ChrisATX
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Can anyone help me with an acceptable relay (HVAC/Chiller)?

Fri Mar 06, 2015 8:41 pm

Hi all, I'm looking for a single channel relay module that will run on 5VDC, and is switchable using the Adafruit PCA9685 I2C module. Here's the kicker, the relay itself is a beast. 250 VAC, 30 Amps NO:

http://www.electronicnetwork.ca/media/d ... ou/sli.pdf

It switches a 1 horsepower chiller.

Thanks!!!!

Chris

toxibunny
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Re: Can anyone help me with an acceptable relay (HVAC/Chille

Fri Mar 06, 2015 9:08 pm

that adafruit thing is apparently a servo/pwm driver. I don't think it's relevant to relays at all.


Relays are easy though. find a 5v relay that fits the bill, an optoisolator, a small transistor and a resstor or two. you can the wire those up to safely trigger the relay from a single gpio+ground.
note: I may or may not know what I'm talking about...

klricks
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Re: Can anyone help me with an acceptable relay (HVAC/Chille

Sat Mar 07, 2015 1:35 am

Here is a SSR that has opto isolation built in and can be triggered directly by a GPIO (3.3V @ 2.3mA)
http://www.opto22.com/site/pr_details.a ... =480D45-12
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.

ChrisATX
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Re: Can anyone help me with an acceptable relay (HVAC/Chille

Sat Mar 07, 2015 2:17 pm

Thanks! The reason for the I2C bus is I will be using a RTC unit as well as other items (dimmable LED drivers) that need PWM-capable outputs.

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mikronauts
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Re: Can anyone help me with an acceptable relay (HVAC/Chille

Sat Mar 07, 2015 5:36 pm

You do realize relays have different switching characteristics.

You must read the data sheets for the relays you are considering, and make sure they can handle the PWM frequency... pay careful attention to the switching characteristics.

Mechanical relays likely won't do - some SSR's will be fine.

You could always just use a MOSFET.
ChrisATX wrote:Thanks! The reason for the I2C bus is I will be using a RTC unit as well as other items (dimmable LED drivers) that need PWM-capable outputs.
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toxibunny
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Re: Can anyone help me with an acceptable relay (HVAC/Chille

Sat Mar 07, 2015 9:34 pm

PWMing stuff plugged into the mains is not at all the same as dimming an. LED. you should probably use an SSR (and a hefty heatsink) and the pulses should be looooong. 30 seconds at an absolute minimum...
note: I may or may not know what I'm talking about...

ame
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Re: Can anyone help me with an acceptable relay (HVAC/Chille

Sat Mar 07, 2015 11:46 pm

toxibunny wrote:PWMing stuff plugged into the mains is not at all the same as dimming an. LED. you should probably use an SSR (and a hefty heatsink) and the pulses should be looooong. 30 seconds at an absolute minimum...
Actually, it's exactly the same. The pulses can be a few milliseconds to turn on a triac or something for all or part of the mains cycle. If the switching element is turned on for only part of the cycle then the controlled device will only receive a percentage of the total power available for that cycle.

Of course, not all devices can be driven this way (incandescent light bulbs are the usual candidates), and you really need a zero-crossing detector to start your timer at the right point in the mains cycle. So, it's PWM synchronised with the mains frequency.

I suppose a heater or a chiller needs a duty cycle measured over minutes, not milliseconds however.

ChrisATX
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Re: Can anyone help me with an acceptable relay (HVAC/Chille

Sun Mar 08, 2015 1:49 pm

Thanks everyone. This is the setup I'm roughly trying to emulate:

http://www.reefpi.net/smf/index.php/topic,90.0.html

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mahjongg
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Re: Can anyone help me with an acceptable relay (HVAC/Chille

Sun Mar 08, 2015 2:43 pm

ame wrote:
toxibunny wrote:PWMing stuff plugged into the mains is not at all the same as dimming an. LED. you should probably use an SSR (and a hefty heatsink) and the pulses should be looooong. 30 seconds at an absolute minimum...
Actually, it's exactly the same. The pulses can be a few milliseconds to turn on a triac or something for all or part of the mains cycle. If the switching element is turned on for only part of the cycle then the controlled device will only receive a percentage of the total power available for that cycle.

