Inductor Calculation?

Discussion in 'General Electronics Chat' started by apprenticemart2, Oct 22, 2013.

  1. apprenticemart2

    Thread Starter Member

    May 7, 2013
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    Hi All,

    I have a broken inductor in a Roberts radio that's on the input of a Sharp 1CZ41H chopper regulator.

    It's involved in generating 3.3v dc. The input voltage that goes through the inductor is 9v DC.

    The inductor looks like it's been getting hot at one end as the plastic shrouding was discoloured.

    The enamelled wire is about 0.09mm in diameter and the length is about 128cm.

    The body of the inductor is 4mm in diameter and 5.62mm long.

    I'm guessing that this one is somewhere around 2.5uH.

    Any problems with this, sound about right to you?

    Cheers
    Martin
     
    Last edited: Oct 22, 2013
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Are their any colored dots on the end?

    How many turns did you unwind? You need to wind that same number back on.
     
  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Your photo is not great but diode D371 looks badly cooked from the inside, and is probably dead.

    Your inductor should have a 3 digit number printed on top, or on the side (or maybe dots like SgtWookie said). That will tell it's value. If it is on the Vin input and is just an input filter, you need to replace it with an inductor of the right value AND of the same (or higher) rated DC current.
     
  4. apprenticemart2

    Thread Starter Member

    May 7, 2013
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    EDIT:EDIT: I've been measuring current drawn by the 3.3v rail as around 250-260mA while at its hardest working. I don't know how much this figure is affected by the burden voltage of my 8060A which is 0.9v on the 2000mA range.

    This amount of current seems far too high for the physical dimensions of the inductor that was installed in my Roberts radio or am I imagining things? (a bit less than 4mm x 6mm)

    Could I replace it with something like this http://uk.mouser.com/ProductDetail/JW-Miller/RL855-391K-RC/?qs=sGAEpiMZZMsg%252by3WlYCkU0kzpPlkT1F4iYxPVMb4ztg%3d

    I'm limited by dimensional constraints. The inductor can't be more than 8mm in height.

    EDIT:
    I have managed to solder some leads on to the ends of the wire and re-wind the ferrite.

    It's not a very good job unfortunately.

    I'm now measuring around 360uH and DC resistance of 4.6Ω (probably bad soldering?) with a Peak LCR40.

    Can I rely on these figures at all?

    Sorry about the photos, It was dark here and I had to try and get a halogen beam in there as compact fluorescent is abject gash for lighting quality.

    Diode D371 is fine as far as I can tell, just some scum on its surface looked bad under the light in last nights photo.

    There were no identifying marks or dots on the inductor. Its ferrite? fell apart after I took the shrunken plastic off it to try and see what was going on.

    I'm have counted about 150 turns on the inductor.

    I'm guessing that it is for input filtering then? as I soldered in a wire instead and the radio happily powered up and has been connected to the internet and playing mp3's off a USB memory stick. The 3.3v was just a smidgen over.

    The example circuits in the PDF for the Sharp 1CZ41H didn't show any inductor on the input as far as I could see.

    I would like to put a new inductor in as a wire jumper is probably not the right way to do it.

    Does this mean that the type of inductor that I need is called something else like an RF choke rather than a power inductor?

    Cheers
    Martin
     
    Last edited: Oct 23, 2013
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Thanks for the better photos. :)

    Your little handwound inductor repair looks fine. But the DC ohms should be far less than 4.6ohms, and probably is, I think you have a measurement issue.

    If you connect the inductor and a series resistor (and series ammeter) to a variable power supply, you can run a measured 100mA through it, and measure the DC volts across its legs. Then calc ohms as R = Volts/0.1 amps.

    The photo seems to be showing the 470 (47uH) inductor as the main inductor, so I think you maight be right this little inductor is just an input filter, probably to stop RF being radiated out the DC input power lead.

