CCTV Rocks! 0 Posted June 25, 2012 I have a great Sony SSC-DC10 camera that must have 12VDC. I want to mount it on my 24VAC Pelco PS7-24 Pan Motor. I built a circuit that this great website recommended and am experiencing thermal shutdown from the 7812. It seems that 24VAC into this circuit is excessive. I hooked up 12VAC into the same circuit and it works fine. Of course this defeats the whole purpose as I only have 24VAC at the pan unit. Current draw of the working circuit is around a quarter amp so the 7812 should cruise along if I can figure out how to not overdrive it with too many volts. There is a little extra room in the enclosure to fit extra stuff that you fellows may recommend. I perf boarded the the first attempt. Thanks for helping. Share this post Link to post Share on other sites
NotoriousBRK 0 Posted June 25, 2012 Not knowing what you wired up it's hard to recommend suggestions... If you google "24vac to 12vdc converter" you'll find a bunch of products in the $9-$20 range that solve this problems directly. Doesn't seem like a wheel worth re-inventing. Share this post Link to post Share on other sites
ak357 0 Posted June 25, 2012 Most likely you exceeding couple parametres for LM7812 Max input voltage and Max power dissipation across LM7812 Look for LM 317 and good heat sink Share this post Link to post Share on other sites
Horizon 0 Posted June 25, 2012 Hi there. The most important problem is that the regulator is operating at or beyond it's maximum input voltage. When you rectify the 24VAC, you get 34VDC. The 7812 can only handle a maximum of 27 to 35V, depending on manufacturer. Please don't use this regulator - it will fail, and probably kill your camera! The thermal shut down is happening because the regulator is not very efficient. It is a linear series regulator, and has to drop a lot of voltage across itself in order to provide the regulated 12 volts. I calculate that it will be dissipating 5.5 watts, so you'll need a heatsink. The math: (34V - 12V) * 0.25A = 5.5Watts What you could use is a switch-mode regulator. They are more expensive to build, and it's usualy easier and cheaper to buy one. Although if you're cunning, you can scavenge and modify a cell-phone charger. Here's a link: http://makeprojects.com/Project/Repurposed-Switch-Mode-Power-Supply/233/1 You want to find a 24V charger (for use in 4 wheel drives and trucks), as these chargers are more likely to use switch-mode, rather than linear regulators. You should also look into installing a "crowbar" across the camera input, to protect the camera incase the regulator fails and dumps 34VDC into your 12V camera. Follow the link: http://www.radio-electronics.com/info/circuits/scr_overvoltage_crowbar/scr_over_voltage_crowbar.php Another helpful hint is to use a dummy load (rather than your camera) to test out the power supply. Something like a bulb out of a car dash board would do the trick - cheaper to replace than the camera if something goes wrong Share this post Link to post Share on other sites
Soundy 1 Posted June 25, 2012 The LM78xx series specifies a maximum input of 24V - and while the datasheet doesn't state it outright (http://www.fairchildsemi.com/ds/LM/LM7805.pdf), if memory serves, they're designed to take DC input, not AC... so at the very least you'd have to rectify (and preferably filter) the input voltage first: Also, if it's going into thermal shutdown, that indicates there's not enough cooling. I wouldn't run more than 100mA through one of those without SOME sort of heat sink attached, even if it's just a little piece of aluminum that's not attached to anything else: As Alex notes, an LM317 is a better bet: designed for input voltages of up to 40VDC and output current up to 1.5A. http://www.fairchildsemi.com/ds/LM/LM317.pdf HOWEVER - note that you'll still need to rectify the input voltage - if you're just connecting it straight to the AC, I'd pretty much guarantee that's most of your problem. Brian makes the best point though: there are lots of these kinds of products out there already that are just plug'n'go. We've used this one extensively, as they're cheap from our wholesaler, and absolutely rock-solid: http://www.electronicproductonline.com/catalog/product_info.php?products_id=2237 - AC input, jumpers for 6/12/24VDC fixed outputs as well as a variable output, built-in fuse, built-in support for battery backup... 1A and 3A (http://www.electronicproductonline.com/catalog/product_info.php?cPath=31_42&products_id=1770) versions available. Share this post Link to post Share on other sites
CCTV Rocks! 0 Posted June 26, 2012 Where do I begin? This is without a doubt the most knowledgeable, friendly, and quick to help group of people I have ever had the pleasure of replying to! May I reply to you all below? NotoriousBRK: I apologize for not being clearer on the schematic that this site provided. It was very close to the schematic that “Soundy” provided for us to review. ak357: I had a bunch of 7812’s lying around from my days of video game / pinball repair so I thought it may work. Horizon: I will look into your suggestions – I thought this little bitty camera would not be a current gobbler – I also suspect the lens is adding its own load. Soundy: I think you posted a reply to another newbie that illustrated the schematic I followed. That schematic only showed ½ wave rectification, but I went ahead and went full wave (I have all these parts laying around just wanting to do something interesting!) Photo attached at my prototype and a photo of the great camera that Sony made. Hope the photos show up - I may not be uploading correcting. BTW; Mr. Sony has survived all my power supply “tests”!! I will follow all the great advice you experts have provided and promise to provide an update on the final solution. Share this post Link to post Share on other sites
NotoriousBRK 0 Posted June 26, 2012 The 78xx series regulators are simple devices, but they're really a poor choice for anything that draws more than a very small amount of current. 78xx regulators essentially work by converting the excess voltage into heat, so the greater the difference between your input voltage and their regulated output, the more heat they put out (which is also impacted by current draw). The recommendation for most 78xx regulators is to keep the input voltage not more than ~3 volts higher than the regulated output. These days it's better to use a switcher style regulator instead of a linear like the 78xx, even if it requires a couple of additional external components since the switcher will run much cooler. Share this post Link to post Share on other sites
mike_va 0 Posted June 27, 2012 The recommendation for most 78xx regulators is to keep the input voltage not more than ~3 volts higher than the regulated output. The 78xx has a "typical" dropout of 2V, so needs at least that much to maintain regulation... Share this post Link to post Share on other sites
Horizon 0 Posted June 28, 2012 The recommendation for most 78xx regulators is to keep the input voltage not more than ~3 volts higher than the regulated output. The 78xx has a "typical" dropout of 2V, so needs at least that much to maintain regulation... ... maintain regulation at full load. Yep, being pedantic At full load, you will be dropping extra voltage across the mains transformer, and the rectifier diodes, so need to compensate for this. And extra ripple. Also then there's the +/- 5% or so variation in the mains supply to take into account. Maybe make that +/- 10% if you're feeling paranoid...). So it's a bit of a balancing act between designing for efficiency (close to the regulator's drop-out voltage) and reliability. Sorry, I'm being very picky, but there's all kinds of annoying stuff to take into account. Share this post Link to post Share on other sites
mike_va 0 Posted June 28, 2012 The point was that it should be more than 3V as a rule of thumb, not less than 3V. If you want less than 3V you need to use an LDO like the LT1083 etc. Share this post Link to post Share on other sites
