Inexperienced circuit design beginner here. I would like some feedback on this circuit to switch a 1/2HP 120 VAC motor under the control of an ESP32 development board. Have I done this design right?

Please ignore the pin numbers on the ESP32 schematic -- they're for bare ESP32 modules not for ESP32 dev boards.

Beginner's stupid questions:

I'm planning to use a Mean Well RD-35A dual-output power supply to provide both 5v and 12v power. Will I need decoupling caps to prevent relay coil transients on the 12v rail from disrupting the 5v rail that powers the ESP32?

Is it reasonable to expect the Omron G7L relay to cope with on-times of several hours? It's not explicitly rated for continuous duty but the datasheet doesn't specify a maximum duty cycle, ether.

Any other pitfalls I haven't thought of?

Thanks in advance.

 

Inexperienced circuit design beginner here. I would like some feedback on this circuit to switch a 1/2HP 120 VAC motor under the control of an ESP32 development board. Have I done this design right?

Please ignore the pin numbers on the ESP32 schematic -- they're for bare ESP32 modules not for ESP32 dev boards.

Beginner's stupid questions:

I'm planning to use a Mean Well RD-35A dual-output power supply to provide both 5v and 12v power. Will I need decoupling caps to prevent relay coil transients on the 12v rail from disrupting the 5v rail that powers the ESP32?

Is it reasonable to expect the Omron G7L relay to cope with on-times of several hours? It's not explicitly rated for continuous duty but the datasheet doesn't specify a maximum duty cycle, ether.

Any other pitfalls I haven't thought of?

Thanks in advance.

I've done similar with an ESP32 and it worked well.
I even added PWM to reduce the coil current of the relay after turning it on to reduce power consumption.
If you add PWM, change the coil flyback diode to a fast switching and add an RC snubber to dampen coil oscillations if needed.

Rich

 

Just eliminate the Relay altogether and get a bigger SSR rated for the Motor-Load.
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.
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Just eliminate the Relay altogether and get a bigger SSR rated for the Motor-Load.
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.
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And while he's at it might as well use one with zero cross in it which will be kinder to the motor and its start/run capacitor. Like an opto22 or compatible but make sure the one has the zero cross. You can eliminate the opto coupler and replace with a PN2222 and ~3.3K base resistor. You need between 5 and 20 ma to drive the led in the opto22 brick. At 5v it's closer to 5 ma. Maybe the micro can handle it directly. You'll need to check the specs and do a few rudimentary calculations. The opto bricks come in 10, 25 and up amp ratings. Probably the 10 would be adequate, but the 25 would be robust. You can buy these on ebay cheap. I'd google for a p/n to make sure you get the one with the zero cross. They all look the same and ebay sellers don't know the difference. The same devices are also made by Gordos and a host of others.

 

0.5 HP is < 400W, and thus about 3A on a 120Vrms.

Just eliminate the Relay altogether and get a bigger SSR rated for the Motor-Load.

would be handy if the requirements were clear that it's a simple ON/OFF switching, not PWM or cycle chopping, etc. Using a relay of course wouldn't be ideal if you were doing PWM.

I agree with the contributor that posted about zero cross being easier on the motor - having the µC be aware if of the phase would make it better all around when switching the motor on.

An optotriac (with zero cross detection, meaning the µC need't care about the phase for simple ON-OFF operation), plus a suitably rated triac. would be easy, they're common and inexpensive parts.

What is the purpose of R2? I grok a pulldown if you're driving a FET. Is there some reason you expect a floating GPIO to drive current through the LED?

 

@splud - your HP to watts conversion id off by an order of magnitude.

1HP = 745.7 watts

So, full load steady state current is ~3.1A.
but you need a larger rating depending on the startup torque. So, like I said the 10A is probably ok and the 25 is robust, maybe even handle the locked rotor current, but the breaker needs to kick it out before that.
Anyhow why would he want to go to all that trouble when there is a solution in a box. And it has terminals which will take electrical stake-on terminals for the 14 or 12 ga wire that will be used. And, oh, by the way it's UL. They take 3-32 in so the relay could even be at the motor to keep the big wiring short, and the driver transistor could send the 12VDC to the relay. Depending on distance 24 is probably better.

 

Thanks everyone for your feedback.

This is a homebrew project to replace a purely mechanical controller which is broken and no replacement is available. The existing hardware just does on-off switching.

Soft start is an interesting idea but I don't have enough electronics experience to feel comfortable trying to implement that.

I did consider using a high-current SSR instead of a relay but it was cost prohibitive. UL-recognized SSRs specified to handle a motor of this size cost at least $80 (Canadian), which is about double the cost of a relay and its support electronics. This adds up as I need to build three channels.

@splud:

I've read that the ESP32 MCUs can float their GPIO pins during bootup. The pulldown resistor is there in the hopes of keeping the SSR and relay off until my MCU code takes control of the GPIO pin. I don't think a floating pin can source enough current to turn on the SSR, but I figured adding a resistor was cheap insurance.

 

You need to go to ebay for such things.
OPTO 22 240D10 SOLID STATE RELAY 3-32 VDC 10 AMP 120AC /240AC 240D10-17 Refurb 689741011737 | eBay

$1.90 US + $2.99 shipping (probably combines for multiple items)

You can't put ANYTHING together for that.

 

@splud - your HP to watts conversion id off by an order of magnitude.

1HP = 745.7 watts

So, full load steady state current is ~3.1A.
but you need a larger rating depending on the startup torque. So, like I said the 10A is probably ok and the 25 is robust, maybe even handle the locked rotor current, but the breaker needs to kick it out before that.
Anyhow why would he want to go to all that trouble when there is a solution in a box. And it has terminals which will take electrical stake-on terminals for the 14 or 12 ga wire that will be used. And, oh, by the way it's UL. They take 3-32 in so the relay could even be at the motor to keep the big wiring short, and the driver transistor could send the 12VDC to the relay. Depending on distance 24 is probably better.

Yes, in a perfect world. The actual power expressed in watts is a function of motor efficiency. A typical 115 VAC 60 Hz motor 1/2 HP will typically draw about 8.9 amps or 115 * 8.9 = 1023.5 watts. Nominal Efficiency is about 58.8 % under full load.

Typical Data sheet

• Typical Motor

Motors like this are far from efficient so the 746 watts per HP really doesn't work out well. Also if I wanted to run this motor using a SSR I would want a 15 Amp SSR and many of the cheaper ones on Amazon or Ebay seldom meet their published specifications.

Just My Take...
Ron

 

You need to go to ebay for such things.

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