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Why is this Inductor burning?

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EE-2002

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Please see the attachment.
+12V comes into the board through a pin on PCI connector. When I power up the board, the plastic wrapper on the 3.3mH inductor starts melting. I replaced both capacitors and the inductor and it still happens.
With another board ( which is good) inductor does not burn, so there is no problem with the +12V source
datasheet for inductor: **broken link removed** (#187LY-332J)
Can anyone please give suggestions on why this might be happening?

Thanks for your time.
 

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  • Inductor.pdf
    12 KB · Views: 283

Is this a trick question? Because much more than 100 ma is flowing through it.
 

No trick !
On a good board, this inductor does not burn.
V1 (+12V) is used by 2 VCOs ( rated 20mA max. per VCO)
Do you think one or both of the VCOs could be bad and both VCOs combined could be trying to pull more than 100mA?
 

There are only 2 ways to burn an inductor.

95% are simply over currented. Short on the board, etc. Power it all up with a milliamp meter on the 12v line and see what it says. twist the board around, tap/push on the components to get whatever it is that is short circuiting to do it. It is probably a good idea to turn down the current limit on the power supply (assuming it is a lab supply) so you do not fry the inductor again when troubleshooting.

5% of inductors overheat and the insulation burns off simply because you are varying the magnetic field too much and not heat sinking the coil--as in a switching regulator. You are not using a switcher though.
 
biff44 said:
There are only 2 ways to burn an inductor.

95% are simply over currented. Short on the board, etc. Power it all up with a milliamp meter on the 12v line and see what it says. twist the board around, tap/push on the components to get whatever it is that is short circuiting to do it. It is probably a good idea to turn down the current limit on the power supply (assuming it is a lab supply) so you do not fry the inductor again when troubleshooting.

5% of inductors overheat and the insulation burns off simply because you are varying the magnetic field too much and not heat sinking the coil--as in a switching regulator. You are not using a switcher though.
Click to expand...

I strongly agree with biff44's comment. The majority of times I've seen this has been something like a solder bridge between power and a nearby GND trace, a diode installed backwards, part installed backwards, etc. Start by checking any parts on the 12V node, verify diode orientations, looking for solder bridges, etc. If there are only a few devices on the PCB that use 12V, remove them (or lift/disconnect the supply pin from the board). If that doesn't fix it, remove the parts and check for shorts underneath them. Failing that, maybe start cutting the 12V traces to isolate portions of the line, then use an ohmmeter to check for high resistance... if it's low, then that section of trace has a problem.
 
biff44 and enjunear: Thanks a lot for taking the time to respond.
If there is a short, shouldn't I be able to see it by checking between +12V and GND using a multimeter? Unfortunately, I cannot power up the board directly with a lab power supply. It is powered up through a PCI Backplane.
There are several components that use 12V but only 2 VCOs that use the V1 (Shown in Schematic). So the current flowing through Inductor is going to only 2 VCOs which are rated as having 20 mA max each.
Maybe I should pull out both VCOs and put a new Inductor and power up the board and see if it will burn again ?

Thanks for your time.
 

Do you own a DC current meter?

To make it hot you need to be drawing >0.3 amps. To make it burn, maybe 3 to 5 times that.

Do you have a filter cap (220 uF) on output that can't take 12vdc?
 
Last edited:
As a start, just use an ohmeter and measure resistance from the 12v pin to ground (with the power supply turned off and disconnected, of course). 100 ma = 12V/120 ohms. So if you have a resistance anywhere less than 120 ohms in the steady state, you know there is a problem.

This will not catch every sort of possible failure or start up condition, but it will certainly find the dead shorts!
 
A totally off the wall suggestion, check for a high frequency AC signal on the 12V line, using an oscilloscope.
 
EE-2002 said:
biff44 and enjunear: Thanks a lot for taking the time to respond.
If there is a short, shouldn't I be able to see it by checking between +12V and GND using a multimeter? Unfortunately, I cannot power up the board directly with a lab power supply. It is powered up through a PCI Backplane.
There are several components that use 12V but only 2 VCOs that use the V1 (Shown in Schematic). So the current flowing through Inductor is going to only 2 VCOs which are rated as having 20 mA max each.
Maybe I should pull out both VCOs and put a new Inductor and power up the board and see if it will burn again ?

Thanks for your time.
Click to expand...

Like biff4 suggested, isolate the board (disconnect it from backplane), and use an ohmmeter from V1 (filtered 12V) to GND. If it's low, start pulling off the loads (VCOs) one at a time and recheck the resistance. If both loads are gone, then remove the caps one at a time (might have a shorted cap... check voltage ratings, per RCinFLA). Lastly remove the inductor. If everything is removed and it's still low resistance, then hunt for shorts. Once you find the culprit, power up the board with the parts off. Add them back one at a time (coil, caps, then VCOs) to make sure you don't have more than one bad part.
 

klystron said:
A totally off the wall suggestion, check for a high frequency AC signal on the 12V line, using an oscilloscope.
Click to expand...

There is a very small chance that there is some sort of latch-up problem in the 2 oscillators, where they take a lot more current at start up. But the much more likely case is that there is a dead short on the board somewhere.
 
klystron said:
A totally off the wall suggestion, check for a high frequency AC signal on the 12V line, using an oscilloscope.
Click to expand...

On this board, that is highly unlikely, even if there is one, the caps should take care of it, right ?

---------- Post added at 20:51 ---------- Previous post was at 20:48 ----------
biff44 said:
There is a very small chance that there is some sort of latch-up problem in the 2 oscillators, where they take a lot more current at start up. But the much more likely case is that there is a dead short on the board somewhere.
Click to expand...

I checked with Mini-Circuits ( VCO is POS-150) and they said all VCOs together will not take 100mA

---------- Post added at 21:04 ---------- Previous post was at 20:51 ----------
RCinFLA said:
Do you own a DC current meter?

To make it hot you need to be drawing >0.3 amps. To make it burn, maybe 3 to 5 times that.

Do you have a filter cap (220 uF) on output that can't take 12vdc?
Click to expand...

No. 220uF cap shown in the attachment is rated 16V.
 

Jeez, looks like a waste of your time with all that. What happened with the ohmeter test???
 

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