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Old 15th February 2012, 11:13 PM   #11
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Originally Posted by ChrisA View Post
A big part of the design is the opto-couplers. I CERTAINLY want optos between the high voltage and the Arduino. It takes some effort to pass a linear analog signal through an opto. However serial data like I2C can work easy with an opto. I have yet to work out a parts count and cost for each method.
We use the IL715 and IL711 chips at work for isolation. Pretty interesting because they are coupled magnetically. These days digital is the way to go.

http://www.nve.com/Downloads/il71x.pdf


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The other part of design is the user interface. I'm thinking of using a 2x16 character LCD and a rotary encoder, the kind with a push button on the shaft.

One more idea: Build a "library". ("Library" as a rather exact meaning when using an Arduino.) To abstract away difference between Serial and parallel DACS and so on. Just have exposed functions like "set_plate_voltage()" or "get_cathode_current()". Then if people modify their hardware the top level control routines don't change.
I think an application running on a PC is going to be better than doing a local control. Scrolling through menus on a small LCD is not fun. TCP/IP is nice but adds some complexity.
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Old 15th February 2012, 11:42 PM   #12
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Thanks for all the comments.

I've thought a lot of this through and concluded that using return side current measurement makes isolation not an issue IMO.

The beauty of this approach is that the horsepower of the engine is scalable from Uno level up to the Maple and other 32 bit variants. As I mentioned, I think most of the heavy lifting in SW belongs on a PC, but it could easily be done the other way.

Also I don't think Arduino has a built in D/A at all; there are some PWM channels. I plan to add a $20 8 channel 10 bit D/A that allows synchronous control of all outputs simultaneously. It hooks up to the Arduino using 3 signals.

I think that the regulator is also not so difficult to implement with a control voltage in and current measurement out. Attached is a basic design I'm using as a starting point. It can be configured for either bipolar operation for control grid, or unipolar operation up to 600V for plate or g2. I do think the range selection is needed to accommodate different power tubes and draw smooth curves with 10 bit A/D resolution. I need to measure plate current from 1mA to almost 1A to cover the full range of tubes.
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Old 15th February 2012, 11:50 PM   #13
ChrisA is offline ChrisA  United States
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Originally Posted by tomchr View Post
It's not that hard to get linear signals through opto couplers. Use two identical opto couplers (a dual type is preferred). Hook the LEDs in series and drive the current through them with an op-amp. Use one of the photo transistors as the feedback to the op-amp, use the other to drive the isolated side. I can draw a schematic for you if you're not familiar with the circuit.

~Tom
No, not a "dual type opto" because think of the chip as having a "hot" side" and a "safe side". Using a dual chip means you have a wire or a PCB trace that crosses the opto's center line. I'd prefer a slot milled in the PCB right down the center of the opto chip. Or at least a ground trace with no mask over it. Think of dirt and crud as a conduction path. They make an IL300 opto that has two receivers and one transmitter, They are matched receivers. So, yes it can be done. But then you also do the design using serial. Then you compare the two BOMs, look at cost, square inches and performance.

I still think it could go either way. But if a library abstracted the hardware then it hardly matters. You call "get_plate_voltage()" and don't worry to much the details of how it works after it's built.
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Old 16th February 2012, 12:10 AM   #14
ChrisA is offline ChrisA  United States
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Originally Posted by Michael Koster View Post
...
I think that the regulator is also not so difficult to implement with a control voltage in and current measurement out. Attached is a basic design I'm using as a starting point. It can be configured for either bipolar operation for control grid, or unipolar operation up to 600V for plate or g2. I do think the range selection is needed to accommodate different power tubes and draw smooth curves with 10 bit A/D resolution. I need to measure plate current from 1mA to almost 1A to cover the full range of tubes.
One thing I'd add is "voltage measurement out". Yes you know the control voltage you send using your serial DAC but knowing the volts dropped would not be exact. It depends in the setting of the pots that control the feedback to the op amp. It would be easy to place a voltage diver (10:1) across the output and send it to an ADC.

I agree about a range selection. A relay and a multi-tapped power transformer would be ideal. My plan however is to use an existing bench B+ power supply and with the above "voltage measurement out" I would measure the incoming voltage from the bench supply.

If you publish this for others to build, everyone will use a different power transformer and you'd like the software to auto-scale.
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Old 16th February 2012, 12:33 AM   #15
ChrisA is offline ChrisA  United States
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Originally Posted by Michael Koster View Post
Thanks for all the comments.

I've thought a lot of this through and concluded that using return side current measurement makes isolation not an issue IMO. ...
Yes it will be 100% safe until a component fails or a wire breaks some other thing happens that you did not plan for.

Remember the Ford Pinto. It was perfectly safe as long as you never had an accident.

The purpose of the opto is to be fail safe in the advent of an accident, dropping a screw into a chassis and not noticing should not cause a fatal injury to the user. same if a mosfet overheats and melts pvc insulation off a wire.

