Parallax RFID power issue. Please help!

eagletalontim

I was using the SX28 programming board for testing purposes and moved my chip to my project board for further testing. Everything works perfect but the RFID reader is causing my 7805 regulator to get very HOT. I have used 7805's on all of my projects so I figured it would be fine for the new application. Currently, I have the RFID reader, 1 SX28, 1 256K EEPROM, and a 1 wire serial LCD display from parallax connected to the power source...a 12 volt 7.5Ah battery.

So far, I have tried several different things. I went to radioshack and picked up a 7812 (12v 1.0A) regulator and connected the 7805 to that hoping to drop some of the wasted power. I have connected the 7805 directly to the battery and still had no change. I have even put in a second 7805 just to run the RFID and it still got hot. The other 7805 stayed cool. I have tried several different resistors before and after the regulators which did not work either. Since radioshack only has 7805's and 7812's regulators, I am really limited on what I can use. This is a one time project and I can't afford to order 1 or 2 small parts on the internet and pay for shipping, so I am hoping I can work with what I have on hand.

Does anyone have any ideas on how I can keep the heat down but still use the 12v battery. The battery is required since the project is for an access control for my shed. If the power goes off, I still want to be able to get into it.

Edit : Just checked the actual temp of the regulator with my IR temp gauge. The temp got up to 110º F

dandreae

Are you able to use a heat sink to help reduce the heat?

Dave

eagletalontim

No, I tried a large heat sink and the temp of the regulator after 3 minutes was 120º. I was given a link to a regulator which is a direct drop in for the 7805 which is supposed to work. I may try it out.

Here is the product :

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=94126117&uq=634277877058700405

sam_sam_sam

I have found that you have to have the regulator run just RFID reader it dose run very warm to the touch
What you can do is use a mini power board like this one to power RFID reader at this site http://www.futurlec.com/Mini_Power.shtml

This board incorporates the high-speed National Semiconductor LM2575T switching regulator to produce a steady +5V output, suitable for driving most microcontrollers.

It can run both the RFID reader and the SX28 programming board and will be cool to the touch

These are very good little boards to use

I hope this helps

erco

110-120 F isn't all that hot. Clearly your RFID sensor is pulling some current. You might check to see if that's in spec. Is it an option to switch to a 6 volt battery? You'll have a lot less heat dissipated if you drop your supply voltage.

eagletalontim

Considering the max operating temp of the 7805 is 150º, I thing that is way too hot for it. That was the temp of the regulator after 3 minutes of running....it was still climbing, but I turned it off to prevent any component damage. As for using a 6 volt battery, I would not be able to do that since the Maglock that I will be using needs 24 volts. If I run 2ea 12v 7.5 Ah batteries in series, I can get the 24 volts, then tap off of one battery's neg and pos leads, I can get the 12 volt source.

Beau Schwabe

Does your RFID reader need to be run continuously? ... On the older versions of the RFID reader it was required that they be cycled intermittently. This was more to protect the drive circuitry and NOT necessarily to reduce current consumption. The same technique however could also be used in this case to reduce the average amount of current consumed.

Just a thought.

erco

Two 6V in series with a 12V is another option to get 6 and 24 volts.

7805s have thermal shutdown built in. From http://www.fairchildsemi.com/pf/LM/LM7805.html :

The LM78XX series of three terminal positive regulators are available in the TO-220 package and with several fixed output voltages, making them useful in a wide range of applications. Each type employs internal current limiting, thermal shut down and safe operating area protection, making it essentially indestructible. If adequate heat sinking is provided, they can deliver over 1A output current. Although designed primarily as fixed voltage regulators, these devices can be used with external components to obtain adjustable voltages and currents.