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Protecting power supply from back EMF

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LAN_403

E.TexasTesla

Sat Jan 06 2018, 04:10AM

Registered Member #4362 Joined: Sat Jan 21 2012, 03:44AM
Location: Texas
Posts: 98

Correct me if I'm wrong but early 50's induction coils were powered by batteries.
Just use a battery.

StaticBuildup

Sun Jan 07 2018, 12:22AM

Registered Member #61695 Joined: Sun Jul 16 2017, 11:22PM
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Posts: 31

I will try the T filter with those values and damping resistor and see what happens. Does the resistor have to be more than 1/4W as it might carry about 1 or 2A?

I will probably be using a linear power supply. I do sometimes use batteries, but some of those coils use more current than normal alkaline batteries can supply, about 2A. I don't have larger batteries than that.

2Spoons

Sun Jan 07 2018, 07:39PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615

At 2A a 1 ohm series resistor will be dissipating 4W if its in the DC path. Put it in series with the capacitor and it only has to handle AC current, which should be much lower. My gut feel is a 10ohm 1W in series with the C.

StaticBuildup

Fri Jan 12 2018, 11:56PM

Registered Member #61695 Joined: Sun Jul 16 2017, 11:22PM
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Posts: 31

Thank you for the replies.

I looked for 47uH inductors and noticed there are different types, such as power inductors and choke coils, and different W and A ratings. I guess that the rating would have to be for the maximum current it would be used with? I would probably be using up to 3A.

And does the capacitor voltage rating have to take into account the voltage of the back EMF?

profdc9

Sat Jan 13 2018, 03:44AM

Registered Member #58522 Joined: Tue Mar 15 2016, 08:33PM
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Inductors that tend to work at low frequencies use laminated steel cores. These are typically good to < 10 kHz.
Inductors that work at higher frequencies tend to use ferrite or iron powder cores. Up to 1 MHz, typically MnZn ferrites are used, which are the typical ferrite materials used in switched-mode power supply transformers. Above 1 MHz, NiZn ferrites are used. Powdered iron cores of various mixes are also used as well as kHz and MHz range frequencies.

You can look at the self-resonance frequency (SRF) of the inductor if you have a datasheet. The self-resonance occurs because there is a capacitance between the turns of the inductor that are effectively in parallel with the inductance, so you get a parallel LC circuit, and near or above this frequency, the inductor doesn't behave like an inductance, and it will pass high frequencies because the capacitance dominates. The SRF should be at least twice the high frequency of use.

Like there are capacitor types that tend to be used for different frequency bands (for example electrolytic for low frequencies, ceramic or film capacitors for higher frequencies), you may have to use inductors for different frequency bands if you want to block a wide range of frequencies, especially going from kHz to low MHz. A high inductance lowers the SRF, so if you want to block many decades of bandwidth, you may need to have several filters in cascade for different frequency bands. You should probably figure out what the bandwidth of the pulse you expect to block is.

And the capacitor will have to be rated for the peak voltage (or well above it) that it will endure. You could consider using a zener, TVS diode, or MOV if you think that the voltage spikes are going to be significantly above the working voltage and so can be absorbed by these devices.

StaticBuildup

Tue Jan 16 2018, 12:05AM

Registered Member #61695 Joined: Sun Jul 16 2017, 11:22PM
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Posts: 31

Thank you for the explanation. Would inductors like these be OK? I couldn't find the SRF but they seem to be for EMF suppression.

https://www.ebay.co.uk/itm/DPT047A3-Inductor-wire-47uH-3A-10m-THT-FERROCORE/173093512464?epid=577390885&hash=item284d2d6510:g:XZcAAOSwbWZaWLMo

https://www.ebay.co.uk/itm/DTPU047A3-Inductor-wire-L47uH-3A-48m-THT-L-I-0A75uH/162847997813?hash=item25ea7f3f75:g:Vs8AAOSwKwVaWLP4

I don't know what the peak voltage of the back EMF is, would it be measurable with an oscilloscope? I guess it couldn't be very high because it hasn't ever zapped me when I adjust the interrupter contacts, even when it is arcing.

If I used a MOV, where would it be in the circuit? In parallel with the load, or somewhere else?

2Spoons

Tue Jan 16 2018, 08:29PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Don't use a MOV - they wear out. They're a bit like fuses in that every time they fire they get damaged (how much depends on the impulse energy of course). They're really only meant as protection from major events, such as lightening surges.

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