Arouse1973
Adam
I tell you what. This HOP bloke is good aint he?
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hevans1944
Hop - AC8NS
Isolation transformers make some measurements possible, but not necessarily safe. And, yes, working with electricity can be dangerous. You have to know the consequences of "floating" a circuit if, by mischance, you make a connection that defeats the isolation.
Really high-quality isolation transformers will have a Faraday shield connected to earth-ground between the primary winding (one end of which is invariably connected to earth ground in the United States and most, but not all, other countries) and the secondary winding. This shield helps to minimize capacitance coupling between the primary and the secondary that acts to couple high-frequency power-line noise into the secondary. There are other reasons (which I won't go into here) to use this Faraday shield to keep the parasitic capacitance of the secondary to a minimum.
It is really hard to say whether an earth-grounded Faraday shield is really needed for most applications requiring an isolation transformer. Early in my career, while working as a technician, my supervisor purchased the fancy version with Faraday shield for me to use on the bench. IIRC it was large, heavy, and weighed in at about twenty or thirty pounds. It was only rated for about one kilowatt. It came with solder terminals which made it a PITA to use. Later, I purchased much less expensive, ordinary 1:1 isolation transformers, without a Faraday shield but equipped with a power cord and a convenience outlet. I often used either type transformer to "float" my Tektronix tube-type oscilloscope so I could attach the probe's "ground lead" anywhere I wanted to in a circuit, without fear that it would short something out.
The third-wire "ground" prong on the o'scope power cord had to be defeated of course with a "cheater" adapter plug that disconnected the "green-wire" safety ground. Most of the time the equipment that I was working on had an earth-ground connected to the power supply common, so if I needed to, say, float the o'scope "ground" up to several hundred volts above earth-ground, I needed to be damn careful not to place any part of my body between earth-ground and the oscilloscope chassis.
So, by all means purchase an isolation transformer. Just be aware that galvanic isolation is never perfect. There will always be some leakage resistance and parasitic capacitance between the primary and secondary windings. It is up to you to determine how this will or will not affect your "isolated' measurements. An "isolation" transformer that seems to offer good galvanic isolation from DC up to power-line frequencies can look like a dead short at kilohertz frequencies. That sort of problem can usually be solved by placing choke coils in the secondary leads, but be aware of what you are doing. It isn't always just "plug and chug" when it comes to using isolation transformers.
Last year I purchased some Cree power MOSFETs and Cree Schottky diodes from Digi-Key, intending to use these to power a homemade PWM stepper-motor driver/controller that will be used to tune a pair of trombone capacitors on a magnetic loop antenna I am building for the 80m and 40m Amateur Radio bands.
There are integrated circuits available that allow "micro-stepping" stepper motors, thus allowing direct-drive to the antenna tuning capacitor, and that was (and is) my intent. However, the MOSFETs and the Schottky diodes are now packed away until I get a basement workbench cleared off after several decades of neglect. When that happens, I will have an electronics lab (and a ham radio shack) again and can continue pursuing this electronics hobby in retirement. Meanwhile, you are on your own selecting appropriate diodes. If the switching frequency is in the low kilohertz range, I would try 1N4007 diodes first before venturing to the much more expensive Schottky diodes. It is really expensive to get a high PIV (Peak Inverse Voltage) rating for a Schottky diode. I think I purchased the Cree C4D02120A silicon carbide Schottky diodes for my project.
73 de AC8NS
Hop
Really high-quality isolation transformers will have a Faraday shield connected to earth-ground between the primary winding (one end of which is invariably connected to earth ground in the United States and most, but not all, other countries) and the secondary winding. This shield helps to minimize capacitance coupling between the primary and the secondary that acts to couple high-frequency power-line noise into the secondary. There are other reasons (which I won't go into here) to use this Faraday shield to keep the parasitic capacitance of the secondary to a minimum.
