Trying to control a 12-24VAC relay

Thread Starter

tocxx

Joined Oct 29, 2021
5
Hey!
I'm extremely new to electrical engineering and trying to make some of my circuits a bit smoother with PCBs. My goal is to input 12-24VAC and output a potential free relay output. This is my second circuit that doesn't work, and I'm kind of running out of ideas.

So my first circuit used a bridge rectifier, a mechanical switch and a 12VDC relay. The switch switched between stepping down 24V to 12V, and just keeping 12V, depending on input. I quickly noticed that didn't work, as I measured the voltage out of the bridge rectifier circuit to be variable (like AC is). DC worked fine.

The second is the circuit I believe should've worked, but doesn't. The main difference is the regulator that steps 10-35VDC down to 5VDC for the new 5VDC relay, solving the switch+variable voltage problem. However the measurements of the regulator output, while powered by AC is much higher than 5V, closer to 20V actually, as well as my instrument measuring it as AC..

So my question - Is there an obvious problem with the circuit, or are any of the components wrong?

Thanks a lot for the help!
1635512975129.png
 

crutschow

Joined Mar 14, 2008
28,521
You need a lot more than 330nF to filter the rectified DC from the bridge.
Try at least 470µF.

And you likely zapped the LM7805 since the peak output voltage from the rectifier with a 24Vac input is near 33V, which is more than its maximum rating of 25V (did you look at the data sheet ratings?).
You need a regulator with a higher voltage rating, such as the LM317HV.
 
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sagor

Joined Mar 10, 2019
534
You should also have a back EMF diode across the relay coil... The relay "may" have something built into it, but best to have an external one in case you change the relay to a different part.
EDIT: Corrected spelling mistake
 
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crutschow

Joined Mar 14, 2008
28,521
You should also have a back ENF diode across the relay coil.
That's needed if you are rapidly switching the relay coil off, but that's not the case here.
When the power is removed the output voltage will just decay slowly with no negative transient generated.
 

MisterBill2

Joined Jan 23, 2018
10,500
Is the intention to operate the relay with 12 volts? or with 24 volts?.
AND I have a much simpler way to run a DC relay on AC with no buzz at all. You can even do it with that bridge rectifier.
Connect the AC source to the two AC terminals, then connect one relay coil lead to the DC+ terminal and the other relay coil lead to on AC terminal. The relay will be running on DC and not buzz at all That is because on one half cyclethe relay is powered by the AC, whikle on the other half cycle the collapsing magnetic field keeps the current flowing. It can also be done with two discrete diodes, which works the same and is cheaper, but using the bridge package is "cool."
 

Thread Starter

tocxx

Joined Oct 29, 2021
5
You need a lot more than 330nF to filter the rectified DC from the bridge.
Try at least 470µF.
Alright thank you! I will try a 470µF 35VDC one.
The LM I got has a max of 35V, which I thought would be more than enough, but I can upgrade.
 

MisterBill2

Joined Jan 23, 2018
10,500
Alright thank you! I will try a 470µF 35VDC one.
The LM I got has a max of 35V, which I thought would be more than enough, but I can upgrade.
With the regulator there is a "safe operating area" specification that matters also.
I did make a suggestion in post #8 that will avoid needing a filter capacitor. It uses that collapsing magnetic field to keep the relay current flowing. It really does work.
And I do wonder about why the need for dual voltage operation.
I suggest an investigation into how those switcher power supplies would cover such a broad voltage range. That may provide some useful insights as to how to handle the dual voltage requirement.
 

Ian0

Joined Aug 7, 2020
4,082
I suggest an investigation into how those switcher power supplies would cover such a broad voltage range. That may provide some useful insights as to how to handle the dual voltage requirement.
I like that idea. Use the cheapest switchedmode IC you can find - maybe one for LEDs - and place the relay coil where the inductor is in the application note circuit.

Another point, don’t use too large a smoothing capacitor, otherwise it will take ages to turn off. Don’t forget a 12V relay may stay closed until the voltage gets as low as 3V.
 

MisterBill2

Joined Jan 23, 2018
10,500
Good thinking! but there might be some delay in the response time, both opening and closing.
And still we have no explanation as to the reason for the dual voltage requirement.
And it will be rather handy to know what sort of current the relay is expected to be switching. Is the load a few milliamps, or a few amps? That does make some difference.
If the application is for a really small relay then a really cheap and simple way is to put a 12 volt Zener diode across the relay coil and a series diode and resistor to limit the current if 24 volts is applied.
 

Thread Starter

tocxx

Joined Oct 29, 2021
5
And still we have no explanation as to the reason for the dual voltage requirement.
And it will be rather handy to know what sort of current the relay is expected to be switching. Is the load a few milliamps, or a few amps? That does make some difference.
If the application is for a really small relay then a really cheap and simple way is to put a 12 volt Zener diode across the relay coil and a series diode and resistor to limit the current if 24 volts is applied.
The simple explenation is; I use a lot of 12VDC and 24VDC relays for projects. Sometimes AC, if I'm forced to use some old AC systems (basically it's smarthouse applications). So I'm just playing around with the idea of having one relay 12-24VAC for flexibility.
Usually the biggest ps is rated 1A @ 24V. Smallest is like 400mA @ 12V.
If your method satisfies that requirement, I'm all for it.
 

MisterBill2

Joined Jan 23, 2018
10,500
When "one size fits all" it seldom fits any of them well. The two diode scheme will allow quiet operation of a DC relay on AC, and a simple resistor will llet you use a 12 volt relay on 24 volts.. That can work , unless you have a whole bushel of 12 volt DC relays to use up.
 

Ian0

Joined Aug 7, 2020
4,082
Two suggestions:
1) bridge rectifier, 330uF smoothing cap, the drive the relay with a NSI45060DD. (needs a diode across the relay)
2) bridge rectifier, 330uF smoothing cap, then drive the relay with a ZXLD1350. Use the application note circuit with Rs=1.5Ω to give 67mA. The relay coil replaces the inductor. Ignore the LEDs, but it still needs the Schottky diode.
 

MisterBill2

Joined Jan 23, 2018
10,500
Two suggestions:
1) bridge rectifier, 330uF smoothing cap, the drive the relay with a NSI45060DD. (needs a diode across the relay)
2) bridge rectifier, 330uF smoothing cap, then drive the relay with a ZXLD1350. Use the application note circuit with Rs=1.5Ω to give 67mA. The relay coil replaces the inductor. Ignore the LEDs, but it still needs the Schottky diode.
That smoothing capacitor will certainly slow the release time for the relay, and promote contact arcing if there is much load current. The double-diode half-wave scheme needs no capacitor. And it is cheaper, as well.
 

Thread Starter

tocxx

Joined Oct 29, 2021
5
2) bridge rectifier, 330uF smoothing cap, then drive the relay with a ZXLD1350. Use the application note circuit with Rs=1.5Ω to give 67mA. The relay coil replaces the inductor. Ignore the LEDs, but it still needs the Schottky diode.
Alright, is this what you mean?
Edit: I also want a diode to see if the relay is switched, I'm guessing I can put a 5V diode like the circuit in the docs?
1635853286169.png
 
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