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Increasing current on 78xx series regulators
by Talking Electronics on March 2, 2006 Table of Contents intro: Increasing current on 78xx series regulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . step 1: Preparing the Components & Circuit Board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . step 2: Drawing in the Copper Clad Board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . step 3: Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . step 4: After Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . step 5: Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . step 6: Placing and Soldering the Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . step 7: Testing and other Mods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Related Instructables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advertisements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 3 3 3 4 4 6 6 7 7
http://www.instructables.com/id/Increasing-current-on-78xx-series-regulators/
intro: Increasing current on 78xx series regulators
Typically 78xx series regulators have a maximum load current capacity of 1 to 1.5 Amperes. Using this design you can double the maximum current of your 78xx regulator. This design was posted on the Net by I Hakki Cavdar of Karadeniz Technical University, Trabzon, Turkey. I have revised some of the components values due to heating concerns and to suit my intended application. Picture #2 is the schematic diagram.
step 1: Preparing the Components & Circuit Board
Component List: IC1 and IC2 - 78xx series regulator IC ( 7805 for 5V, 7812 for 12V etc.) D1,D2 & D3- 1N4003 ( 3 Amp Diodes ) D4 & D5 - Light Emitting Diodes (LED)** R1 & R2 - 4.7 K , 1/2 watt resistor ** C1 & C2 - 4700 uF / 16V electrolytic capacitor C3 - 47,000 uF / 35V electrolytic capacitor Printed Circuit Board ( PCB ) Etching Solution WaterProof marker ** - optional components Cut the Circuit Board using a hacksaw, click the picture for a better view. Using the waterproof marker, draw this to the copper side of the circuit board- copy the RED diagram. Take note of the pin distances of the components so that placing them afterwards will be a breeze. Put the PCB in the Etching solution and wait until you'll see the copperless plate ( around 20 minutes ). Rinse the PCB with water. Clean the Marker Ink with Acetone to expose the copper. Drill the holes for the components and your PCB is ready to go.
http://www.instructables.com/id/Increasing-current-on-78xx-series-regulators/
step 2: Drawing in the Copper Clad Board
Draw the circuit pattern on the copper side using a waterproof marker. The other picture is what it looks like in the other side.
step 3: Etching
After proof reading your drawing,soak it in the Etching Solution.I am using Ferric Chloride to do it.
step 4: After Etching
The copper drawn with the marker remains. Clean it with Acetone to get rid of the marker ink and expose the copper.
http://www.instructables.com/id/Increasing-current-on-78xx-series-regulators/
step 5: Drilling
Drill the component holes and your done with the PCB.
step 6: Placing and Soldering the Components
In placing the components, I always place the resistors first, in this case R1 and R2. Next are the capacitors C1, C2 and C3, please always check the polarities of their pins (you can check this by reading the plastic covering of the capacitor, usually its there) to avoid your capacitor blowing up. You might not want a hot liquid and lots of paper shreds in you face. Next, is to insert the LEDs D4 and D5, again take note of their pin polarities (anode and cathode), this won't blow if polarities are not correct, only that it won't light up. Lastly insert diodes D1,D2,D3 and the 2 regulators. Once all the components are in place, double check their polarities again and your ready. Place the PCB upside down exposing the copper side with the component pins. In my experience, its better to solder the components first before cutting the excess pins but some people I know are more comfortable cutting the pins first before soldering, so this one depends on your liking. Clean all excess pins protruding from the soldered area and your ready for testing your project.
http://www.instructables.com/id/Increasing-current-on-78xx-series-regulators/
http://www.instructables.com/id/Increasing-current-on-78xx-series-regulators/
step 7: Testing and other Mods
This circuit is very easy to test, just connect a power supply to the input at C1. Note that input voltages should be higher than your desired output.For example, if you want a 12V output, your input voltage should be like 16 Volts or higher -- the 78xx regulator can handle input voltages up to 35V. If everything goes well, your 2 LEDs will light up. If it doesn't, check if there are any voltage coming from your output with a Multimeter then check the LED's pins. The output of this circuit is dependent on your 78xx series regulator, say you connected a 7812 regulator, output should be in the range of 11.3 to 11.5 Volts. I have added sufficient heatsink to prevent overheating of the regulator. I connected this to my wireless router and stayed stable after powering it up for 2 days straight. I found a small CPU fan and added it to reduce heat even further, although its not necessary, might as well make use of it.
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http://www.instructables.com/id/Increasing-current-on-78xx-series-regulators/
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Comments
17 comments Add Comment
0_Nvd_0 says:
Jul 20, 2009. 11:06 PM REPLY D1 & D2 are dropping 0.7 volts. So you get output voltage that is less by 0.7 V than the desired value. The diode also has feeble internal resistance, which makes it drop more voltage when higher amount of current is drawn. I suggest the use of schottky diodes with very small internal resistance.