It is really hard to say whether an earth-grounded Faraday shield is really needed for most applications requiring an isolation transformer. Early in my career, while working as a technician, my supervisor purchased the fancy version with Faraday shield for me to use on the bench. IIRC it was large, heavy, and weighed in at about twenty or thirty pounds. It was only rated for about one kilowatt. It came with solder terminals which made it a PITA to use. Later, I purchased much less expensive, ordinary 1:1 isolation transformers, without a Faraday shield but equipped with a power cord and a convenience outlet. I often used either type transformer to "float" my Tektronix tube-type oscilloscope so I could attach the probe's "ground lead" anywhere I wanted to in a circuit, without fear that it would short something out.
The third-wire "ground" prong on the o'scope power cord had to be defeated of course with a "cheater" adapter plug that disconnected the "green-wire" safety ground. Most of the time the equipment that I was working on had an earth-ground connected to the power supply common, so if I needed to, say, float the o'scope "ground" up to several hundred volts above earth-ground, I needed to be damn careful not to place any part of my body between earth-ground and the oscilloscope chassis.
So, by all means purchase an isolation transformer. Just be aware that galvanic isolation is never perfect. There will always be some leakage resistance and parasitic capacitance between the primary and secondary windings. It is up to you to determine how this will or will not affect your "isolated' measurements. An "isolation" transformer that seems to offer good galvanic isolation from DC up to power-line frequencies can look like a dead short at kilohertz frequencies. That sort of problem can usually be solved by placing choke coils in the secondary leads, but be aware of what you are doing. It isn't always just "plug and chug" when it comes to using isolation transformers.
Last year I purchased some Cree power MOSFETs and Cree Schottky diodes from Digi-Key, intending to use these to power a homemade PWM stepper-motor driver/controller that will be used to tune a pair of trombone capacitors on a magnetic loop antenna I am building for the 80m and 40m Amateur Radio bands.
There are integrated circuits available that allow "micro-stepping" stepper motors, thus allowing direct-drive to the antenna tuning capacitor, and that was (and is) my intent. However, the MOSFETs and the Schottky diodes are now packed away until I get a basement workbench cleared off after several decades of neglect. When that happens, I will have an electronics lab (and a ham radio shack) again and can continue pursuing this electronics hobby in retirement. Meanwhile, you are on your own selecting appropriate diodes. If the switching frequency is in the low kilohertz range, I would try 1N4007 diodes first before venturing to the much more expensive Schottky diodes. It is really expensive to get a high PIV (Peak Inverse Voltage) rating for a Schottky diode. I think I purchased the Cree C4D02120A silicon carbide Schottky diodes for my project.
73 de AC8NS
Hop
Well, anything to reduce the chance that I accidently kill myself I suppose.
Were required to use them at work / school, for any testing purposes, they sound pretty much like what you described, big and heavy, these had adjustable voltage output and power cord outlets, no soldering or anything like that here, dont know how much those things run at, but im pretty sure it would melt my bank account if I was to even look at the price of a new one.
A used one might be possible though..
That sounds like quite the project you got there
I'll easily try those 1N4007's first then, I think I even have some in a box somewhere..
Also.. a friend had one of these laying around for some reason, I could get it pretty cheap.
http://www.antekinc.com/an-10464-1000va-64v-transformer/
That's a huge step up from the one im using now, though I wonder what voltages other low/medium powered induction circuits run? google is abit scarce with information there
hevans1944
Hop - AC8NS
That toroid power transformer is really nice. Price seems reasonable, too, if you need 1000 VA capacity.
You could use the two 64 V windings to power your MOSFETs and have two lower-voltage windings left over for +5, +12, or whatever power supplies you need for supporting electronics. I think you could connect the 18 V and 12 V secondaries in series-opposing to get 6 V @ 2A, relieving some of the burden on a 5 V regulator. However, don't even think about connecting the two 64 V windings in parallel in an attempt to increase the current capability. Unless the two windings produce absolutely identical voltages, connecting them in parallel will result in large circulating currents because of a slight voltage mismatch. Don't ask me how I know this. There are specific transformers, called control transformers, that are designed to allow their secondary windings to be connected in series or in parallel without creating humongous circulating currents in the parallel configuration.
You could even use the two primary windings as an isolation transformer, albeit with a relatively large amount of capacitance coupling between the two primary windings. I really like toroid transformers because they are very efficient. A real bitch kitty to wind though. And usually quite a bit more expensive for a given VA rating than an equivalent E-I or C-core construction.
