I am in the process of upgrading some older circuits with newer components. One of which components is a 150uH iron core inductor. For reference, this is the part at DigiKey:


The circuit that this inductor is in is a UV Lamp drive circuit that is powered by a LT1172 100kHz constant current driver. According to DigiKey, the resonant frequency of the inductor in the circuit is 3.2MHz, with a maximum current draw of 1.15A.

  • I imagine the resonant frequency is quite important, but can i use an inductor with a different harmonic perhaps?

  • Also, the quality rating DigiKey lists 20 @ 796kHz, could i choose an inductor with a lower quality rating because the circuit is running at a much lower frequency?

  • The DC Resistance is listed as 370mΩ max. I'm assuming that as direct replacement the DC Resistance is pretty important.

  • Is it important that the replacement inductor has the same core material? Will a ferrite core inductor suffice in place of an iron core?

I'm guessing that a lot of these properties are somewhat application specific. It doesn't seem like there are a ton of inductors on DigiKey that have the same set of properties.

What properties do i have some wiggle room on? I suppose the 150uH is pretty set in stone, and anything with the same current rating or above; other than that I'm in uncharted territory.


Resonant frequency is the point where the inductor starts to behave like a capacitor so ideally you should find a replacement with a resonant frequency not less than the current device. However, if the fact that the inductor resonates is useful to the circuit this can be very bad advice. We need a circuit to tell!

Q factor is pretty much the same as resonance - higher is usually better BUT, like resonance, the circuit may be relying on a Q factor that is not that great.

DC resistance - normally lower is better in general and this also means Q factor is improved too but the same small print applies. No circuit, no can tell!

Probably the most reliable part of the answer is the core material. If all other requirements are met the only disadvantage I can see with ferrite is that it's permeability changes with operating temperature and this may cause a problem. Material specifications for both core materials are the best way to judge.

Sorry, it's not an easy answer; a circuit analysis to understand what the inductor does and how temperature may affect performance is a basic requirement for judging this one.

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    \$\begingroup\$ Ferrite tends to saturate at a lower flux density than iron, so I would expect an "equivalent" ferrite (same current at same frequency) to be physically larger - maybe 3x the cross sectional area. So I'd be reluctant to substitute without understanding that... \$\endgroup\$ – user_1818839 Sep 7 '13 at 10:39
  • \$\begingroup\$ @BrianDrummond thanks Brian; I should have mentioned in the answer that I would expect any replacement inductor to be rated to use the same amperage to avoid saturation problems. \$\endgroup\$ – Andy aka Sep 7 '13 at 16:09

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