and capacitor EXACTLY as it was with the original fan ?

Les.
 

djsfantasi

Joined Apr 11, 2010
5,617
In the picture of the original capacitor, it would be instructive if we could see more of the top wire. Center the shot so that this top wire is not cut off at the top and show more of the wire to the right. If this is possible. This is to provide further confirmation of LesJones hypothesis.

If is indeed a diode, the power supply is supplying half wave DC to the fan at about 6VDC on average. The capacitor smooths that out and raised the average VDC somewhat.

If the new cap was connected backwards, since it appears to be an electrolytic cap, in effect you inadvertently made a tiny bomb.

This article may help you understand what I’m talking about.
http://www.bristolwatch.com/ele/basic_ac_rectification.htm
 

AlbertHall

Joined Jun 4, 2014
8,381
If the new cap was connected backwards, since it appears to be an electrolytic cap, in effect you inadvertently made a tiny bomb.
Given that the presumptive diode is still attached to the old capacitor, then it seems likely that the new capacitor was connected to AC and they really don't like that.
 

Thread Starter

nailleb

Joined Jan 24, 2019
8
Lol, I truly am an idiot. I do not know why I did not see the diode underneath the shrink tubing. It is indeed there. I will proceed to put it in the same way it was previously. It all makes sense now.

I must admit, my first experience at building a tiny bomb was less difficult than previously imagined.... and no, I don't sit around imagining making bombs of any sort. :)

I will put it together and test it and let you all know how it goes. You guys are great!
 

Thread Starter

nailleb

Joined Jan 24, 2019
8
Well folks, we have a moral to this story. D.I.O.D.E.

Dumb
Idiot
Omits
Diode from
Electronics

Now that I have a diode in the circuit, I no longer have combustion. I let it run for about 10 minutes and it works beautifully. I thank you all for weathering my moment of stupidity. Your eagerness to help is much appreciated and I've learned a new lesson about creating tiny bombs. I intend to retire from my bomb making at this point having learned a valuable and slightly amusing lesson.

Thank you all!
 
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