12vac to 5vdc = smoke? Help please

[jbeucus]

Hello,

I've built a firefly simulator based on the instructable: **broken link removed**

It's comfortable from 3-5.5 vdc and I'd like to connect it to my low-voltage landscape lighting system. I tried creating a rectifier using 1n4001 diodes rated @ 50v/1amp and passing the resulting dc into a 7805 voltage regulator. So far all i've accomplished is creating 4 SED's.

The 12vac transformer isn't much help, it says 12vac maximum 300watts.

If I haven't already said something to identify me as a total amateur, then let me point out that I'm a complete noob and by the lingering smell of fried electronics I have to admit I might even be a bit dangerous.

Please someone, help me before I kill another component.

thanks

John

 

[Papabravo]

A schematic would be helpful in figuring out where you are going wrong.

 

[chemelec]

Hello,

I've built a firefly simulator based on the instructable: **broken link removed**

It's comfortable from 3-5.5 vdc and I'd like to connect it to my low-voltage landscape lighting system. I tried creating a rectifier using 1n4001 diodes rated @ 50v/1amp and passing the resulting dc into a 7805 voltage regulator. So far all i've accomplished is creating 4 SED's.

The 12vac transformer isn't much help, it says 12vac maximum 300watts.

If I haven't already said something to identify me as a total amateur, then let me point out that I'm a complete noob and by the lingering smell of fried electronics I have to admit I might even be a bit dangerous.

Please someone, help me before I kill another component.

thanks

John

300 watts at 12 volts is 25 amps.
That will Definately Smoke a lot of parts.

 

[Speakerguy]

Make absolutely sure your bridge rectifier is wired up properly first. Even I get that confused sometimes and I'm an electrical engineer.

Make sure you're using the transformer correctly. If it's got dual secondaries, is centertapped, etc you'll need to make sure you wire everything up right to get 12VAC. Otherwise you might be getting 24VAC, which when recified will blow your voltage regulator more than likely and give you less safety room with those 1n4001's (I always always at least use 1N4002's, even on voltages like this). Make sure you have the primaries and secondaries correctly identified, otherwise you'll be getting a LOT of voltage at the rectifier and the diodes are smoke no matter what you do The transformer being oversized is a good thing, you will get better voltage regulation from a bigger transformer generally, but yeah if something is wrong downstream you've got a big power source upstream waiting to dump current into it.

 

[audioguru]

If the regulator is connected backwards then the load will probably smoke.

 

[Speakerguy]

Yeah that too What's a SED btw? smoke emmitting diode?

 

[Super_voip]

Don't build the recifier, buy one, that would be the safest option. Then just connect the AC to the ~ symbols

 

[Speakerguy]

Those are so expensive compared to 1N400X diodes though...

 

[Blatman Bond]

You should make it step by step, and check it if it is work properly.

 

[jbeucus]

Thanks everyone

Wow thanks for the enthusiastic and speedy replies!

I've taken the best of your suggestions and rewired the thing, this time using a 200v/4a bridge rectifier that I purchased. I've also added a 100uf cap accross the bridge DC outputs.

Voila, no smoke, perfect 5v output and happy fireflies. BTW speakerguy79, yes a SED is a smoke-emitting device.

Score one for the home team.

Next question; the 7805 gets smokin hot. So hot that even with a heatsink I'm not comfortable putting this thing in a box and setting it out in my flammable landscape. Any thoughts on a way out of this problem?

I saw a diagram somewhere that used no bridge but just one diode on one side of the ac effectively halving your voltage which is of course perfect for me. I tried that but couldn't get voltage out of my regulator and 6v is just a smidge too much. I suspect this may have something to do with the regulator not liking the frequency of the 1/2ac. Just guessing, total noob here.

any suggestions?

Oh, and I'd just like to repeat that I succesfully did not ignite any innocent components this evening. Thanks again.

John

 

[kchriste]

Next question; the 7805 gets smokin hot. So hot that even with a heatsink I'm not comfortable putting this thing in a box and setting it out in my flammable landscape. Any thoughts on a way out of this problem?

According to the site, the FireFlys should only draw around 0.5ma, so the 7805 should be cool to the touch. Make sure you have a 0.1uF cap on the output of the 7805. It may be oscillating. How much current is it drawing.
You could also try reducing the current consumption of the FireFly circuit by increasing R2 & R3 to 470hm: because you are running it on 5V instead of the 3V which it is designed for. The Leds will be a little dimmer, but it may just do the trick for you especially if you use high efficiency LEDs.

 

[jbeucus]

unsend, unsend

Man, why isn't there an unsend button?

Ok, before anyone else points it out, I'd like to point out that I'm an idiot.

Half of the Ac wave is still 12v peak

doh.

Maybe a 9v regulator feeding a 5v?

How about a "switching" regulator? I don't really know what that is but I've seen it mentioned.

The objective is to run my 5vdc fireflies off of my 12vac landscape light transformer in a manner that's cheap and wont ignite dry grass. 7805, switching, witchcraft or any other means I haven't thought of would be fine.

suggestions?

Thanks again

John.... still no smoke tonight

 

[kchriste]

Maybe a 9v regulator feeding a 5v?

You'll still have the same amount of heat to dissipate. It'll just be spread around more.

How about a "switching" regulator? I don't really know what that is but I've seen it mentioned.

Google for R-78xx-0.5 it is a drop in switching regulator replacement for the 7805, but as my edited post states above, you shouldn't need it.

 

[audioguru]

According to the site, the FireFlys should only draw around 0.5ma, so the 7805 should be cool to the touch.

Six LEDs each at about 20mA each equals 120mA but they aren't lighted continuously.
The 12VAC has a peak of 17V and after the rectifiers is 15.5V so the 5V regulator has 10.5V across it.

Maybe the hot 7805 and PIC are the same ones that were used when the rectifier diodes were backwards and now they are broken.

 

[Speakerguy]

A pre-regulator before your final 5V regulator is a good idea. It will also decrease the size of the filter capacitors you need significantly. Also consider adding a fan to your system to keep the cool.

A better solution though would be a switching regulator.

https://focus.ti.com/docs/prod/folders/print/ptn78000w.html

Order a couple of free samples. They are wide-ranging input to adjustable output and very high efficiency.

 

[jbeucus]

Cool running

I actually had only a 25ma LED and my multimeter hooked into the power supply circuit when it was cooking the 7805. I added the .1uf cap across the 7805 output as suggested by kcriste and since the voltage seemed good I decided to risk another microcontroller and wired my firefly into the supply.

The fireflies are blinking merrily and the regulator is running cool. I don't understand why, but hey.. it works. Could the one LED and the multimeter really have been the culprit or was it the addition of the capacitor? Maybe just bad wiring?

my working circuit includes a 100v/2a bridge rectifier, a 100uf/50v electrolytic cap, a 7805 and a .1uf ceramic cap.

I can't wait to see my wife's reaction when fireflies come to life in Nevada. Thanks for everyone's help on this, I'd have been lost without you.

John

 

[kchriste]

Could the one LED and the multimeter really have been the culprit or was it the addition of the capacitor? Maybe just bad wiring?

It wouldn't have been the one LED (Assuming you had a resistor in series with it). It was most likely the lack of the 0.1uf on the output, but bad wiring is a possibility also. It's good you've got it working.