You could use the two 64 V windings to power your MOSFETs and have two lower-voltage windings left over for +5, +12, or whatever power supplies you need for supporting electronics. I think you could connect the 18 V and 12 V secondaries in series-opposing to get 6 V @ 2A, relieving some of the burden on a 5 V regulator. However, don't even think about connecting the two 64 V windings in parallel in an attempt to increase the current capability. Unless the two windings produce absolutely identical voltages, connecting them in parallel will result in large circulating currents because of a slight voltage mismatch. Don't ask me how I know this. There are specific transformers, called control transformers, that are designed to allow their secondary windings to be connected in series or in parallel without creating humongous circulating currents in the parallel configuration.
You could even use the two primary windings as an isolation transformer, albeit with a relatively large amount of capacitance coupling between the two primary windings. I really like toroid transformers because they are very efficient. A real bitch kitty to wind though. And usually quite a bit more expensive for a given VA rating than an equivalent E-I or C-core construction.
The early schematic that included the load inductor showed one end of the inductor tied to the negative power supply. The FETs can only drive the other end of the inductor positive. This will result in a really big DC component that will just make things get hot.
I don't have the time to offer a comprehensive suggestion at the moment, but feel I should ask if several problems might be solved by using a single positive high current supply with capacitive coupling between the FETs and the inductor.
The coupling capacitor must be large relative to the frequency and the output current, and must have a sufficient current rating.
The IRS2153 common and the lower FET source pin would then both connect together to the negative side of the single supply.
Another point is that the white plastic breadboard will not be happy at the sorts of currents I think you will see in the FETs.
Ted
I don't have the time to offer a comprehensive suggestion at the moment, but feel I should ask if several problems might be solved by using a single positive high current supply with capacitive coupling between the FETs and the inductor.
The coupling capacitor must be large relative to the frequency and the output current, and must have a sufficient current rating.
The IRS2153 common and the lower FET source pin would then both connect together to the negative side of the single supply.
Another point is that the white plastic breadboard will not be happy at the sorts of currents I think you will see in the FETs.
Ted
hevans1944
Hop - AC8NS
Hi Hop,
I kinda imagined you must have been an amateur enthusiast back in the 60s-70s (it just fits you totally
),as so many electronics lovers were at those days.
I would think with the Web of today that would be a "redundant hobby" ,but looks like it is still alive and kicking.
Never heard of trombone capacitors tuning till now,
the closest I new about was cavity tuning
(for much higher frequencies than 14Mhz).
In the old days they were called antenna-tuners,
some were fully hand cranked others were automatic-motor driven.
Are you into Morse as well?
What kind of Transmitter are you using,
I guess it is an "all Analog" old one...
hevans1944
Hop - AC8NS
I received my Novice Amateur Radio License (KN8UTJ) while serving in the Air Force from 1963 to 1967. IIRC the "ticket" was issued in the summer of 1966 and expired in the summer of 1967, soon after my enlistment was finished. I should have traveled to Columbus OH and taken the tests for either Technician (5 wpm code test) or General class (13 wpm code test) licenses, but didn't. Back in those days it was upgrade or out. The Novice "ticket" was not renewable.Hi Hop,
I kinda imagined you must have been an amateur enthusiast back in the 60s-70s (it just fits you totally
as so many electronics lovers were at those days.
I would think with the Web of today that would be a "redundant hobby" ,but looks like it is still alive and kicking.
Never heard of trombone capacitors tuning till now,
the closest I new about was cavity tuning
(for much higher frequencies than 14Mhz).
In the old days they were called antenna-tuners,
some were fully hand cranked others were automatic-motor driven.
Are you into Morse as well?
What kind of Transmitter are you using,
I guess it is an "all Analog" old one...
