Inductive kickback voltage

If you don't have an oscilloscope and you are not game to put a multimeter across the inductor, is there a neat electronics trick you can use (with some additional components perhaps) to get an estimate of the kick back voltage level?

The greater the henry value, the more intense the kickback.

The higher the impedance seen by the coil, the more intense the kickback. It may even generate such a high volt level as to penetrate the wire insulation, and spark among the coil windings, or even to nearby objects including your hands. Windings can be ruined by arcing.

Try hooking up an led (or bulb) to the coil. Wave a magnet around the coil and see if you can make the led flash.

BradtheRad said:

The greater the henry value, the more intense the kickback.

The higher the impedance seen by the coil, the more intense the kickback. It may even generate such a high volt level as to penetrate the wire insulation, and spark among the coil windings, or even to nearby objects including your hands. Windings can be ruined by arcing.

Try hooking up an led (or bulb) to the coil. Wave a magnet around the coil and see if you can make the led flash.

Click to expand...

I have a flyback from a telvision and, on the exposed part of the ferite core, I have wound my own primary. I measure an inductance of 0.2uH or something like that.....if that is what this trick is meant to gauge.

I was thinking more along the lines of getting an estimate of the kick back voltage while it is operating.

I guess you mean momentary voltage spikes?
Greater than 15V?

Consider neon bulbs. They are is designed for higher volt levels than an led or flashlight bulb.
You can easily see a momentary flash.
By looking closely at the two terminals inside the bulb, you can see if it's DC only, or AC.

A neon bulb needs about 60V or greater to turn on. Current draw about 1 mA.

There is (or used to be) a gizmo made from a ladder array of neon bulbs, for an electrician to measure AC mains.
If one is lit then it is 120V. Two lit means 240V. Three lit means 480V. Etc.

BradtheRad said:

I guess you mean momentary voltage spikes?
Greater than 15V?

Consider neon bulbs. They are is designed for higher volt levels than an led or flashlight bulb.
You can easily see a momentary flash.
By looking closely at the two terminals inside the bulb, you can see if it's DC only, or AC.

A neon bulb needs about 60V or greater to turn on. Current draw about 1 mA.

There is (or used to be) a gizmo made from a ladder array of neon bulbs, for an electrician to measure AC mains.
If one is lit then it is 120V. Two lit means 240V. Three lit means 480V. Etc.

Click to expand...

Yeah the transient voltage spikes.

Can those little neon bulbs suffer over voltage?

If you understand the circuit, you'll know the kickback voltage.

FvM said:

If you understand the circuit, you'll know the kickback voltage.

Click to expand...

Most people measure the kickback with an oscilloscope. But I don't have one so I need some other means of measuring it, at least roughly.
There are forumulas for calculating it that I have found. But they all seem fairly imprecise with multiple variables that you can't necessarily be sure about.

A RCD snubber with sufficient high load resistor will capture the peak flyback voltage and allow to measure it as DC voltage.

boylesg said:

Yeah the transient voltage spikes.

Can those little neon bulbs suffer over voltage?

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They are destroyed by too much current. As for voltage, they trigger at a certain voltage and then regulate it very close to that voltage.

When hooked up to 120 VAC, neon bulbs are typically put in series with a 220k resistor.

A coil produces greater inductive kick when it sees high impedance. The kick is a momentary spike, and sometimes a spark.

When the coil sees low impedance, it generates a lower level emf, and for a longer period of time. It's less like a kick.

BradtheRad said:

They are destroyed by too much current. As for voltage, they trigger at a certain voltage and then regulate it very close to that voltage.

When hooked up to 120 VAC, neon bulbs are typically put in series with a 220k resistor.

A coil produces greater inductive kick when it sees high impedance. The kick is a momentary spike, and sometimes a spark.

When the coil sees low impedance, it generates a lower level emf, and for a longer period of time. It's less like a kick.

Click to expand...

Corresponding to lengthening and shortening of the high voltage spark gap no doubt.

boylesg said:

Corresponding to lengthening and shortening of the high voltage spark gap no doubt.

Click to expand...

Yes, if you place the wire leads close enough together, then a spark may jump the gap. If they are too far apart, the spark may occur in the windings, with the possibility of tack-welding wires together and ruining the coil.

A neon bulb is sort of like a plasma gap inside the glass. If the coil contains a lot of energy, then a xenon flash tube might be in order.

Beware of testing large transformers with an ohmmeter. I read a story (can't remember if it was presented as fact or rumor) about an inexperienced lineman who wanted to check the resistance in a transformer big as a refrigerator. He did it by hooking up his regular ohmmeter. He took the reading and started to disconnect one lead. (He may have had a partner who tried to warn him to leave the leads connected.) There was only a tiny current flowing through the coils, but due to the massive Henry value, it generated a powerful spark as he took off the lead. The spark jumped to his hand, travelled between his arms and through his chest, stopping his heart.