High voltage high freqency measurement - Page 1
Forum section: electronics / projects

author: charon1985
author_rank: Contributor
timestamp: June 10, 2016, 12:47:01 pm
content: Hi, how can i measure voltage (1500V max) that has a high frequency 65-150KHZ? i need to measure it with a microcontroler (0..+5v range input). I have tried to bring the voltage down using a resistor voltage divider, and rectify it with 1n4148 diodes. The voltage divider and the diode work. But i cant see on the oscilloscope the "bumps" from the original sine wave, it looks more like a straight line, like is filtered. Is this because of the capacitance of the prototyping board?After rectifying and filtering(?!) i plan on using an optocoupler and interface it with the microcontroler(?!)Does anyone know another method how i could bring the voltage down in the measurable range?Later edit: i don't make the tests at 1500V (max voltage), i make them at 50V

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: June 10, 2016, 04:12:27 pm
content: the spring clip proto boards are useless for anything even moderately fast and high Z, too much parasitic C between nodesalso diodes come in many sizes, different parasitic C too, you need to use the right part for the V and Z at place you are using it/themyou could read up on 'scope probe design, though your signal is slow by those standards you still probably want high MegaOhm divider not to load the sourceand do pay attention to Resistor, Capacitor max V spec, many R can flash over with high V across them even well below their power rating

author: uncle_bob
author_rank: Unknown
timestamp: June 10, 2016, 04:41:01 pm
content: Quote from: charon1985 on June 10, 2016, 12:47:01 pmHiThere are several parts to this:1) How do you safely bring the voltage down to something you can work with.2) How do you accurately represent the voltage in digital form.3) How do you isolate this / make it safe?The first part is "simply" making an attenuator. It is not a lot different than a scope probe. You need a correct step down ratio. You need a frequency response that is wider than the range of the signals being measured. This could involve R's and C's. It might also involve a broadband transformer. If ground isolation is a requirement (it sounds like it is), the transformer is a good idea. The degree of loading on the measured circuit also gets into this whole process.The second depends a *lot* on what the waveform looks like. Is it a pure sine wave? Is it a square wave? Is it something else entirely? If it's some thing other than a sine wave, how do you plan to represent the waveform? Peak to peak is one answer. True RMS is another answer. There are several other answers. The detector you use *must* match your intended representation and the waveform. Using a peak detector on a composite waveform to get RMS is not going to work.The third part is a bit tricky. It sounds like you need ground isolation. If so you have two basic options:a) Use the transformer mentioned earlier.b) Float the measurement circuit on the "input ground" and then send the data to the "output ground" via an opto isolator. Sending analog signals over an opto is *not* a good idea. It's a disaster if you need good accuracy.So, that's lots of twists and turns. Lots of choices. Lots of things that could be elaborated on. Which direction is this headed?Bob

author: Marco
author_rank: Super Contributor
timestamp: June 10, 2016, 04:51:03 pm
content: Quote from: charon1985 on June 10, 2016, 12:47:01 pmNo, the scope and the probe will have much more capacitance. With a x10 probe you have around 10MOhm in parallel with >10pF after your rectifier, with a time constant of 100 microseconds you will have quite a lot of smoothing.

author: HighVoltage
author_rank: Super Contributor
timestamp: June 10, 2016, 04:54:06 pm
content: Quote from: charon1985 on June 10, 2016, 12:47:01 pmUse a capacitive divider instead and it will work well

author: David Hess
author_rank: Super Contributor
timestamp: June 10, 2016, 04:56:48 pm
content: If the signal is only AC, I might use a capacitive voltage divider followed by some standard method for measuring AC. Otherwise a compensated resistive divider will be needed.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: June 10, 2016, 05:42:53 pm
content: Quote from: charon1985 on June 10, 2016, 12:47:01 pmDo you have a schematic? What resistances did you use? Did you size them for 50V, or did you size them for 1500V and only apply 50V? Are the diodes before or after the resistive divider?