Of course, not all devices can be driven this way (incandescent light bulbs are the usual candidates), and you really need a zero-crossing detector to start your timer at the right point in the mains cycle. So, it's PWM synchronised with the mains frequency.

I suppose a heater or a chiller needs a duty cycle measured over minutes, not milliseconds however.
problem with using the triac as a dimmer is that the trigger pulses for the triac must be synchronized to the mains phase, and must come at a specific short period after each zero transfer of the mains, that means some specific electronics, or sensing the zero crossing, and some very clever software to generate the trigger pulses at reliable intervals.
solving this in hardware would be much easier, but (optical) mains isolation is critical, and mains can kill! So be careful :cry:

In this case a commercial solution would be safer, there indeed are electronic dimmers that accept a PWM control signal.

ChrisATX
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Re: Can anyone help me with an acceptable relay (HVAC/Chille

Sun Mar 08, 2015 5:35 pm

Yes, in the case of LEDs, the Meanwell LDD series takes a PWM signal.

ChrisATX
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Re: Can anyone help me with an acceptable relay (HVAC/Chille

Mon Mar 09, 2015 3:40 am

OK, still researching.

The typical Arduino 5vdc relay modules (up to 10 amps), e.g.:

http://www.amazon.com/Isolated-Optocoup ... B00GYUR09E

Have a grnd, Input and the Vcc

I'm confused on compatible wiring on a typical solid state with only two contacts on the switch side.

http://www.opto22.com/site/pr_details.a ... =480D45-12

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Richard-TX
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Re: Can anyone help me with an acceptable relay (HVAC/Chille

Mon Mar 09, 2015 4:22 am

Why would you use a monsterous relay like that when a 40 amp solid state relay would work perfectly and noiselessly too.

Hook up the SSR directly to a GPIO pin and ground and go.
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klricks
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Re: Can anyone help me with an acceptable relay (HVAC/Chille

Mon Mar 09, 2015 12:19 pm

ChrisATX wrote:OK, still researching.

The typical Arduino 5vdc relay modules (up to 10 amps), e.g.:

http://www.amazon.com/Isolated-Optocoup ... B00GYUR09E

Have a grnd, Input and the Vcc

I'm confused on compatible wiring on a typical solid state with only two contacts on the switch side.

http://www.opto22.com/site/pr_details.a ... =480D45-12
Click on the data sheet link in the above link and look at the connection diagram on page 4.
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.

ChrisATX
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Re: Can anyone help me with an acceptable relay (HVAC/Chille

Mon Mar 09, 2015 6:16 pm

Thank you.

So it's simply the control current going through the two control contacts. I guess I'm confused as to why the breakout board module relay has a Vcc and inputs.

klricks
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Re: Can anyone help me with an acceptable relay (HVAC/Chille

Mon Mar 09, 2015 10:58 pm

ChrisATX wrote:Thank you.

So it's simply the control current going through the two control contacts. I guess I'm confused as to why the breakout board module relay has a Vcc and inputs.
The VCC and ground is used to power the relay coils independent of the control input. A transistor switch is used to do the switching from very low current outputs such as GPIO on the RPi.
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.

ChrisATX
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Re: Can anyone help me with an acceptable relay (HVAC/Chille

Tue Mar 10, 2015 5:29 am

klricks wrote:
ChrisATX wrote:Thank you.

So it's simply the control current going through the two control contacts. I guess I'm confused as to why the breakout board module relay has a Vcc and inputs.
The VCC and ground is used to power the relay coils independent of the control input. A transistor switch is used to do the switching from very low current outputs such as GPIO on the RPi.
Ahhhhhh. Thank you. That makes perfect sense.

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