I am learning electronics and for my bread board I have build myself a voltage regulator to get 9V using the following circuit :
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I am using 220V-12V 2A AC transformer. The only deviation from the above circuit is the use of a bridge rectifier W02 8169, Cermet Preset Pot. Also the capacitors C1 and C3 are disk and Tant. respectively, but values are the same.
At what voltage the Pot. should be set so that when I connect it to the bread board, I would get 9V throughout. Thank you for your help.
Try presetting the pot to 744 ohms with a digital meter. OR, power it up and adjust the output to 9 volts. 744 assumes the 120 ohm resistor is exactly 120 -- they rarely are. Either way, you need a meter and if you intend to do more of this, you NEED a meter. Prices start from $5 to ???
Bigdaddy, GG's right. One other thing -- the reason it's called a voltage regulator is that the voltage stays almost exactly the same if the load current is small or large. For the LM317 with a 120 ohm resistor, it stays the same even if there's no load.
So, get a voltmeter. Power up your regulator circuit with *no load* connected, and tweak in the pot to the right output voltage. Then you'll know it will keep 9.0V for any reasonable load (but make sure you put a good heat sink on the LM317 to keep it from getting too hot).
Thanks for your reply. I did that and I do have a digital meter. The problem is after adjusting the pot to 9 volts and then connecting to any circuit on the bread board I do not get 9 volts. In fact it is only about 6.5 volts. But if I attach 9 Volt battery on the same circuit I get 8.47 volts or there about.
That is my point. Its being voltage regulator, it is not doing its job. I do have digital volt meter and I did tweak the pot to 9 volts but when I connect it to the board with a basic 555 circuit I get 6.51 volts at the terminals, but with a 9 volt battery I get 8.47 volts.
The 120 ohm resistor is not exact, but wouldn't the pot compensate for that. There is a heat sink attached to the LM317 firmly with conducting paste in between and the heat sink remains cool.
The regulator is not working correctly. Either it is unstable, because the capacitors are not close enough to it, or it is overloaded by excess load current and is current limiting, or the input voltage is not high enough to feed it throughout the AC cycle (which means that your input capacitor may be faulty or not large enough).
Check the load current with your milliamp meter in series (don't forget to put it back to volt meter operation, afterward, or you will blow the fuse the next time you read voltage).
Hi, Bigdaddy. Check your wiring. If your input cap is backwards, it's not helping you. Look at the voltage across the input cap when you load the circuit. If it bogs way down when you load, that may be an issue.
But it sounds like you might have made the dreaded "wrong pinout" mistake with the LM317. Note the pinout looks like this, with the front (plastic side) of the TO-220 package facing you (view in fixed font or M$ Notepad):
Take heart in the fact that almost everyone's done this once. I smoked by first LM317 that way a few decades ago, and had to pay almost $10 USD (not adjusted for inflation, that was the actual price back then, when dollars actually were worth something) rather than the $0.65 you can pay for one at Mouser these days. And most who say they've never done it are fibbing.
Live & Learn. If this teaches you to always RTFDS (Read The Fine Data Sheet), it's a remarkably inexpensive lesson. And you don't have to do it twice. ;-)
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