During that year "on the air" I became a passable Morse code operator, eventually sending and receiving about 20 wpm. I only "worked" one band: the lower end of 80m with three (maybe four) crystals to set my home-brew CW transmitter frequency. Back then Novices could not use a VFO (Variable Frequency Oscillator), so we were all "rock bound" as far as transmitter frequency. My receiver was a Heathkit SB-301, assembled from a kit while awaiting the arrival of my Novice license, or "ticket" as the amateur radio license is called. My antenna was an 80m dipole assembled on the roof of my barracks, supported on EMT about ten feet above the tar-and-gravel flat roof. Unfortunately it was oriented east-west so it transmitted and received signals mainly from the north and south, and I was in the upper peninsula of Michigan near the Canadian border. Still, I made a goodly number of contacts and became proficient enough on the straight key to purchase an inexpensive Vibroplex clone from Lafayette Radio Electronics. I never much cared for mechanical dot generation, so I soon returned to the straight key, which I still have today. My transmitter was based on the venerable RCA 6146 beam power pentode and had an input power of about 70 watts. It has long since disappeared, but I still have the Heathkit receiver.
Flash forward to 2013. I returned to amateur radio and obtained an Amateur Extra License (AC8NS) on April 1, 2013. To celebrate, I purchased an Elecraft KX3 rig that is extensively digital (but NOT an SDR) with 10 W output power. Lots of control knobs, which IS very old-school. Later I added the Elecraft 100 W KXPA-100 with built-in antenna tuner.
I still aspire to resuming CW (as Morse code transmissions are called) on 80m, hence the construction of an 80m magnetic loop antenna requiring a tuning capacitor as an integral part of the loop. Normally, hams use a Russian military-surplus vacuum variable capacitor for this, but I decided to construct two coaxial capacitors, connected in series, with common tubes moving in and out of two outer tubes... hence the name trombone capacitor. Later, I plan to unsolder the trombone capacitor and fit a vacuum variable in its place. But until that antenna is finished, I have only made a few SSB contacts to nearby states on 40m using an ad-hoc end-fed inverted "L" antenna. I am re-learning Morse code using a Koch trainer app installed on my Android Galaxy 4 cell phone, so hopefully I won't embarrass myself the next time I call CQ using Morse code and failing to recognize the response. Yikes! There are Morse decoders available (one is built into the KX3) but where's the fun in that?
Amateur radio is alive and growing. Since the code requirement was eliminated from the license exam a lot of older hams have gotten back into the game, and younger folks are being recruited in increasing numbers every year. CW ops is great for DX: the narrow bandwidth really punches through the noise. Low power (QRP) operation has become very popular, especially with SOTA activations (Summit On The Air) in places world wide. A popular portable antenna is the Buddipole and the Alexloop, but the KX3 works fine with just two hanks of wire about fifty feet long, one tossed up into a tree, the other laid out on the ground as a counterpoise. It's a fun hobby at any wavelength, almost from DC to Light™. And if you really want a challenge, "moon bounce" or EME (Earth-Moon-Earth) is available, as are amateur orbiting satellites and a whole bunch of digital modulation modes. Twenty-first century technology has really opened up a lot of possible communication paradigms. Or you can just use the Internet, your cell phone, or any of several services such as Skype for world-wide conversations. I don't find that to be as much fun as contacting total strangers having a common interest.
That sure brings up memories...
I never had a license myself ,but some friends did let me CQ a bit in the 20m band using USB,
yep not that of today kind but the Uper-Side-Band one...
The station used a 100W transceiver RF-301 which had terrible reliability and was hard to fix.
They had other stuff too.
I remember a Collins R-390 receiver, which was a great receiver although it was mechanically tuned and was build like a very complicated mechanical watch.
I never had a license myself ,but some friends did let me CQ a bit in the 20m band using USB,
yep not that of today kind
but the Uper-Side-Band one...The station used a 100W transceiver RF-301 which had terrible reliability and was hard to fix.
They had other stuff too.
I remember a Collins R-390 receiver, which was a great receiver although it was mechanically tuned and was build like a very complicated mechanical watch.
hevans1944
Hop - AC8NS
I had my first introduction to ham radio visiting a mentor in my youth while living in Morristown TN with my grandparents in the mid 1950s. He was a single man in his twenties and he owned a Radio and TV repair shop. He managed to tolerate my questions about electronics, probably because he had no one else to talk to about it. He also did odd jobs, mainly audio installations for public address systems and intercoms and such.
