Rectify 12v ac for relay supply

[KarenColumbo]

I need your help again. I'd like to supply a 12v dc relay with a 12v ac tranny secondary I happen to have hanging about.
The relay is content with any dc >8.4v, so I think it's alright if I feed it with the 10.4 I get when I (bridge-)rectify it.
1. Dat true?
2. Would simple 4007s (or similar) suffice?
3. How much filtering/smoothing would I give the dc? 10uF? 100uF? Will I need several of those smoothing caps? Should I add a small one for the heck of it (100n or so)?

[antonis]

<12v ac tranny secondary>
BJT or JFET..?? 
(to the best of my knowldge, "tranny" is exclusively used for transistors..)

1. How come 10.4 VDC..??
Full-bridge rectification w filter cap should be (12 x 1.414) - (2 x Diode forward voltage drop) which is about 15.5VDC..
(may be even higher with light load so you might need a series resistor..)

2. Of course, for up to 1A relay current..

3. It depends on specific load regulation demands..
If you tell us your relay specifications we'll be happy to tell you about the size of reservoir cap..
(100nF should do no harm but it should be overkill for relay RF shunt..)

P.S.
You didn't refer on reverse diode across relay for inductive kick prevention..

[KarenColumbo]

This is what I get when I post something from the top of my muddled head without thinking    

Yeah, it's a transformer. So maybe Transy?

Of course! The 1.414 thing. Completely forgot.

Ach!

Can't I just diode rectify the bugger, filter it and be done with it? Relay will take 26.5V dc max acc. to data sheet (https://produktinfo.conrad.com/datenblaetter/500000-524999/507425-da-01-en-RELAIS_ZETTLER_AZ822_2C_12DE.pdf

Do I have to regulate it? Would save space if I Wouldnt ...

[ElectricDruid]

Can't I just diode rectify the bugger, filter it and be done with it?

I would. That's not necessarily a recommendation

You've got plenty of voltage headroom on the transformer output, so use a full wave bridge rectifier with your 4007s, and then put plenty of uFs of smoothing on the end. The 100nF is "best practice" and won't hurt, but you won't miss it either. I'd say 470uF is a minimum, and 2000+ is better, but it really *does* depend on how much ripple is too much ripple. Have you got a 'scope to look at the final output voltage?
Adding a resistor and another cap for an RC filter on the DC is going to make the output voltage *very* load-dependent. Whether that's an issue or not with a relay connected to it, I don't know. I stay away from mechanical things

[merlinb]

Yes you can just rectify and use a smoothing cap. Or rectify, add some series resistance, then the smoothing cap. This may get you closer to 10V so you can run the relay cooler and with less ripple. Either way there is no need to regulate.

3. How much filtering/smoothing would I give the dc? 10uF? 100uF? Will I need several of those smoothing caps? Should I add a small one for the heck of it (100n or so)?

You didn't say how much current the relay coil needs, but if yours is a 98mW coil then about 8mA at 12V. You'd be OK with 100uF or more. No you don't need several caps.

[antonis]

Relays tend to be "immune" to voltage variation (ripple) as far as their coil power isn't violated..
(e.g. a relay of 15V nominal voltage and 10mA working current can safely work either on 12V with 12mA or 18V with 8mA which count for 20% voltage ripple..)

So, estimate your coil resistance (load) from nominal current (10mA typical) and coil power (about 100mW) [easily resulting to 10R coil resistance  ] and throw it togheter with your wished voltage regulation to the calculator below..

https://www.changpuak.ch/electronics/power_supply_design.php

Or better, blindly pick from the some hundrends of μFs drawer a cap and then open one eye to verify its voltage rating.. 


edit: Merlin has proven by far faster..!! 

[PRR]

NO-capacitor will "work", but the to-zero ripple will make the relay buzz and will throw electro-buzz all over what you are relaying.

Yes, 100u is plenty for a few mA of uncritical load, and you can use a resistor to drop the 17VDC down in the 10V-14V range for happier relay.

[GibsonGM]

Transformer....sometimes  "Trafo".... 

[Sooner Boomer]

This might help (or it might just confuse...).  For a quick answer, jump to the end.

https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3250_AppNote&DocType=CS&DocLang=EN

One other thing to be aware of is that when you add in a filter cap(s), you have to take into account the charging/discharging times.  This will affect pull-down and release timing of the relay (and switched circuits).

[KarenColumbo]

okay, since I have no reputation here that can be tainted, I'll jump into the deep water and cry "help!"
Explanation: I have NO technical background whatsoever  , I went to a gymnasium with a humanistic background (philosophy, ancient greek and latin, arts, literature and such. So there's HUGE gaps that probably never will be closed in my lifetime.

But I guess I try to tackle this, maybe you will (as always since) steer me into the right direction.

