Suggestions for supporting 400 Hz 115Vac on embedded controller PCB

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I am working on designing a circuit card using one of our favorite AVR controllers.  One of the many functions for this controller design will be to switch 115VAC (400Hz) power to a heater circuit that will be located remotely.  The 115VAC current will be fairly low (< 750 mA).  I would like to get a discussion started as to opinions regarding design techniques (aka precautions) for ensuring safety for both the user and the low voltage circuitry.  The plan is to use an opto-isolated solid state relay to do the switching.  Obviously the 115VAC power will be fused as it comes on to the PCB.  What other suggestions would our resident engineers recommend?

 

Thanks

Jim

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I switch two 120vac/15A circuits using a mega324 and 12v relays.  I too use a Solid State Power Relays from Omron that has a big heatsink, and its mounted in a metal cabinet with two cooling fans underneath it blowing ambient air through the box.  THe 5 - 24vdc control input is fed by the 12v relay which is controlled by the M324 through a relay driver.

 

The SSR(Solid State Relay) gives you the isolation you need depending on the manufacturer specs of course. 

 

Heres two pics of my setup:

 

These pictures remind me that I need to do some clean up in the cabinets  blush

 

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

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You can easily use triacs...many are avail the can be driven directly by the AVR...However this is non-isolated & would be suited for systems that have no connections to the outside world, or use insulated switches (like you'd have in a space heater, power saw, etc).

Is the card enclosed an insulated (floating) system?

 

In lieu of that, optocouplers are good, but care must be used in the PCB process.  If production cost is not relevant (does this driver need to cost $2 or $20)  the solid state relays are good & a lot of the safety factors have been designed for you and certified.

Even if relays are used, at some point you actually deal with line voltage & need sufficient insulation at that point.

 

These are only 25 cents:

https://www.digikey.com/product-detail/en/ween-semiconductors/BT136-600D-DG127/1740-1018-ND/2779834

 

 

These pictures remind me that I need to do some clean up in the cabinets  blush

That is a top-of-the-line setup  yes 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Tue. Aug 6, 2019 - 01:31 AM
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rfdes wrote:
I would like to get a discussion started as to opinions regarding design techniques (aka precautions) for ensuring safety for both the user and the low voltage circuitry.
Are there safety standards for aircraft 115Vac/110Vac 400Hz?

Reasons :

  • 28Vac 400Hz already bites operators
  • 115Vac/110Vac 400Hz may exceed extra low voltage (ELV) standards
  • such flows to an aircraft passenger's seat (and flight crew, and flight attendants, and the avionics bay)

 

Are there 115Vac/110Vac 400Hz GFCI?

 

"Dare to be naïve." - Buckminster Fuller

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400Hz suggests aircraft electrics. A relay doesn’t really care if the ac it switches is 60Hz or 400Hz. A triac might.
Sharp have a range of opto triacs - not sure if they’re good for 400Hz.

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avrcandies wrote:

These pictures remind me that I need to do some clean up in the cabinets  blush

That is a top-of-the-line setup  yes 

 

In what way?  Other than the SSR's are one of the best on the market.

 

 

 

Kartman wrote:
400Hz suggests aircraft electrics.

Learn something new everyday.

If this is the case, I would assume you need FAA certifications for the thing.  I can remember when we made the fiber modems that went into the runway lighting vaults for Denver International Airport.  Had a lot of certifications to meet before we could put them out there.

 

JIm

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB user

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jgmdesign wrote:
If this is the case, I would assume you need FAA certifications for the thing.
or military specifications though there may not be a link between USA FAA and USA DoD for aircraft power.

Safety standards and system specifications are ideally listed in the bid request from the customer.

 

"Dare to be naïve." - Buckminster Fuller

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avrcandies wrote:

 

These pictures remind me that I need to do some clean up in the cabinets  blush

That is a top-of-the-line setup  yes 

 

 

 

In what way?  Other than the SSR's are one of the best on the market.

 

That is very well done!  You seen plenty of pics around here of the bottom barrel technique.  

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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avrcandies wrote:
That is very well done!  You seen plenty of pics around here of the bottom barrel technique. 

