Author Topic: Oh Lawd, I got dem Op-Amp blues!  (Read 692 times)

Offline abstruse1

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Oh Lawd, I got dem Op-Amp blues!
« on: August 27, 2022, 08:22:08 PM »
I want to measure voltage output from flybacks.  I've tried HV probes with DVMs, but at around 30 kV the readout disappears gradually, I assume from the electrostatic fields.  So I want to try an analog VM.  The probem is, the current output of a FB is so small that the low impedance of the analog meter drains a lot of the current and voltage.

From what I read, op-amps are exactly what I need, but I can't get an op-amp to work.  There are many schematics on line, but they're abbreviated, so I've had to guess a bit.  Maybe I guessed wrong.

Here's what I'm doing:  Using LM-1458N chip, 9v battery for chip power, pins are

1.  not used
2.  connected to pin 6 and -voltage input
3.  voltage +input
4.  ground, voltage output pin 6, and -pwr input
5.  not used
6.  connected to pin 2 and +voltage output
7.  +power input
8.  not used.

What's wrong with this?  I'm not using any resistors like some schematics show for multiple voltage input ranges.

I'll use a voltage divider to get the max input to the Op-Amp around 1 or maybe 10 volts, whatever my meter likes.

It seems weird to be working with tiny wires and very low voltage!
Abstruse1
   Been working with electricity for 60 years and still getting a tingle!

Offline klugesmith

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Re: Oh Lawd, I got dem Op-Amp blues!
« Reply #1 on: August 28, 2022, 06:50:20 PM »
You gave a schematic, in words, of a partial circuit whose purpose isn't clearly stated. 
Are you trying to make a voltage amplifier or voltage follower?

Also I need to pick on your statement: >>So I want to try an analog VM.  The probem is, the current output of a FB is so small that the low impedance of the analog meter drains a lot of the current and voltage.

Analog multimeters on 1000 V scale generally have higher impedance than digital meters.   Work it out, for analog meter with 20 uA or even 50 uA movement.   Most DMM's have input impedance not higher than 10 megohms.
The difference (less current for analog metering) also applies to solutions with external HV scaling resistor up front.

AC voltage measurement presents special challenges when working with external HV scaling resistors.   
Analog meters generally have weird, nonlinear impedance in AC volts modes.   Digital meters (the better ones) go to AC coupling, so any DC component of the HV under test will appear unattenuated at DMM terminals.

Offline abstruse1

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Re: Oh Lawd, I got dem Op-Amp blues!
« Reply #2 on: August 28, 2022, 08:03:02 PM »
A voltage follower, Kluge.  1:1 input:output voltage.

What did you mean by "partial circuit?"  What's missing for a follower cicuit?

You said that 1kV analog multimeters have higher input impedance than DVMs.  Is this also true of panel analog meters that have no power supply or internal circuitry?

I have an old Triplett analog multimeter.
« Last Edit: August 28, 2022, 08:07:01 PM by abstruse1 »
Abstruse1
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Offline Twospoons

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Re: Oh Lawd, I got dem Op-Amp blues!
« Reply #3 on: August 28, 2022, 10:36:03 PM »
It would help if you could specify how much load your flyback can handle.
I drew a sch of your description  - took me  about 5 mins, so I dont know why you didn't do this. The left sch is what you described, but given the resistors are likely to be in the mega/giga ohm range, the right hand version is what you should do (R5=R3). And you need to pick a much better opamp, one with an input bias current in the pA range. LM1458 is absolutely the wrong choice for buffering a high impedance divider.
« Last Edit: August 28, 2022, 10:37:43 PM by Twospoons »

Offline klugesmith

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Re: Oh Lawd, I got dem Op-Amp blues!
« Reply #4 on: August 29, 2022, 12:08:15 AM »
My "partial circuit" comment was a bit pedantic.  The schematic-in-words talked about things like -voltage input, +voltage input, +voltage output, voltage output pin6, ground, etc.  Leaving reader to guess about two connections to HV divider and two connections to meter.   I guessed same as Twospoons drew, and guess that Gnd symbol has implicit connection to negative side of HV supply and negative side of VOUT meter.

Twospoons offered a more immediately useful comment, about minding op-amp details like bias current.

As for analog meters, I was sitting within reach of the two in picture here.  Soltec scale card says "20K ohm/V DC". .   Micronta has a Range Doubler switch that applies to all ranges.  Scale card says 50K / 25K ohm/V DC.   
Soltec input resistance, measured with a DMM, is 9.91 megohms on 500V range and 20.18 megohms on 1000V range (with dedicated input socket). Micronta input resistance appears to be infinite; I think I blew it up in recent years by some electrical abuse.

Analog panel meters come with different coil sensitivities, usually not matching the printed scales in mA or A or V or kV.  I'd say that 1 mA movements are very common, and 100 uA movements are more common that the 50 uA or 20 uA movements in multimeters.   As Abstruse said in OP, 1 mA is lots more current than most DMM's draw on 1 kV range.   

The battery-powered digital panel voltmeters I've seen and used have input impedance around 100K, and would be fine for use with the voltage follower op-amp. 
« Last Edit: August 29, 2022, 12:18:37 AM by klugesmith »

Offline abstruse1

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Re: Oh Lawd, I got dem Op-Amp blues!
« Reply #5 on: August 29, 2022, 03:02:53 AM »
Thanx,guys.  Twospoons, can you recommend a better op-amp? 
Abstruse1
   Been working with electricity for 60 years and still getting a tingle!

Offline Twospoons

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Re: Oh Lawd, I got dem Op-Amp blues!
« Reply #6 on: August 29, 2022, 03:42:34 AM »
Go to Digikey, use the filters.  There are hundreds of possibilities for you to sort through. I'm not going to make suggestions, because you probably have a raft of other parameters that need to be met. Or at least you should do. Its far more beneficial for you (long term) to choose a part yourself than for me to toss part numbers at you.  You've already tried one part, found it doesn't work as expected, and most importantly found out why.
Run through the math, maybe run some simulations, whittle the list down, then pick something once you're at the point where all the options look the same.

Offline AkashaStar

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Re: Oh Lawd, I got dem Op-Amp blues!
« Reply #7 on: November 26, 2022, 06:49:16 PM »
Simplest  is to make a simple voltage divider and take your measurement from that.

Get highest voltage resistors avail and if necessary build a series chain to whatever decade of kV you approx need. I’ve  used Gigohm values; they may go higher, I haven’t checked.

But as I say, really simple method.

For RF, I built an EHT oscilloscope probe with all in mineral oil and encased.
Akasha

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Re: Oh Lawd, I got dem Op-Amp blues!
« Reply #7 on: November 26, 2022, 06:49:16 PM »

 


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