- The transformy (see? I won a single letter) secondary puts out 12vac and is rated for 1A.
- I bridge rectify the hell out of this ac via a DF06M i happen to have scattered around (https://www.vishay.com/docs/88571/dfm.pdf)
- So there's 16,968 vdc at the output, says my calculator.
(- I stick 100n, 100uF and 220uF in series, just for the heck of it)
- the relay coil wants a dc voltage between 8.4 and 26.4 to operate (according to data sheets).
- You gentleman suggested a vdc in the range of 10-14 vdc to prolong the coil's life and minimize probable damage
- Ohm says R = V / I
- I have V, I need R, everybody mentions I, but the only I there is is the Imax of the transformer sec. And I guess there is a (small) Imax for the coil lest I burn it to buggery.

So I'm completely stuck here. (I never grabbed the concept that there's something beside voltage, current that is, that is irrevocably bound to it and changes with it).

[merlinb]

I have V, I need R, everybody mentions I, but the only I there is is the Imax of the transformer sec. And I guess there is a (small) Imax for the coil lest I burn it to buggery.
So I'm completely stuck here. (I never grabbed the concept that there's something beside voltage, current that is, that is irrevocably bound to it and changes with it).

'I' is the current that flows into the relay coil, which from your datasheet will be about 8mA.

[KarenColumbo]

Thank you Merlin. So that I get a picture of this: Those 8mA are the current the coil "draws"? Or the max current that the coil can suffer before it explodes? In other words: Do I have a say in how much mA the coil takes? Or is this a constant I can calculate with under my particular circumstances? If so, I calculated a resistance of 625 Ohms to drop those 17V to 12V

[merlinb]

Thank you Merlin. So that I get a picture of this: Those 8mA are the current the coil "draws"? Or the max current that the coil can suffer before it explodes? In other words: Do I have a say in how much mA the coil takes? Or is this a constant I can calculate with under my particular circumstances? If so, I calculated a resistance of 625 Ohms to drop those 17V to 12V

The 8mA is the current at the rated coil voltage of 12V. The coil is basically a resistor equal to 147 ohms, so the current will indeed vary with the applied voltage, but you don't need to be very exact with relays. Especially since we're making a lot of assumptions about the transformer voltage to begin with, i.e. it is likely to be more than 12Vac since you are hardly loading it, and we didn't subtract diode drop across the rectifier, and what is the true wall voltage? Too many variables. Just slap in a 560R resistor or whatever, and see what you get.

[antonis]

I went to a gymnasium with a humanistic background (ancient greek...)

Τούτου δοθέντος, θα ηδυνάμην όπως καταδειξω υμάς την ορθολογιστικήν χρήσιν του, υπό του Merlin προαναφερθέντος, σειριακού αντιστάτου, ωμικής τιμής 560R, διά την δευτερεούσης σημασίας ωφέλειαν, όσον αφορά στην δημιουργία RC βαθυπερατού φίλτου κατωφλίου συχνοτήτος αποκοπής 2,8 κύκλων ανά δευτερολέπτου σε συνδυασμό μετα του συμπυκνωτού, τιμής 100μF, ομού και τατυτοχρόνως μετά της αρχικής προθέσεως οσον αφορά στον υποβιβασμό της τάσεως λειτουργίας του ηλεκτρονόμου..

(Breef translation for the rest of forum members from slangy ancient greek..)
You can simultaneously use the 560R voltage drop series resistor, refered by Merlin, for forming RC low-pass filter of 2.8Hz cut-off frequency together with 100μF reservoir cap..

[KarenColumbo]

Thx again. I'm beginning to understand 

[antonis]

I'm beginning to understand 

That's exactly the time for an argument about the use of transistor switch instead of relay...!! 
(what switched load you intend to drive..??)

[KarenColumbo]

Heh They relay will switch an AX7 gain stage in and out of my tube amp project (there is a thread going on in the members only area).

[antonis]

That said, we could shake hands for a 470 - 560 R series resistor, a 100 - 220 μF shunt capacitor and also an - optional - 100nF shunt ceramic cap.. 

[GibsonGM]

I went to a gymnasium with a humanistic background (ancient greek...)

Τούτου δοθέντος, θα ηδυνάμην όπως καταδειξω υμάς την ορθολογιστικήν χρήσιν του, υπό του Merlin προαναφερθέντος, σειριακού αντιστάτου, ωμικής τιμής 560R, διά την δευτερεούσης σημασίας ωφέλειαν, όσον αφορά στην δημιουργία RC βαθυπερατού φίλτου κατωφλίου συχνοτήτος αποκοπής 2,8 κύκλων ανά δευτερολέπτου σε συνδυασμό μετα του συμπυκνωτού, τιμής 100μF, ομού και τατυτοχρόνως μετά της αρχικής προθέσεως οσον αφορά στον υποβιβασμό της τάσεως λειτουργίας του ηλεκτρονόμου..

(Breef translation for the rest of forum members from slangy ancient greek..)
You can simultaneously use the 560R voltage drop series resistor, refered by Merlin, for forming RC low-pass filter of 2.8Hz cut-off frequency together with 100μF reservoir cap..

The ancient greeks had 100uF capacitors??  WOW! 

Don't forget your reversed diode across the coil to catch back EMF   

[KarenColumbo]

Already done, sirs! And I stole the driver circuit from Rod Elliott, I must admit.
Whpiped up a quick board -> AllPCB