If optos are to expensive then at least place a zenier diode on every conductor leaving the metal high voltage enclosure so it is clamped to about 7V. I'd do both

Also and this is a big deal. It's a tube tester right? So before long someone will try to test a bad tube that has shorted elements. You have to make the power supply "short safe".

The short might be to ground or between elements. Also I assume there are some switches or a plug board so different pin-out tubes can be tested and maybe there are many sockets but in any case there is chance of a user error that could do ANYTHING like place 500 volts on the heater you just can't know.

What if you have a software bug that sends 500 volts from a 1 amp supply to a small preamp tube and it melts the physical structure inside the envelope? Any random combination of pins might now be shorted together.

No you can't test for shots in advance of applying power, the power might cause the short.

Bottom line is that you have to plan for things you'd can't foresee

Last edited by ChrisA; 16th February 2012 at 12:38 AM.
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Old 16th February 2012, 01:05 AM   #16
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Great comments, ChrisA. I'm not building a Pinto

I am planning for current limiting the power supply such that a short will not blow anything up. Pulse measurement allows me to limit the continuous current to the tens of mA range in the event the SW tries to continuously apply voltage to an electrode. Switching the range could also switch the current limit.

With proper current limiting and zener clamps there should be no problem with shorted tubes causing damage or safety issues. It should be pretty hard to even blow up a tube.

Readback of the electrode voltage would be useful to diagnose shorts or miswires. The additional A/D channels are available and it would only need another opamp for scaling the measurement.

I'm planning a single voltage range scaled by the feedback resistors for plate and g2, and 2 voltage ranges for the g1. Note that the g1 voltage scaling isn't shown on the regulator circuit yet. Someone building this would be able to scale things to their own needs.

Heaters will be powered by dedicated transformers which allows for isolation and a safe return path in the event of shorts or miswires to heater pins. The cathode will be grounded through a small value resistor for current measurement.

Additionally, with this design a low voltage can be applied to test for shorts before blasting the tube with 600V.

If you have any more safety comments please refer them to the specific design being proposed.

Thanks!

Michael

Last edited by Michael Koster; 16th February 2012 at 01:07 AM.
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Old 16th February 2012, 01:42 AM   #17
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Default Isolators?

Can someone show me a rough sketch of how one of these isolator components being proposed would actually be used (for example) in a plate current measurement circuit?

It occurs to me that using the pulse measurement technique would enable a simple transformer to be used as long as the measurement interval is short enough to keep the core from saturating (!)

Cheers,

Michael
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Old 16th February 2012, 02:48 AM   #18
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Please pardon me for butting in, here, but...

Some of you guys are getting down into the weeds way too fast.

Planning comes first and then, much later, implementation details are considered, only after everything has been planned: required capabilities are selected, then required functions are derived from capabilities, then specs are derived from functional requirements, then a top-level functional modules' architecture is selected after analysis of alternatives, then the top level functions are decomposed to smaller or lower-level functional modules, which all should be traceable back to capability requirements. Then the same entire process is applied recursively to each of the lower-level modules, until finally there is a set of lowest-level modules, that are implementable. THEN you can discuss how to implement them as circuits (or whatever they need to consist of). (And then eventually you start going back up the other way, from components to circuits to modules and finally to a complete system, verifying specs and functionality of lower-level modules as you go, before combining them into larger modules or eventually into a complete system.)

You will consider alternatives based on some of the details of how they might be implemented as circuits, before that, of course, but even at that point you're still not ready to go as deep as where you're trying to already jump to.

Doing things properly, in sequence, and spending more time planning at the beginning, can definitely save both time and money, and should get the best end result.

Maybe the first thing for everyone to come to terms with is: Whose thread is this and what is its purpose?

Michael Koster is the thread starter. So he can decide that, if he wishes. And he does seem to already have some things planned and some implementation details more-or-less decided.

So if there are others who would like to start back at the initial planning stage, it wouldn't hurt to start a different thread. It would probably be beneficial to both.

Last edited by gootee; 16th February 2012 at 03:00 AM.
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Old 16th February 2012, 02:56 AM   #19
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Quote:
Originally Posted by Michael Koster View Post
Can someone show me a rough sketch of how one of these isolator components being proposed would actually be used (for example) in a plate current measurement circuit?

It occurs to me that using the pulse measurement technique would enable a simple transformer to be used as long as the measurement interval is short enough to keep the core from saturating (!)

Cheers,

Michael
The Application Note from Vishay appears to be updated from the version which I have -- see figures 8 and 10.

http://www.vishay.com/docs/83708/appn50.pdf

The material is equally applicable to the HCNR200 -- there's a spice model for this part on the Avago datasheet (Avago Technologies Semiconductors Analog, Mixed-signal and Optoelectronic Components and Subsystems) -- used to be Hewlett Packard's semi biz.
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Old 16th February 2012, 03:16 AM   #20
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Wouldn't it be a lot simpler to just put one opto set in the serial USB link?
I guess there was (is?) some backward compatibility issue with analog impedance on the USB line, but can't that be dispensed with now?
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