Hello everyone!
I’ve recently bought some Chinese voltage regulators L7800 series and this might be a rookie question but excuse me in new in electronics 🙂
So I’ve plugged in the voltage regulator I have kind of high Vi(36v) but I’ve tries with lower voltage as well when the Vo is open circuit I have nice 12v but when I plug a load it goes down instantly like it’s not regulated at all. When I tried the 7812 I count even draw current and it instantly dropped to 1v. Is my regulators cooked or I’m doing something wrong?
Hi, because you say you’ve purchased Chinese L7800 series Regulators it could well mean that they might be fake and therefore not working as intended. But let’s say for a minute that the IC is (or was ok ) to begin with. A good starting point is to always consult the datasheet PDF documemnt first. Now again, referring to the datasheet that I’ve just looked at, on page 3 it clearly states that for regulators with an output voltage range between 5 and 18V the input voltage should be a maximum of 35V but in your case, you have fed your IC with 36V !!!. So chances are, you might have fried your IC. Another thing to consider is the pinout. Again, you need to look at the pinout that is shown in the datasheet to see if you didn’t somehow mix up the legs and connect your input supply to the output pin instead. Also, keep in mind that the GROUND pin (pin 2 in the center) is common for input and output. You also need to make sure that you’ve fitted some capacitors in parallel with the output and input pins to the ground (as shown on pages 3 and 4 of the datasheet by ST). ST is a manufacturer’s name by the way. I suggest that you start from the beginning with a brand new IC (if you have another new one) and follow the above making sure that your input voltage does not exceed around 30V (to provide you with a safe margin). But if it does the same thing the conclusion is clear and simple: you’ve just got yourself some nice FAKEs (Hopefully is not the case). PS, keep in mind that depending on what version of IC you’ve purchased the maximum current delivery capability is limited and in the range of between 1 to 1.5A so, make sure that your load does not exceed that as this will trigger the protection circuit and the IC will go into the shutdown mode to protect itself from burning out. Also is good practice to fit an aluminium HeatSink on it before using it.
@ovi4 Yeah you was right I did try the 7805 with Vi 12v and I got a stable output. I also used a load and it stayed stable so i guess i have to find some sort of step down circuit before i use the 7812’s (if i haven’t fried them already). Anyway thanks for the reply!
Vaggelis Derdelis,
Don’t forget about the device maximum power. and series dropping resistor. The 7800 series of regulators are Linear regulators. In short, the voltage gets dropped(regulated) according to Ohms Law. As an extreme example, suppose you have a L7812 regulator that is operating from a 22V source and a 1 amp load. In this extreme case, the regulator must drop the source voltage 10V (22v – 12v) and deliver 1 amp. The L7812 will have to expend 10 Watts of power (10v x 1A = 10W) to regulate down to 12v. Not sure what your regulator package is, (Assuming TO220) the most power it can dissipate without a heat sink is probably about 1.5W. If you are drawing high currents, you must include a series dropping resistor to take some of the heat away from the regulator to keep it within limits.
Frogman
Hello!
Thanks for the information i will be using it for probably 1A max. It will power a 44W amplifier and I know 1 A is not enough for a 44w amp but it will be running lower within the l7812 limits. I didn’t exactly understand about the serie resistor you told me to use though. My though is since I’m planning to run the amp at 1A draw max I think a current limiting resistor wouldn’t be useful, but i might be wrong.