Triac Circuit Advice for Zero Crossing

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Hi

 

I am planning to develop a PCB, which needs to control power (24V AC 2A(MAX) and 240V 5(MAX)).  The applicance will be 24VAC solenoids and 240V FANS/Motors.

 

Instead of using relays, I would like to try to use a Triac circuit. I aim to place about 8 triacs.

 

Would like to have some advice from anyone who has some experience. 

 

The device needs to controlled by GPIO either from AVR or buffer IC, but from my understanding a Zero Cross might need to be also implemented.

 

If I am correct Zero Crossing is only required if you aim to create some type of dimmer switch?  I have seen some some Triac Drivers come with Zero Cross and others do not.

 

Can Triacs be used for DC controlling?

 

What is the heat dispatation like from a Triac, in comparsion to a relay?

 

Thanks

 

 

 

 

Thanks

Regards

DJ

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Zero cross not needed, but preferred, especially for noise emissions (such as for PWM)...zero crossing lets the current ramp up slowly as the sinewave goes up.  For something that is rarely going on /off (not 500 times a second), is is that less important....if you turn it on every hour, is a surge ok??? You be the judge.

 

Yes, DC is fine, though turnoff is not possible (or difficult).   You can use a triac or SCR as a simple/clever latch (till the current drops really low).

 

A heavy relay has "zero" voltage drop, hence little heat (what is your contact resistance??).

 

An SCR/Triac  has Vdrop, hence losses...dig out your calculator.  If switching every minute, switching losses are minimal (other than perhaps things like motor or lightbulb large surges).

 

A relay provides isolation--perhaps an important consideration.  Triacs make that  a little more complicated, not too bad.

 

 

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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I do zero-cross with relays, too.  Makes the relays happier (for basically the same reasons).  S.

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I do zero-cross with relays, too

That's fast---at least coming close. is better than nothing

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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djoshi wrote:
Can Triacs be used for DC controlling?

Yes for turning on, no so much for turning off! devil

 

Scroungre wrote:
I do zero-cross with relays, too

Helps to prevent arc over when the points separate! (i.e. opening at or near the ZC)

 

Jim

 

Keys to wealth:

Invest for cash flow, not capital gains!

Wealth is attracted, not chased! 

Income is proportional to how many you serve!

 

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djoshi wrote:
Instead of using relays, I would like to try to use a Triac circuit

Have you considered Solid-State Relays (SSR) ?

 

https://en.wikipedia.org/wiki/Solid-state_relay

 

djoshi wrote:
Can Triacs be used for DC controlling?

What kind of "controlling" ?

 

eg, you could rectify the the controlled AC output of a triac ...

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I have had a look at SSR, but aim to use something much smaller like a Triac.

 

The DC device, would be a 12V FAN. 

 

I guess i can avoid using Triac for DC powering and maybe use a relay when required for the DC device.

 

 

 

 

Thanks

Regards

DJ

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I guess i can avoid using Triac for DC powering and maybe use a relay when required for the DC device.

Don't get nutty, use a mosfet...relays are good for severe overloads/environments that might suddenly "blow" a mosfet  

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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or a protected MOSFET; very common for 5V USB, common for 12Vdc, some for 24Vdc automotive.

MIC2514

[top of page 5]

"Dare to be naïve." - Buckminster Fuller

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The protected mosfets are really good....relays are great when there is some super severe things going on & a heavy pair of contacts can take the beating (like a contactor). 

 

Low side protected fets are a good fit many times...take a look at ZXMS6004

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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Thanks

 

I normally use the following: TPS27081ADDCR. Which i believe is very similar in functionality. 

 

 

Thanks

Regards

DJ

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I will keep this Triac  PCB purely for AC power.

 

I will also research into the PCB layout side with AC power to ensure that copper thickness on PCB is correct and layout is right. Is there is anything i should keep in mind?

Thanks

Regards

DJ

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 Is there is anything i should keep in mind?

If dealing with line voltage (or possibility of line connection via failure), the triac should be isolated with an optocoupler.

 

Unless you must, use a low side switch instead of a high side...save $$.   Many auto use high side, since the module is bolted to gnd

The hi side does have the advantage, if the V+ out is shorted downstream along the wire, the high side driver will take care of things.

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

Last Edited: Fri. Jul 16, 2021 - 09:32 PM
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Triacs have their own set of requirements. Voltage drop across a triac is around 1-2V. Multiply that by 5A you have a heatsinking requirement. The I2t rating is normally fairly low, so an overload can easily blow a triac. The triac will fail way before the fuse blows (unless you have expensive high speed fuses). Then there’s snubbing required to filter out high voltage transients. A relay is so much simpler. 

As for pcb layout - track sizing is critical. Ensure you have adequate fusing. Creepage distance is also a legal requirement.