Derin says:
Hakk? abi, bu projeye destek için çok sa? ol. Derin Erkan
Jul 17, 2009. 8:45 AM REPLY
recuat says:
May 10, 2009. 10:14 AM REPLY THIS IS VERRY USE FULL PROJECT FOR MY CIRCUIT BUT IT ONLY CAN HOLD UP TO 3A IF I RUN FOR 24 HOURS WOULD IT BE BURN? OR CAN YOU POST ME A 4 AMP UNIT, BECAUSE I NEED 3A LOAD TO RUN MY CIRCUIT.IF THE MAX LOAD IS 3A THIS WILL CAUSE TOO HOT FOR THE HEAT SINK AND ALSO EASY TO BURN OF. PLEASE ADVICE,THANKS.........
eight says:
Just remember that whatever voltage you "Lose" in a 78xx Reg will come out as heat. E.G. Say you pop 12V through a 7805. 12v IN; 5v OUT; 7V disipates as heat. The 7V will turn the 7805 Regulator into a toaster. A rather large heatsink is mandatory and I have found that one twice the presumed size is a minimum. I'll upload a nifty project I did using the above example, so you can have some fun. BTW, makes a nice hand warmer on a cold day. This is my 1st post here, so in advance, sorry if I pissed anyone off. That is/was not my intent. :p
Jul 15, 2006. 2:02 AM REPLY
mctylr says:
It is not the 7 volts that dissipates as heat, it is the power (Watts) that is converting into "waste" heat.
Dec 14, 2007. 12:00 PM REPLY
So that is 7V times the current draw (up to 1A for TO-220 package or 0,1 A for the TO-92 package), which is 7W or 0.7W depending on which 7805 regulator you are using. At 7 watts you would certainly need to attach a heatsink for the supply to be reliable and stable.
eight says:
Dec 14, 2007. 6:54 PM REPLY Indeed I do hear where you are coming from. Of course I understand the relationship between Voltage Amperage and Wattage. My point here was that the differential of 7 Volts goes somewhere and that somewhere... is in heat. I acknowledge that heat is of course noted in terms of watts and Volts x Amps = Watts. I happen to use 220's all the time, so I did not differentiate the power handling of a 92. Merry Hannukah and Happy Christmas !
Talking Electronics says:
No Harm done, opinions are welcome here.
Oct 23, 2006. 4:16 AM REPLY
ratgod says:
anyone know a way of putting 7812's to make about 4A output? I have a LCD I need to make a PSU for.
Jun 6, 2007. 11:28 PM REPLY
Also a "Sharpie" permanent marker is good as an etch-resist pen, we get them in canada at walmart, though I have recently found a pack of 5 at the dollarstore (I will not let you know how much they cost though).
mctylr says:
You could simply use a LM338 which can handle up to 5A continuous output with adequate heatsinking. See LM338 from National for datasheet and example schematic.
Dec 14, 2007. 11:54 AM REPLY
Or you could use a 7812 with a series pass transistor that can handle the higher current. Examples are given in any manufacturers' datasheet. See the High Current Voltage Regulator example schematic on page 13 of the National's LM340 / LM7812T datasheet.
http://www.instructables.com/id/Increasing-current-on-78xx-series-regulators/
For a 4A power supply I would suggest using a single 2N3771, 2N5301, or 2N5302 for the pass transistor with a large heatsink attached. Personally I don't recommend using this parallel 7812 regulator trick, as the 7812's have an risk of thermal run-away, if the current handling is not evenly shared between them. It may work, but it is a hack, and may not work with all 7812s, and has a higher risk of failure than using a just as easy alternative solution.
ratgod says:
Thanks, I will look into them
Dec 14, 2007. 12:37 PM REPLY
mctylr says:
Normally rather than using a second 7812, you would use a power bipolar transistor like the 2N3055A (NPN) http://www.onsemi.com/PowerSolutions/product.do?id=2N3055A and 2N2955 (PNP) and related. See Figure 2 at
Mar 8, 2006. 10:56 AM REPLY
"LM317T Voltage Regulator with Pass Transistor" at http://www.users.qwest.net/%7Eptaylor/Electronics/DC-PCsupply/PC-DC-AT-supply.jpg http://www.zen22142.zen.co.uk/Circuits/Power/1230psu.htm
Talking Electronics says:
Yes, those are alternative circuits if you really want high current outputs.
Mar 9, 2006. 6:48 PM REPLY
mctylr says:
Dec 14, 2007. 12:15 PM REPLY colinbeeforth is correct, the parallel regulators circuit is an hack (hey it's neat, but it has caveats), and while it may of worked for you, you cannot be certain that all individual 78xx regulators will perform identically. Using a series pass transistor is no more complicated that wiring up this parallel regulator circuit. See High Current Voltage Regulator sample schematic on page 13 of the LM340 / LM78xx datasheet from National. That circuit only needs 5 components versus 7 required (not counting either LED & their current limiting resistors) in your circuit. You can substitute the pass transistor with a different NPN transistor with adequate current and power ratings. For 2-3A possible easy to find transistors would include 2N3055, 2N3771, 2N3773, 2N5301, TIP41, TIP120, and many other NPN power transistors.
mctylr says:
Oops, that should be LM340 / 78xx datasheet.
Dec 14, 2007. 12:17 PM REPLY
g7mjv says:
Nice one
Nov 29, 2007. 11:11 AM REPLY
offlogic says:
Mar 25, 2006. 9:24 AM REPLY If you use a shallow glass dish (like a cassarole dish) for etching your PCB, you can shine a flashlight up through the bottom to check the progress of the etching easily.
colinbeeforth says:
Mar 8, 2006. 6:15 PM REPLY Unfortunately, you can't rely on this circuit to work every time. If the regulators are very well matched (same batch) it might work. If they have significantly different output voltages, one will volunteer more current than the other and you will find it shuts down well before the circuit gives the X2 maximum current. It is a perennial problem with regulated supplies, if you want to parallel them, you need some sort of active current sharing circuitry as well. The diode isolation circuit you've shown works well to provide X1 current with redundency if one regulator should fail, another common technique, but it can't reliably ensure load sharing between regulators.
http://www.instructables.com/id/Increasing-current-on-78xx-series-regulators/