author: uncle_bob
author_rank: Unknown
timestamp: June 10, 2016, 05:58:40 pm
content: Quote from: charon1985 on June 10, 2016, 12:47:01 pmHiYes, it looks like a DC voltage with only minor ripple due to the stray capacitance on the board, the scope probe C, and the capacitance of the MCU input.Bob

author: charon1985
author_rank: Contributor
timestamp: June 11, 2016, 04:23:50 am
content: I need it for an induction heater. I need to measure the voltage from a Series LC circuit so i can implement some over voltage protection, the tank capacitors are rated to 1600V but from my calculation if no workpiece is in the coil the voltage will be 1800+ V at resonance.The precision is not that important if i can measure the voltage with in +/- 50V is OK. The wave form is obviously sine wave. Peak voltage is what i need (i guess). I also could use an analog comparator to trigger only when the voltage is above 1450V so that the microcontroler knows to stop shifting down the frequency (stop the power increase). I do not need to reconstruct the waveform shape or anything closer to that... only a simple over voltage protection.

author: charon1985
author_rank: Contributor
timestamp: June 11, 2016, 04:56:32 am
content: Regarding the idea of using a transformer to step down and have a separation. At 1500V, 75khz minimum, 1cm2 (square) core, and 1000Gaus flux density, i will need to put 500 turns in primary. And considering that i need a wire with at least 1500V insulation, well.... i don't know if it is practically feasible

author: danadak
author_rank: Super Contributor
timestamp: June 11, 2016, 10:02:06 am
content: If you have a resonant tank in general you do not want to placea load on it with a Resistive component in the Z as that affects fstability, power loss, oscillation loop, etc...Why not modify the L component of the tank and tap it like an autotransformer. You still have to design for load effects but that effectshould be minimal if done properly.Regards, Dana.

author: uncle_bob
author_rank: Unknown
timestamp: June 11, 2016, 11:46:58 am
content: Quote from: charon1985 on June 11, 2016, 04:56:32 amHiIf you need the isolation a transformer provides, there are cores that you can put the windings on and get the insulation separation. High voltage transformer based supplies have operated in the same frequency range you are talking about.====A far more practical way to solve your problem:In any circuit, you can either measure voltage or current. Putting in a sense coil gives you a transformer (it's a coil), a step down (it's loosely coupled), and a relative indication of what is going on (that can be calibrated).Bob

author: charon1985
author_rank: Contributor
timestamp: June 11, 2016, 01:43:05 pm
content: I think that my calculation are partial wrong because i assumed that the circuit had a 0,01 ohm resistance but the small voltage tests showed that the LC circuit has a 0,035 ohm +/- 20%... so at resonance and at full 14 volts applied to the LC the voltage across the capacitors will be around 350-400V.Maybe the resistance 0,035 ohm comes from the capacitors legs, in the future i will put more capacitors in parallel (to resolve the heating problem of the capacitors) and thicker buss bars so its possible that the resistance comes down to 0,01ohm so i think that decreasing the coil inductance and increasing the capacitor bank value will resolve my problems.I need to study to see if the lower inductance will lower the heating effect on the workpiece.

author: uncle_bob
author_rank: Unknown
timestamp: June 11, 2016, 01:51:05 pm
content: Quote from: charon1985 on June 11, 2016, 01:43:05 pmHiIn a very simple view:Heating is about ampere turns. More inductance generally is more turns. As soon as you put the fry pan on the heater, the loss in the pan is what should dominate the whole process.You have to look at the entire system.Bob

author: charon1985
author_rank: Contributor
timestamp: June 11, 2016, 01:57:29 pm
content: If the calculation show a 1400A in the LC circuit at resonance, that means that the primary winding of the transformer would absorb 1400A/22 wire turns = 63 A? is that correct?If is correct then i could monitor the current consumption of the primary winding which is safer and a little more simple because i have a RMS to DC converter that works up to 500khz with 1% errorBob, more turns = bigger magnetic field, more amps = bigger magnetic field this is what you want to say? this will increase the heating effect? (i think i know the answer but i want the confirmation).

author: uncle_bob
author_rank: Unknown
timestamp: June 11, 2016, 04:35:16 pm
content: Quote from: charon1985 on June 11, 2016, 01:57:29 pmHiYes, you can increase the field either by adding turns or by adding current. Inductance goes up as the turns increase, there is no free lunch.The frying pan forms a single shorted turn on the "other side" of the transformer. Ideally all the power should go into it's resistance.Keep in mind this is all the "simple" version. There are always going to be secondary issues.Bob