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.


2 Answers 2


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.

  • 1
    \$\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\$
    – user16324
    Sep 7, 2013 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, 2013 at 16:09

The frequency you are quoting is the self-resonant frequency. In your application, you might want to avoid the self resonant frequency if the circuit doesn't rely on it.

DC resistance basically dictates how much joule heating there will be. For this parameter, the smaller, the better, but usually inductors have small DC resistances well below 1 Ohm, and usually they are not critical unless designing with high Q-value or extremely low power dissipation, like those running in a dilution fridge.

Core materials are important. Iron cores have higher saturation but loss of efficiency is greater at higher frequencies due to eddy current. Ferrite core is the opposite: it's lighter in terms of density, has lower saturation levels but performs better at high frequencies.

In some niche applications where stray magnetic field is an issue, toroidal inductor creates less stray B field compared to open cores such as rod shape, since magnetic field is confined in the loop.

To summarize, I would say in the order of importance:

  1. inductance
  2. current rating
  3. core material => it determines effective inductance at higher current load, and high-frequency loss
  4. DC resistance

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