 

Ahh, thanks!  The Unit on the right is a PCB of my own design.  Controls the Irrigation on the property, and the pool filter pump and the pool heater.  Theres a serial port that connects to my Crestron system for commands, and an RS-485 port for connection to the pool RTU, the property lighting modules, and the lab lights.  THere is also an I2C port(the cat5 connector) that feeds a 4x20 lcd with three buttons on the front panel for local control of the unit. The can on the left houses the SSR's for the pump and the heater, along with two small cooling fans at the bottom of the unit.  Theres a cover that I removed for the picture, and the top sides are slotted to allow the warm forced air to escape.  I do need to clean up the thin control wires though.  Originally I had an LM7812 to supply power to the fans as the input to the relay control was between 15 and 24vdc.  two diodes with their cathodes connected supplied power to the 7812 and it in turn fed 12v to the fans when either the heater or the pump were turned on.  When I finally added the PCB controller the control lines were all 12vdc so there was no need for the 7812 anymore.  So, I suppose I should remove the clutter....or at least make it picture presentable.

 

Sorry for the brief hijacking of the thread "rfdes"  I was originally trying to simply show you one way I used an AVR to control High Voltage/Current with an actual example....OOPS! surpriseblush

 

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB user

Last Edited: Tue. Aug 6, 2019 - 04:07 AM
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In terms of safety, you need to comply with the likes of NEC (if you're in the USA) and UL. If it is for an aircraft installation or other specialised environment, there most likely will be specific standards you need to comply with. These standards will drive your circuit and layout as they will specify things like creepage distance, insulation, temperature rise etc.

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Good Ol Relays work too! You can use an n-fet or opto to drive the coil depending on how much isolation you desire.  I do not envy having to work within DO-178 guidelines ever again! 

Good luck with your project.

 

Jim

 

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So Jim, (OP), building 1 or 1 million?

 

As mentioned, what is the environment, (factory, train, aircraft, test lab, Mars rover, etc)?

 

Switching the power supply might, I think, be the "easy" part of the project.

 

Dealing with water proofing, vibration, temperature ranges, connector breakage, etc., might be other issues to think about.

 

Depending upon the specifics of the project you will also want to think about system failure modes and automated shutdown.

For example, what happens if the wire to the remote device is accidentally sheared and the power leads are short circuited?

 

JC  

 

Edit: Typo

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Hi -

I appreciate all of the comments and suggestions.   The application in my case is a legacy military test set upgrade for which we are designing a touchscreen panel.  The display's touchscreen has a lower limit temperature issue which will require the use of an external heater.  Thus our custom display controller board will support a PID controlled heater circuit.  The test set uses 400 Hz power for other, higher power,  functionality and we are stealing a small about(30W) of power for the heater.  We planned on using a small SS relay due to the low power requirement.

 

Jim

 

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rfdes wrote:
The display's touchscreen has a lower limit temperature issue which will require the use of an external heater.  Thus our custom display controller board will support a PID controlled heater circuit

PID control in this case seems to be overkill, a simple P control should be good enough, does it really need PID?

 

Jim

 

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The display's touchscreen has a lower limit temperature issue which will require the use of an external heater

Interesting, since I've designed that for LCD in extreme cold...what type of heater are you using (enclosure or the display itself)?

 

 

probably not good for touchscreen?

https://www.heatron.com/products/details/clearview-transparent-heater/

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Tue. Aug 6, 2019 - 05:56 PM
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rfdes wrote:
The application in my case is a legacy military test set upgrade for which we are designing a touchscreen panel.  The display's touchscreen has a lower limit temperature issue which will require the use of an external heater.
and the operator is wearing gloves; IIRC, one is taught that during Arctic safety training.

LCD with HID is available off-the-shelf for Arctic; those have a heater for the LCD though the one I operated had sealed tactile switches on the display's bezel (no touchscreen)

Though the following would be out-of-spec, could place it and you in a temperature chamber and trial it :

Grove 12 Channel Capacitive Touch Keypad (ATtiny1616) - Seeed Wiki

 

"Dare to be naïve." - Buckminster Fuller