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How to measure flyback transformer output voltage

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LAN_403

MRMILSTAR

Fri Nov 02 2018, 09:12PM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

I want to measure the output voltage of a flyback transformer. I have a Tenma 72-6530 0 to 40 KV high voltage probe with an integral analog voltmeter that I want to use to make the measurement. The instructions for the probe say to insure that the ground lead of the probe is grounded. Neither one of the two flyback transformer output terminals is connected to ground in my project because everything is battery powered.

Likewise, I would also like to use this probe to measure the output voltage of a 1000 volt power supply that I have. As with the flyback transformer, neither output terminal is connected to ground.

Do I need to connect the negative terminal of the flyback transformer output to ground before I use this probe? My thinking is that I do. Likewise, do I need to connect the negative terminal of the 1000 volt power supply to ground before I use this probe?

klugesmith

Sat Nov 03 2018, 04:53AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Can you draw a simple schematic of the things you want to measure?
Let me take a stab at it first:

Think of that as a captcha image. Circle all the grounds in the picture.

Hint: there is no such thing as universal ground, and no such thing as voltage at a single point. Voltage, by definition, is a potential difference between two nodes. It's often handy to adopt some node, call it "ground", and have that be an implicit reference node so you can talk about voltage "at" other nodes.
For a color CRT television set or monitor, "ground" is usually sheet metal chassis parts. It might well be at 60 volts AC with respect to electric outlet ground, if you use a two-wire power cord.

Your voltmeter probe measures the voltage between the pointy end and its "ground" clip.
A better name for that would be voltage reference connection.
Since the meter scale only shows positive kV, the reference had better be the negative side of your supply. Unless you reverse the internal wiring, in order to measure negative kilovolts.

Aside from functionality, there are safety considerations. Damage, injury, or death could result if you use that probe with its "ground" clip at some voltage radically different from you and the floor, and pay no attention to which parts of the probe are not insulated for 40 kV.

To define the voltage "with respect to you and the floor" of any V_out terminal on the power supply,
or any terminal of your battery powered flyback circuit, we need to know more about what's inside and around the boxes.

Sulaiman

Sat Nov 03 2018, 01:29PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I just googled '=earth =ground =voltage =potential' and got 13 million hits,
indicating the importance and diversity of the topic.

As a general rule, the magnetic core of an iron or MnZn ferrite transformer is an electrical conductor so it is best kept at at potentials near 'earth' to reduce electric shock hazard and rfi.
The inner windings of a transformer are closest to the core so are usually the windings with potentials nearer earth.

In this specific case there are two key things;
1 the probe only indicates positive voltages with respect to the clip/'safe' end of the probe
2 the probe only measures d.c. whereas the flyback transformer output is a series of pulses

#1 can be 'solved' by carefully insulating the probe from earth - and sentient beings.

#2 needs a peak hold function
this is easiest accomplished by a simple half-wave rectifier with smoothing
so you will need ;
. a rectifier of suitable voltage and speed rating
. a suitable capacitor, of sufficient capacitance to hold the peak voltage as the probe resistance drains the charge.

e.g. a flyback transformer producing 10 us wide 10 kV pulses at 20 kHz,
. the diode(s) need to be much faster than 10 us, say 100 ns or faster
. 10 kV / 600 MOhm = 16.7 uA
16.7 uA x 50 uS = 833 pJ drained from the capacitor by the probe each cycle.

As the probe has +/- 2% lets allow +/-1% ripple voltage which is +/- 100 V, a change of 200 V per cycle.

Using C = dQ/dV, C = 833pJ/200V = 4.17 pF which is similar to the 'stray'/parasitic capacitance of the setup.

The capacitance should also be orders of magnitude greater than the capacitance of the rectifier or you will just have an un-specified capacitive divider probe.
e.g. suppose you have 10x 10 pF diodes in series, the diode capacitance will be 1 pF,
so about 100 pF of 'smoothing' capacitance would be required, rated for >10 kV..

Patrick

Wed Nov 07 2018, 08:01AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

if your low end (ground clip, wire, whatever) becomes loose your whole measurement set up will float towards your HV potential. Be careful. Thats how youll kill laptops and USB oscilloscopes.

I have never used a physical ground rod for my SMPS HV work so i dont think there needed in HV often unless its something like a tesla coil that keeps trying to influence its surroundings at distances of feet and meters, which flybacks dont do. so if your careful i think youll be ok.

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