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avrcandies wrote:

 Is there is anything i should keep in mind?

If dealing with line voltage (or possibility of line connection via failure), the triac should be isolated with an optocoupler.

 

Unless you must, use a low side switch instead of a high side...save $$.   Many auto use high side, since the module is bolted to gnd

The hi side does have the advantage, if the V+ out is shorted downstream along the wire, the high side driver will take care of things.

 

I am thinking of using something such as

 

https://www.mouser.co.uk/datashe... and

 

https://static6.arrow.com/aropdf...

 

The low side switch you are referring to, i am guessing this is for the mosfet?

 

Thanks

Regards

DJ

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Kartman wrote:

Triacs have their own set of requirements. Voltage drop across a triac is around 1-2V. Multiply that by 5A you have a heatsinking requirement. The I2t rating is normally fairly low, so an overload can easily blow a triac. The triac will fail way before the fuse blows (unless you have expensive high speed fuses). Then there’s snubbing required to filter out high voltage transients. A relay is so much simpler. 

As for pcb layout - track sizing is critical. Ensure you have adequate fusing. Creepage distance is also a legal requirement.

 

Thanks

 

5A is worst case, i am thinking for 240V it would be best to use a Relay, but for 24VAC i can use a triac as i can use less space on a PCB.

 

Do we require a fuse and filters, even for 24VAC? I am to use PCB base transformer. 

 

Triac come in various SMD packages, such as a  SC-73 and  DPAK, i am thinking DPAK would be a better choice as it popular package among multiple companies?

 

 

In regards to the PCB

Thanks

Regards

DJ

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such as a  SC-73 and  DPAK...

 

DPAK is MUCH beefier than SC-73 , use that, the others are for children.

 

Do we require a fuse and filters, even for 24VAC? ​​​​​​​....

 

is it a 2 amp transformer or 30 amp transformer??   For lower currents try a polyswitch.  Generally, fuse the transformer input.

 

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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djoshi wrote:

5A is worst case, i am thinking for 240V it would be best to use a Relay, but for 24VAC i can use a triac as i can use less space on a PCB.

Do we require a fuse and filters, even for 24VAC? I am to use PCB base transformer. 

Triac come in various SMD packages, such as a  SC-73 and  DPAK, i am thinking DPAK would be a better choice as it popular package among multiple companies?

 

You need to check the thermal numbers. 5A is about 5 watts which is quite a lot of heat to manage and may be above a DPAK.  Generally with Triacs you 'work backwards' based on heat, and that may mean many more amps, to get a cool-able package.

 

To keep 23VAC heat down, you could look at SSR's for the 24VAC, or use MOSFETs to make a SSR.  

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Do you really need 5A? . ...also 5A avg, vs 5A RMS vs 5A peak are all different

 

see last example here (example of 5A & DPAK)

 

https://www.farnell.com/datashee...

 

A very popular 4A RMS Triac   (by convention 4A means 4A RMS)

https://www.ween-semi.com/sites/...

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

Last Edited: Sun. Jul 18, 2021 - 03:09 PM
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Thanks

 

I will read the documents in detail when i get the chance. 

 

I am planning to use Triacs for only the 24VAC and use a individual relays for when i need to use it for 240V. 

 

I can confirm that 24VAC solenoid that will be used will use 250-350mA,worst case is 4 solenoids could be operated at one time, but can request to operate one at at time.

 

I guess due to the heat dissipation and to keep the PCB costing/design simple i should avoid designing some universal style PCB but keep it constraint to what is necessary, which is 250-300mA solenoid need power, instead of trying to design a PCB power 5A.

 

Would i be correct in saying there should not be much heat at 250-350mA @24V?

 

Does it make difference if i was to use a Triac that could be rated to 8A compared to 4A, in terms of heat dissipation or anything else?

For example a 1.2A version is higher in price compared to a 4A, but a 8A version is more a less costs the same as a 4A RMS version. So i am thinking it might be better to use a 8A.

 

I done a quick search for DPAK triac on mouser, i did multiple version for example  Rated Repetitive Off-State Voltage 800V vs 600V, would this make any difference for my requirement?

 

I have not yet gone through all the Datasheets, but would i be correct in saying all DPAK triacs all use the same pin out?

 

In regards to Zero Cross, if there is no Zero Cross, i understand it would introduce some noise emissions, can this effect RF performance in the same design?

 

 

 

 

Thanks

Regards

DJ

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In regards to Zero Cross, if there is no Zero Cross, i understand it would introduce some noise emissions, can this effect RF performance in the same design?

   not once it is on--no noise

 

Low voltage might be better in terms of other parameters, such a perhaps trigger or maybe surge

 

Does it make difference if i was to use a Triac that could be rated to 8A compared to 4A, in terms of heat dissipation or anything else?

  prob lower die & lead resistance...every milliohm adds up.

