hello, I am working on a new bot and I am planing on using a 9.6V 1000mAh Ni-Cd battery with a 7805 regulator. As I was making the small board to regulate the power to a picaxe friendly 5V I hooked up power to the board and was soldering the battery on (battery is allays to be used at 5 volts. I needed cheap on hand method because I had no connectors for the battery) when the 7805 blew up. i checked the board and there appears to be no visible shorts on it (will have pictures when I have a camera to use tomorrow after noon) the multimeter says that there is a short but could it be from the blown 7805 still in circuit would anybody know is is about .5 M ohm of resistance.( after checking again it appears that ere is no short in the circuit) the switch is also bad but is should not have shorted it out because it is not conned to ground at all.
I don't know what it could be it appears to be hooked up correctly and there is a switch but forgot to check to see if it was off as i was focused on getting it together. upon further investigation it appears the switch is no good anyway ( again after rechecking to day it appears fine).
Did you really have power applied to the board while you were soldering, as you describe in your post, or did I misunderstand? This is really poor practice, and could have caused you some trouble. Operating temperatures for these chips is up to 125C. You can easily get it a lot hotter than that while soldering. Make sure you have power to your circuit turned off an the battery disconnected before you solder anything.
I am sorry about doing this but once in a while these things get to me…
Hello,
I am working on a new bot and I was planning on using a 9.6V 1000mAh Ni-Cd battery with a 7805 regulator. As I was making the small board to regulate the power to a picaxe friendly 5V, I hooked up power to the board and was soldering on it (battery is allwasys to be used at 5 volts needed cheap on hand method) when the 7805 blew up. I checked the board and there appears to be no visible shorts on it ( I will have pictures when I have a camera to use tomorrow afternoon). The multi meter says that there is [a short], but could it be from the blown 7805 still in the circuit. Would anybody know if about 500 ohm of resistance [is the correct value]? The switch is also bad, but it should not have shorted [the 7805] out because it is not connected to ground at all.
I dont know what it could be. It appears to be hooked up correctly and there is a switch, but I forgot to check to see if it was off as I was focused on getting it together. Upon further investigation, it appears the switch is no good anyway.
Thanks
mudmule
And… Huh?
(battery is allwasys to be used at 5 volts needed cheap on hand method)
sorry i took so long to reply, yesterday was busier than I expected it to be.
thanks for some grate info. I believe that what MarkusB suggested is right in that it was hooked up the wrong way.
I have learned many things from this 1: do not work when tired more likely to make mistakes.2: always reread a post before posting (thanks for catching the grammar errors) and 3: to keep power off at all times when working on the circuit and double check everything.
I will be more careful after this
here is a circuit diagram (don’t have any pictures sorry)
Thanks for the drawing. It is still a bit hard to know if you are hooking it up backwards. If you look in the image below, the pin marked 1 should be your input.
I couldn't find any mention on the 780x datasheet, but on many regulators with a T-220 package, the mounting tab/hole for the heat sink is connected internally to GND. So make sure that nothing is shorting that tab to either pin 1 or 3.
I find it very useful when prototyping a circuit to use a breadboard first to get the circuit working and verify all my connections. Only then do I move on to soldering the components together on a board. If you have or can get a breadboard, it is well worth it.
thanks for the picture. I have a bread board and I used it to check the circuit first. I think I just accidentally switched the leads from the battery around (put + where - should have been).
thanks for the extra info It took me a while to find the right way the first time around and ended up having a to just guess and hook up a multimeter.
Ah, the lessons we learn the hard way are the ones that stick with us.
1. disconnect power when fiddling with anything that effects the circuit.
2. remove rings and hanging jewelry before reaching into circuit
3. ground yourself to eliminate static discharge into VERY sensitive components
4. read the specs.
Don’t feel too bad, Almost everyone has hooked something up backwards and fried something as a result. If you’re like most of us and not made of money, you learn not to do that in short order as you watch your money go down the drain. I myself fried a $100 board this month because I was careless. ouch!
A 7805 seems a small price to pay for that lesson.
I also have a few dead 7805’s or 78L05’s in my history.
My least favorites:
(1) Somewhere in one of my mechanically-traced PCB, the 5V output of the 78L05 was shorted to ground. When I turned on the power, the regulator got very hot and presumably failed (I desoldered it before testing). After painstakingly scraping the board for 2 hours while testing continuity, the short finally went away. I didn’t have another regulator on hand, so I put a jumper from my Arduino 5V to the board. When I turned on the circuit, it fried my Arduino ATMEGA328 chip and the FTDI chip. I eventually got new regulators, a new FTDI chip, and a new ATMEGA328 IC, and got it fixed up (about $15 and >4 hours to fix).
(2) I finished soldering a complicated single-sided PCB, but made a big mistake – I had switched the input and output pins of the 78L05 regulator in the schematic. When I switched it on I got a toasty 78L05. The fun part was that I hooked up the “power on” LED to the higher voltage instead of the lower voltage, so the LED indicated that I had power, but the regulator was toasted and quite hot. I think I burned myself on that one. I ended up just turning the replacement 78L05 around, bending the pins the wrong way, and soldering it in, rather than building a new board. It was a prototype, so I fixed the error in the next few boards I made, and the patched-together prototype still runs. (>2 hours to fix, and a month delay on getting the new parts)
These things happen – sometimes it’s the $0.15 regulator that goes, and sometimes it’s the expensive hardware.
