Building a rectifyer circuit, and need some guidance

[Johnnz]

Hi all,
Sorry this project isn't radio control related; actually it is supposed to end up being hot-water controlled, but it does involve a propellor!

Story so far: This afternoon I finished installing an extractor fan above our shower cubicle at home and it is currently switched on and off with the bathroom lights. The fan itself is a 12vac motor working off a 240vac - 12vac (50va) transformer. However, since I dont want to have the fan come on everytime I turn on the lights in there (and I admit I also just want to build an electronics kitset for the enjoyment of it), I bought a "Temperature Controlled Switch" (http://www.jaycar.co.nz/productView.asp?ID=KC5381&CATID) through Jaycar Electronics. My idea being to tape the thermistor alongside the hot water pipe so that the fan will only switch on when the shower is being used. This switch kitset runs off 12v dc, so I will have to rectify the output of the transformer to run it.

I have done some googling, and found this (http://www.electronics-project-design.com/RegulatedPowerSupply.html) diagram which I will try to build using veroboard or suchlike. It is supposed to be able to deliver up to 400mA which should be sufficient to operate the relay in the kit I will build? My main question is what is the purpose of E2?

Is there anything else I need to consider about this project? It's been a while since I have tinkered with this sort of thing but it should be another good learning curve.

Any help appreciated.


Edit: Just looked up the 7805 (http://www.dse.co.nz/cgi-bin/dse.storefront/488ee77b04de04642741c0a87f3b06d5/Product/View/Z6545) voltage regulator on my local electronics retailers website, and it seems that it is designed to only output 5v? Should I use another regulator such as this (http://www.dse.co.nz/cgi-bin/dse.storefront/488ee77b04de04642741c0a87f3b06d5/Product/View/Z6622) one?

[Markz]

The diagram is ok, but you'll want to use a 7812 instead. You can omit VR1 as well

Marc

[phil_g]

To clarify, by 'omit VR1' Marc means to connect the ground pin of the regulator directly to ground rather than via VR1.
Cheers
Phil

[Tomapowa]

C1 and E2 (electrolytic cap) are simply used to filter the output voltage.
As fun as the project might be, it also might be easier and cheaper to buy/use one of those 12v wallwart transformers... like this $4 400ma model:
http://www.allelectronics.com/make-a-store/item/DCTX-1240/12VDC-400MA-WALL-TRANSFORMER/1.html

A 1.5 amp version for less than $9 is here:
http://www.allelectronics.com/make-a-store/item/DCTX-1215/12VDC-1.5A-WALL-TRANSFORMER/-/1.html

[Markz]

...of course! thanks for the clarification Phil!

[Johnnz]

C1 and E2 (electrolytic cap) are simply used to filter the output voltage.
As fun as the project might be, it also might be easier and cheaper to buy/use one of those 12v wallwart transformers... like this $4 400ma model:
http://www.allelectronics.com/make-a-store/item/DCTX-1240/12VDC-400MA-WALL-TRANSFORMER/1.html

A 1.5 amp version for less than $9 is here:
http://www.allelectronics.com/make-a-store/item/DCTX-1215/12VDC-1.5A-WALL-TRANSFORMER/-/1.html

Thanks for the links, but over here in NZ it would cost between $20 - $30 to buy one of those (That sort of thing are sold as high profit margin items in the electronics stores here.)

Thanks for clarifying the role of those components too Mark and Phil. I notice that there is a 7812T and a 7812K out there - is the only difference the packaging?

[Johnnz]

Just trying to get info on 7812's and found this (http://www.dse.co.nz/cgi-bin/dse.storefront/488f721306f67386273fc0a87f3b071a/Export/catalogs/LRN0002273) information. I only have 12vac input voltage not 15vac as recommended there if I use a 7812. How sure are you guys it will still work in my proposed setup?

[phil_g]

When rectified & smoothed, your 12v AC will make 18v DC, ample overhead for a 7812.
Remember the "12v AC" is the RMS value, the capacitors will charge to the rectified peak value which is what gives you the DC output level.
Cheers
Phil

[Johnnz]

Thanks for that Phil. Just adding up now to see if I can make one of these circuits on veroboard from parts from the local electronics store cheaper than ordering this (http://www.jaycar.co.nz/productView.asp?ID=KC5038&CATID) kitset online...

[Johnnz]

Can't find any 1N4003 diodes but can get 1N4004 which as far as I can tell is just rated to a higher max voltage?

Will make it from parts myself since that'll be the cheaper (and more interesting) way.

[phil_g]

4004s are fine, but a little 1A bridge would be easier... half the connections!

[Malc C]

And don't forget a decent heatsink for the 7812 - as you are dropping 18vdc to 12vdc upto half an amp it might get warm (not hot,) so a heat sink may be advisable. I'm sure one of the clever guys will work out the wattage and heat dissapation, and suggest a suitable one.

[Johnnz]

Thanks for all the help chaps :) Got it all together this evening and installed in the ceiling. The rectifier circuit puts out 11.84vDC from 12vac supply. Quite satisfying to sort everything out into position on a piece of prototype circuit board. I should try doing this sort of thing more often... :) Once again, thanks for all the advice, it really makes all the difference in attempting projects like this for me.

[Dan Baldwin]

Thanks for all the help chaps :) Got it all together this evening and installed in the ceiling. The rectifier circuit puts out 11.84vDC from 12vac supply. Quite satisfying to sort everything out into position on a piece of prototype circuit board. I should try doing this sort of thing more often... :) Once again, thanks for all the advice, it really makes all the difference in attempting projects like this for me.

I'm guessing that the 11.84 volts you are measuring is just using the rectifier bridge without a capacitor. Is that correct?

Dan

[Johnnz]

No, that is the output of the Regulator with capacitors attached. I tested it out with the load (temperature switch circuit) attached and the regulator stays at about 23 degrees celcius (tested with infrared temp sensor). So everything appears to work fine even with slightly less dc voltage output from my rectifier circuit than expected.

[Johnnz]

Just another question: Why are there two capacitors attached in series at the output of this (http://www.electronics-project-design.com/RegulatedPowerSupply.html) regulator? Why not just one? I presume it has to do with different properties between the two types? All I know is that Electroyltics are polarized and the ceramic is not. Just wondering :confused:

[phil_g]

Ok, they're not in series, they are in parallel. The large electrolytic provides further low frequency smoothing but because of its construction is quite inductive and therefore cant remove any high frequency noise coming from the regulator. The smaller capacitor does just that. Without the small decoupling caps these regulators can become unstable & oscillate.
In a nutshell, a big one for smoothing ripple, a small one for noise & spikes.
Phil

[Ron W3FJW]

The ceramic cap bypasses any noise and/or hash riding along on the output of the power supply to ground and the electrolytic provides DC filtering...

[Johnnz]

Thanks for those explainations. Oh yes, parallel not series :o