 

I run 2A  all  time ...cool as ice on the pcb.

 

get some copper board & knife out a small copper area, say 1 inch square...solder one down & try it with a load.

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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djoshi wrote:

Would i be correct in saying there should not be much heat at 250-350mA @24V?

As a rough calc, you can assume 1V drop across a Triac so there you have 350mW, which is not much heat. 

 

djoshi wrote:

Does it make difference if i was to use a Triac that could be rated to 8A compared to 4A, in terms of heat dissipation or anything else?

For example a 1.2A version is higher in price compared to a 4A, but a 8A version is more a less costs the same as a 4A RMS version. So i am thinking it might be better to use a 8A.

I done a quick search for DPAK triac on mouser, i did multiple version for example  Rated Repetitive Off-State Voltage 800V vs 600V, would this make any difference for my requirement?

More volts and more amps give you more margin.

Triacs are a NPNP junction, so the on voltage is a diode drop plus a saturated transistor, and that offset effect means power loss does not improve as much as moving from a 4A MOSFET to a 8A MOSFET for example.

 

Yes, you can find the lowest price parts are not always the lowest current ones.

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avrcandies wrote:

In regards to Zero Cross, if there is no Zero Cross, i understand it would introduce some noise emissions, can this effect RF performance in the same design?

   not once it is on--no noise

 

So can there be an issue during the time its in process of switching on or off. Maybe as a good practise is it worth using a Optocouplers which includes a Zero Cross? I am thinking maybe something from the SMD4 package, unless there is better option:

https://www.mouser.co.uk/Optoelectronics/Optocouplers-Photocouplers/Triac-SCR-Output-Optocouplers/_/N-6x5jl?P=1z0s3nm

 

Who-me wrote:

More volts and more amps give you more margin.

Triacs are a NPNP junction, so the on voltage is a diode drop plus a saturated transistor, and that offset effect means power loss does not improve as much as moving from a 4A MOSFET to a 8A MOSFET for example.

Yes, you can find the lowest price parts are not always the lowest current ones.

 

I will use a 4A as a minimum , but at the time of purchase i can compare the best option if 8A is also available. 

 

 

 

Thanks

Regards

DJ

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Do you need zero cross to avoid a 20 us noise when you turn on your beer sign once a month?  How much on/off will you have?

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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djoshi wrote:
... Optocouplers which includes a Zero Cross?
and current gain for insensitive thyristors

djoshi wrote:
... unless there is better option:
ZCD in either discrete or monolithic form.

The Art of Electronics 3rd Edition | by Horowitz and Hill

Download a sample chapter

[page 11, middle of right column]

4.6.6 Zero-crossing detector

269

AC/DC Converter IC Support Page | ROHM Semiconductor - ROHM Co., Ltd.

[near bottom]

AC Voltage Zero Cross Detection

Migration from the megaAVR® to AVR® Dx Microcontroller Families (additional peripherals, ZCD)

 

"Dare to be naïve." - Buckminster Fuller

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avrcandies wrote:

Do you need zero cross to avoid a 20 us noise when you turn on your beer sign once a month?  How much on/off will you have?

 

If avoid able it would be preferred just to ensure no RF interface happens. Have you had any issue with RF signals?

 

Worst case, on/off could happen few times a day. During testing/design it can happen more often.

Thanks

Regards

DJ

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There are many many many systems where no zero crossing is used (occasional on/off), so my worry would be rather low.

 

If you are PWMMing (such as dimming) or switching something continuously like making a switching supply, then pay more heed.

 

You can also use the ADC to sample & find a close position to the zero cross time, however that is not isolated (unless the entire AVR circuit/user controls are isolated).

There are optoisolators that include zero cross trig.

 

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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Ok, that fine as there are many design which do not use zero crossing.

 

I am using the following device with a Triac

 

https://www.mouser.co.uk/datasheet/2/427/vot8026ab-1767667.pdf

 

SMD6 package look popular and it includes non Zero dectection in the same device and pricing is low £0.30-£0.40

 

 

Thanks

Regards

DJ

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This is my schematic, I am following what the datasheet shows.

 

Do the recommended resistors values look correct, I am not going to be using 250V but 24VAC, do I still require any additional passive circuits for filtering etc?

 

Attachment(s): 

Thanks

Regards

DJ

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you are on the good path...here are some notes for your consideration

 

https://www.onsemi.com/support/d...

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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Thanks

 

I have changed R2 100ohms, as my calculations have shown minimum resistor for 24V is 33 ohms but 100ohm is widely used among my PCBs. I guess I have a little extra voltage headroom.