In his shop basement he had a ham radio rig and exposed knob-and-post AC wiring attached to the floor joists. On the only visit I remember, he reached up and bridged the wires with his fingers to show me how he tested to see if they were "live" or not. I was pretty young and naive then, but I decided on the spot this man was a couple pints short of a full gallon. He was an AM operator, and I don't know if he knew any code beyond what it took to get his license. His equipment was pretty old, but not as old as the RF-301 you linked and certainly not up to the capabilities of the Collins R-390. Hallicrafter was closer to his economic level:
My father owned one of these after he returned from WWII and that got me interested in SWL (Short Wave Listening), but I didn't have a clue about amateur radio until I met this ham in Tennessee. But I was already moving in the direction of an electronics career, having built a crystal set (with galena crystal and cat's whisker) and tuning coil wrapped around a discarded toilet paper cylinder. I salvage junked radios and TVs for parts, especially variable tuning capacitors and trimmers, using a Weller soldering gun to remove the parts. It was years before I discovered pencil irons, about the same time I discovered home-made solder suckers. I once built an AM superhet radio kit, purchased from Allied Electronics, using that stupid soldering gun. What a mess I made of that. But it did eventually work.
You must be getting on in years, @dorke, to have experienced what you did. You don't see much of that vintage stuff at ham flea markets anymore. Too hard to maintain, and the performance is usually sub-par compared to what is available today. I would not attempt to restore such equipment today... but that's because I am lazy, not because it wouldn't be satisfying. I like old stuff that still works the same as the day it was built.
In his shop basement he had a ham radio rig and exposed knob-and-post AC wiring attached to the floor joists. On the only visit I remember, he reached up and bridged the wires with his fingers to show me how he tested to see if they were "live" or not. I was pretty young and naive then, but I decided on the spot this man was a couple pints short of a full gallon. He was an AM operator, and I don't know if he knew any code beyond what it took to get his license. His equipment was pretty old, but not as old as the RF-301 you linked and certainly not up to the capabilities of the Collins R-390. Hallicrafter was closer to his economic level:
My father owned one of these after he returned from WWII and that got me interested in SWL (Short Wave Listening), but I didn't have a clue about amateur radio until I met this ham in Tennessee. But I was already moving in the direction of an electronics career, having built a crystal set (with galena crystal and cat's whisker) and tuning coil wrapped around a discarded toilet paper cylinder. I salvage junked radios and TVs for parts, especially variable tuning capacitors and trimmers, using a Weller soldering gun to remove the parts. It was years before I discovered pencil irons, about the same time I discovered home-made solder suckers. I once built an AM superhet radio kit, purchased from Allied Electronics, using that stupid soldering gun. What a mess I made of that. But it did eventually work.
You must be getting on in years, @dorke, to have experienced what you did. You don't see much of that vintage stuff at ham flea markets anymore. Too hard to maintain, and the performance is usually sub-par compared to what is available today. I would not attempt to restore such equipment today... but that's because I am lazy, not because it wouldn't be satisfying. I like old stuff that still works the same as the day it was built.
Yes Hop,
I'm not a spring chicken anymore,
but you have more then a decade on me...
I also built an AM radio receiver in the high-school perriod.
Long time ago I was a signal man myself ,
the technical side .
And talking about a "man who is a couple pints short of a full gallon". Is this the way people say thess things today?
While working on one of the "beasts" we had those days(not even the biggest one).
A guy goes "What is this thing here" and touches the anode of a working Eimac 4CX1000 RF power tube in an open drawer, before anyone could blink an eye.
It was like a tornado hit him on the spot...He was lucky to stay alive with minor bruises and a painful burnt spot on his "finger of god".
As far as I remember,
the RF-301 was late 50's vintage.
The Collins R-390 was WWII.
They were used into the 80's
I'm not a spring chicken anymore,
but you have more then a decade on me...
I also built an AM radio receiver in the high-school perriod.
Long time ago I was a signal man myself ,
the technical side .
And talking about a "man who is a couple pints short of a full gallon".
