# Safely experimenting with inductors without destroying test equipment

#### dcbingaman

Joined Jun 30, 2021
498
I was planning on making my own inductors using ferrite cores and magnet wire. I am aware that any large change in current can generate very large voltage spikes when working with inductors. What safety and or practices should be followed to help avoid destroying test equipment such as scopes, DMM's power supplies etc.? I have no desire to destroy expensive test equipment.

#### Papabravo

Joined Feb 24, 2006
17,310
I was planning on making my own inductors using ferrite cores and magnet wire. I am aware that any large change in current can generate very large voltage spikes when working with inductors. What safety and or practices should be followed to help avoid destroying test equipment such as scopes, DMM's power supplies etc.? I have no desire to destroy expensive test equipment.
What will you be using the inductors for? RF applications would be different than switch mode power supplies.

#### dcbingaman

Joined Jun 30, 2021
498
What will you be using the inductors for? RF applications would be different than switch mode power supplies.
Switch Mode Power Applications

#### dcbingaman

Joined Jun 30, 2021
498
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#### Papabravo

Joined Feb 24, 2006
17,310
Switch Mode Power Applications
In an SMPS you should know where the voltage spikes are and avoid looking at them unless you know the equipment can handle them. There will always be one side of an inductor that can be probed. It might not be useful to do that depending on the topology. If you really must probe both sides of the inductor you may have to invest in more robust test equipment. A 1 MΩ probe with 15 pf of capacitance in parallel will handle pretty much anything in the 0-50 VDC range, so I never worried that much about since everything I worked on was confined to that range. If you're going to work with offline DC-DC converters then you're going to need some high voltage probes.

#### Ian0

Joined Aug 7, 2020
3,773
If you are testing them in conventional switched-mode power supply circuits (buck, boost, flyback, forward converter, LLC resonant etc) then there will always be somewhere for the inductor energy to go, because that's part of the design, usually into a capacitor somewhere, so you are unlikely to generate much in the way of high voltage. Use a x10 probe on low voltage and x100 on offline (and test the offline stuff using an isolating transformer, because your 'scope will be earthed).
You are much more likely to destroy a few power transistors due to overcurrent (if something saturates) than to destroy test equipment due to over-voltage.
You might also run into iron powder inductors, in which case this old DOS software is really useful
http://www.iec-international.com/micrometals/micrometals/software.html

#### Ian0

Joined Aug 7, 2020
3,773
Hope you enjoy my website. Thanks! It has been a while since I wrote any new articles on that. If you want to post anything, I would be glad to do so and give you credit for it of course. If you are interested I also have two books on amazon about microcontrollers/FPGA's. Just type in my name.
Apologies for the pedantry, but the permeability of free space is ${\mu}_0 = 4{ \pi}.10^{-7} H/m$

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#### dcbingaman

Joined Jun 30, 2021
498
Apologies for the pedantry, but the permeability of free space is ${\mu}_0 = 4{ \pi}.10^{-7} H/m$
Thanks, nice catch. I will fix that in the article. By the way, how did you know what text to input to create that nice formula? Is there a tool or documentation about that?

#### Papabravo

Joined Feb 24, 2006
17,310
Apologies for the pedantry, but the permeability of free space is ${\mu}_0 = 4{ \pi}.10^{-7} H/m$
I see your pedantry and raise you a nitpick! The LaTex idiom for a 'dot' used as a multiplication symbol is \cdot, So

$${\mu}_0 = 4{ \pi} \cdot 10^{-7} \;\text{H/m}$$

#### dcbingaman

Joined Jun 30, 2021
498
=4π.10−7H/m
I see your pedantry and raise you a nitpick! The LaTex idiom for a 'dot' used as a multiplication symbol is \cdot, So

$${\mu}_0 = 4{ \pi} \cdot 10^{-7} \;\text{H/m}$$
That's ok, I understood what you meant.

$${\mu}_0 = 4{ \pi} \cdot 10^{-7} \;\text{H/m}$$

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#### Ian0

Joined Aug 7, 2020
3,773
I see your pedantry and raise you a nitpick! The LaTex idiom for a 'dot' used as a multiplication symbol is \cdot, So

$${\mu}_0 = 4{ \pi} \cdot 10^{-7} \;\text{H/m}$$
I concede - it took me long enough to get that far!

#### dcbingaman

Joined Jun 30, 2021
498
I see your pedantry and raise you a nitpick! The LaTex idiom for a 'dot' used as a multiplication symbol is \cdot, So

$${\mu}_0 = 4{ \pi} \cdot 10^{-7} \;\text{H/m}$$
Looks like it might be worth my time to learn LaTex on top of HTML, CSS and JS. Learning never stops

#### Papabravo

Joined Feb 24, 2006
17,310
On this board especially, discovering and improving LaTex knowledge is quite beneficial IMHO. It is too bad we lack a useable reference for this specific implementation.

#### Papabravo

Joined Feb 24, 2006
17,310
I concede - it took me long enough to get that far!
Digging out the nits has not been without cost in time and frustration

#### atferrari

Joined Jan 6, 2004
4,475
Digging out the nits has not been without cost in time and frustration
@Papabravo

Isn't that "sweating the small stuff" after all? Intrigued.

#### Papabravo

Joined Feb 24, 2006
17,310
@Papabravo

Isn't that "sweating the small stuff" after all? Intrigued.
Maybe, but I like for my written communication to be as accurate as possible,

#### atferrari

Joined Jan 6, 2004
4,475
Maybe, but I like for my written communication to be as accurate as possible,
I like your style, concise and to the point. At work I pretend those in my team to be like that all the time.
Be well.

#### Papabravo

Joined Feb 24, 2006
17,310
I like your style, concise and to the point. At work I pretend those in my team to be like that all the time.
Be well.
Thank you for those kind words. They are much appreciated.