 

In regards to resistor R4, the application note says this resistor is only needed when impedance on the gate is high.

 

How can I determine, the impedance of the triac gate, as the datasheet does not show it?

 

I guess as my load will be inductive rc snuber circuit will be needed as well.

 

 

Thanks

Regards

DJ

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djoshi wrote:

How can I determine, the impedance of the triac gate, as the datasheet does not show it?

 The gate looks like a diode drop, and there should be curves of gate voltage vs gate current if you want a more precise load line.

R4 is needed to avoid the leakage current thru the opto side triggering the gate, and it helps keep dV/dt

 

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Boost Switching Safety with a Zero-Crossing Relay Driver | Electronic Design

by Gino Castillo [Field Applications Engineer, Dialog Semiconductor]

Related To: Dialog Semiconductor

Jul 20th, 2021

Prevent arcing and extra power loss in a relay circuit by timing its switching at zero-crossing with a high-voltage GreenPAK IC.

...

 

"Dare to be naïve." - Buckminster Fuller

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This is the datasheet for my device,

 

BT137S-600 (mouser.co.uk)

 

Seems there is not voltage Vs current for the gate.

Thanks

Regards

DJ

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This is an equivalent SCR circuit, the triac is 2 of these back to back - see the Base-emitter junction or diode drop, for the G-K path 

 

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Ok

 

From what i have understood, i will also need to use Vgt and Igt to calculate the ideal resistance value.

 

So am i correct that for the triac selected:

VGT = 0.7V

 

But when it comes ti IGT, there is 4 values. They related to G+- and T2+-. Can you advice how to know which value to use?

 

I presume i would need to use T2+ and G+, which is believe is 5-35mA,

 

 

In addition to the gate resistor, i have used the formula to calculate RC values for my snubber circuit.

 

I have set the Voltage 30V, but kept the inductance to 50uH. I am not sure about the inductance as the final solenoid can be changes between different manufacturers. So i am not sure how this would effect my RC selection.

 

But for C i am getting 0.0072 uF

and for R i am getting 83 ohms

 

So i have rounded them up to 0.01uF(10nF) and 100 ohms. 

 

 

 

 

 

 

 

 

 

Thanks

Regards

DJ

Last Edited: Thu. Aug 5, 2021 - 11:02 AM
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djoshi wrote:

So am i correct that for the triac selected:

VGT = 0.7V

 

But when it comes ti IGT, there is 4 values. They related to G+- and T2+-. Can you advice how to know which value to use?

 

I presume i would need to use T2+ and G+, which is believe is 5-35mA,

Yes, with a triac opto drive, the gate follows the anode, so you have G- T2- and G+ T2+  and the spec for your linked part is 35mA guaranteed, so you design to that.

At some point you need to choose what T2 anode voltage equates to triggering, as the opto triac acts like a 1-2 diode drop when activated.

 

You could experiment with sensitive gate versions of the triac, (5mA)  as that should lower the RFI from a ON-state operation.

Those parts have lower dV/dT, but for 24VAC you are unlikely to be pushing that.

 

 

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So 35mA will grantee that triac will fully open. So i am correct my calculations show a 800ohms resistor would be needed. 

 

Does this seem correct? As AN mentioned 100-500 ohms

 

If i am correct the voltage output from MOC3043SR2M is 20V?

 

Is resistor needs for 24VAC power supply on the gate?

 

Just out of curiosity, am i better off using a relay(low Hight) or Solid State relay for 24VAC, as i have read that there can be voltage drops which are more evident when working with low voltages such as 24VaC?

 

 

 

 

 

Thanks

Regards

DJ

Last Edited: Fri. Aug 6, 2021 - 09:53 AM
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djoshi wrote:

So 35mA will grantee that triac will fully open. So i am correct my calculations show a 800ohms resistor would be needed. 

Does this seem correct? As AN mentioned 100-500 ohms

If i am correct the voltage output from MOC3043SR2M is 20V?

Is resistor needs for 24VAC power supply on the gate?

Not quite, you need the lowest practical resistor, as you do not want to be waiting for a higher voltage before the chain triggers.

 

 

djoshi wrote:

Just out of curiosity, am i better off using a relay(low Hight) or Solid State relay for 24VAC, as i have read that there can be voltage drops which are more evident when working with low voltages such as 24VaC?

Yes, there are voltage drops with triacs. So you budget roughly 1 watt per amp.

Another topology for SSR is series mosfets, with a isolated gate voltage driver. With the low cost of mosfets, that is also useful at lower voltages.

 

If you search Digikey et al for Optoisolators - Photovoltaic Output you can find those isolated gate drivers like TLP3905 etc

 

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You can try a little mosfet ssr, cheap in quantities!

 

https://www.digikey.com/en/produ...

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