Is this the way people say thess things today?While working on one of the "beasts" we had those days(not even the biggest one).
A guy goes "What is this thing here" and touches the anode of a working Eimac 4CX1000 RF power tube in an open drawer, before anyone could blink an eye.
It was like a tornado hit him on the spot...He was lucky to stay alive with minor bruises and a painful burnt spot on his "finger of god".
As far as I remember,
the RF-301 was late 50's vintage.
The Collins R-390 was WWII.
They were used into the 80's
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hevans1944
Hop - AC8NS
"A few pints short of gallon" I just made up to substitute for "a few cans short of a six-pack" since I don't know if "six-packs" are available everywhere or it that's just an American expression.
I got a similar "shocking" experience with an industrial low-frequency (about 450 kHz) induction heater. Hams sometimes used a pencil to draw an RF arc from their final's tank coil to see if they were "putting out" any signal. I had observed this on several occasions and thought it "safe". No one told me to grip the pencil by the wood, so I gingerly approached the induction heater while gripping the metal ferule that held the pencil eraser. Got an fat arc to the tip of the pencil and another one between my finger tip and the metal ferule. Dropped the pencil immediately of course. I think the resistance of the "lead" and the wood between ferule and the core might have tempered the shock, and of course I never did that again. The finger tip wound was round and deep. Took many weeks to heal.
The worst shock I ever received occurred several years ago at work. I laid my forearm (accidentally) across the poles of two 300 V dry-cell batteries connected in series. That caused some pretty severe burns that, again, took many weeks to heal. That prompted me to design and build a floating 1 kV supply to replace the two batteries. This was a commercial off-the-shelf unit powered from two 6 V sealed lead-acid batteries, all mounted on a circuit board inside a polycarbonate box I also built.
I got a similar "shocking" experience with an industrial low-frequency (about 450 kHz) induction heater. Hams sometimes used a pencil to draw an RF arc from their final's tank coil to see if they were "putting out" any signal. I had observed this on several occasions and thought it "safe". No one told me to grip the pencil by the wood, so I gingerly approached the induction heater while gripping the metal ferule that held the pencil eraser. Got an fat arc to the tip of the pencil and another one between my finger tip and the metal ferule. Dropped the pencil immediately of course. I think the resistance of the "lead" and the wood between ferule and the core might have tempered the shock, and of course I never did that again. The finger tip wound was round and deep. Took many weeks to heal.
The worst shock I ever received occurred several years ago at work. I laid my forearm (accidentally) across the poles of two 300 V dry-cell batteries connected in series. That caused some pretty severe burns that, again, took many weeks to heal. That prompted me to design and build a floating 1 kV supply to replace the two batteries. This was a commercial off-the-shelf unit powered from two 6 V sealed lead-acid batteries, all mounted on a circuit board inside a polycarbonate box I also built.
Damn, you guys sure have been busy, I almost feel bad for just jumping back in..
Well since you guys are still around, I got that digitally adjustable frequency working using a digipot, im now trying to move forward with the project and the next step for me is digital output control of my PSU.
With a ton of inspiration from various sources..
But just like so many things in electronics, i've never tried this kind of regulation before either, most i've tried before this is actually just a potentiometer on the base of T5.. Would this kind of circuit work do you think?
I've only done simple op amp circuits before this, so it sure is quite the step up..
The input on IC2A is comming from a DAC IC, which is controlled by my MCU, the other side of IC1 and IC2 will be used as comparators elsewhere in my induction project
As always, thanks guys!
Well since you guys are still around, I got that digitally adjustable frequency working using a digipot, im now trying to move forward with the project and the next step for me is digital output control of my PSU.
With a ton of inspiration from various sources..
But just like so many things in electronics, i've never tried this kind of regulation before either, most i've tried before this is actually just a potentiometer on the base of T5.. Would this kind of circuit work do you think?
I've only done simple op amp circuits before this, so it sure is quite the step up..
The input on IC2A is comming from a DAC IC, which is controlled by my MCU, the other side of IC1 and IC2 will be used as comparators elsewhere in my induction project
As always, thanks guys!
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