    That means its value is not that critical, and the only safety thing you need to check is its DC ohms vs input DC current (you can even its measure temp rise at the typical operating current). That would likely be what killed the first inductor.

    Also you should try to rule out overcurrent faults, in case the first fail was due to an overcurrent situation. Sometimes if a speaker is shorted an radio (audio amp) will draw excess PSU current.
     
  6. apprenticemart2

    Thread Starter Member

    May 7, 2013
    67
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    A few days ago just on the off chance I emailed Roberts radio and it turns out that they have a spares dept with real people there and real spares.

    They had the actual part I needed so I have ordered it. They wouldn't part with schematics of course.

    I was told that they were aware of the occasional failure with this part but that it was a rare occurrence.

    I was quite shocked to get an email back telling me that they would sell me the part I wanted. That's pretty unheard of these days.

    I will post back any developments.

    Thanks to everyone for their advice.

    Cheers
    Martin
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    From my perspective you are better off with an improved part than a direct replacement, which obviously fails in the field. That's bad form in electronics repair.

    If your hand-wound repair has slightly thicker wire, or slightly less turns it may have a bit less inductance than the "proper" part, but will withstand the current and heat better (and last the distance).

    Obviously using an inductor of larger size (or more modern construction?) would allow both the same inductance AND have an improved DC current rating.
     
  8. apprenticemart2

    Thread Starter Member

    May 7, 2013
    67
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    EDIT: Well, unsurprisingly the new original part was getting too hot to hold my finger on it so I just put the 2.2uH 1.9A 0.05Ω DC resistance one I had in instead.Radio has worked fine all day.



    You are correct of course. I should put a beefier part in.

    The replacement arrived this morning and it's measuring around 480uH and 2.6Ω DC resistance. Current carrying capability is unknown.
    I have seen similar Wurth ones on Farnell that are 3.0Ω DC resistance.

    The only spare inductor I have to hand that will fit in the same space is a 2.2uH rated at 1.9A and 0.05ohm DC resistance.

    I will try the original part and keep an eye on the temperature first.

    Thanks for the advice.
    Cheers
    Martin
     
    Last edited: Oct 25, 2013
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    What is the size of that part? I have a bag of 420uH inductors here in similar appearance that are 7mm Dia and 11.5mm high, rated 0.68A and measured at 0.84 ohms.

    You said you tested it (getting hot) but you did not mention the actual DC current?
     
  10. apprenticemart2

    Thread Starter Member

    May 7, 2013
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    The radio is currently reassembled and I may revisit this again (when it breaks) if I get some time.

    The original spec part is 7mm x 4.1mm (in it's shrink wrap).

    I didn't make any new current measurements with the part in place.

    Because all the 3.3v stuff is actually in a shielded can there's only about 8.5mm between pcb and lid/top of the shielded can and then because of the axis of the leads I could only bend a larger inductor to one side (over the top of the output inductor) and I don't think there's enough clearance to do this.

    I have put a 2.2uH 1.9A inductor in there instead (all I had). Radio is still working so far after 2 days.

    Cheers
    Martin
     
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Wow! OK, that really is a small one! :eek: Re-reading your first post I realised you had already stated the size, so sorry for that.

    It's good that the radio is working now, and the only issue is that it might be radiating some PSU noise down the DC power wires (assuming it uses an external plugpack).

    But there's plenty of Chinese SMPS plugpacks etc which are badly filtered so it is probably no worse than many devices people have in their homes. :)

    Congrats on getting a solution!
     
  12. apprenticemart2

    Thread Starter Member

    May 7, 2013
    67
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    It seems to be working fine and with the 2.2uH inductor in and I think there's less noise while the radio searches for a network to connect to than when I just bypassed it with a wire.

    There is a small pcb in the back of the radio with the DC power jack on it and this has a choke on it and there's inductors all over the place inside. The radio seems quite well built apart from the problem inductor.

    Thanks for your advice. I learnt a few things I didn't know.

    Cheers
    Martin
     
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