Snubber Measurement

Snubber Measurement. Part of a series of videos on the design of snubber circuits for power electronics by Rudy Severns. Hello.

You know there's a lot of work can be done with simulation, but ultimately it's truth on the ground, in other words, the actual circuit that really matters.

So when you've done your modeling and your calculations and you're ready to really find out if your snubber circuits work, then you have to be able to go and make measurements.

Of course, actually, initially, you have to start with measurements to understand what you need in the way of snubber, in other words, what's your problem? What is the circuit doing that you don't like?

That's where you start and there you will use voltage probes, but also current probes. Once you have the basic information, you go back to the simulation, do your design work, and you come back to the laboratory and put in your modifications and see just how close are they. Are they good enough or do we need to make some adjustments?

The key item is to look very critically at your waveforms. Understand on your waveforms what every wiggle is about. Do you want that wiggle or don't you? The key is to understand what is causing the wiggle, because if you want to eliminate it, then you have to know what's causing it.

There are two basic tools that go with an oscilloscope for checking and understanding the waveforms. The first of course is a voltage probe. And you can see we can connect this one in right here. We turn on our power supply and we can get some of the waveforms, the voltage waveforms that are in there.

But also, there's a more critical item. It turns out that you not only want to look at the voltage waveforms, but you want to look at the current waveforms. And there are of course many types of current probes. The simplest, you can put a resistor in, a series resistor, in the path you want to measure, and then measure either unbalanced or in a balanced way with voltage probes to determine what the current is at a given point.

And then of course, if you're a little more elegant, you can use various forms of current probes. This example happens to be an AC current probe which is useful, but there are also DC current probes that will allow you to see both the AC and the DC components.

In power supplies, very often you do want to see what is the AC component floating on - a DC component - for example: output filters, input currents, things of that nature.

So these are two fundamental tools.

Now, with voltage probes it's not very hard to clip them various places in the circuit. But current probes you have to be a little more clever. You have to put some loops in usually. Usually you have to actually interrupt the circuit and put a loop in there. And you have to be a little careful of course. That loop will introduce some parasitic inductance and it may modify things a little.

Also, there's the capacitance of the probe itself, the voltage probe. So normally you'd run the voltage probe on high attenuation to minimize its capacitance.

These are just basic ideas to gather the information you need to design your snubber circuit and then in the end, to verify that the snubber really works.

Rudy Severns' book "Snubber Circuits for Power Electronics" is available for purchase on the Internet as an ebook in PDF format at WWW.SNUBBERDESIGN.COM

Category:

Science & Technology

Tags:

• snubber
• measurement
• design
• power
• supply
• electronics

[tesla500]

A very common AC/DC current probe is the Tektronix A6302, coupled with an AM503 amplifier. This can measure 20A cont., and 50A peak. These have been out of production for some time, but are very common on eBay. They typically go for about $500 to $1k for the set.

If you want something new, the TCP312/TCPA300 runs about $3k. This one is rated 30A cont. 50A peak.

I've used both of these, and they work quite well.

[Afrotechmods]

Neat! I would like to hear more about AC/DC current probes, such as which ones you recommend and where to purchase them from.