LM7812 - Page 1
Forum section: electronics / beginners

author: glinjik
author_rank: Regular Contributor
timestamp: March 05, 2022, 02:03:58 pm
content: Hi I'm building a small project that requires a 12v reg lm7812 I know it requires 2 caps one electrolytic the other is a 0.33uf cap I'm assuming that it should be a ceramic disc cap, or can I get away with something else?

author: TimFox
author_rank: Super Contributor
timestamp: March 05, 2022, 02:53:54 pm
content: For the recommended minimum 0.33 uF capacitor at the input to the 78xx series, I usually install a 0.47 uF X7R ceramic (leaded MLCC for through-hole construction), but I have used 0.47 uF polyester (Mylar), which takes up much more space. For the original 78xx, they are prone to oscillation if you omit this part.

author: bdunham7
author_rank: Super Contributor
timestamp: March 05, 2022, 03:14:01 pm
content: The input cap can be almost anything and it can be bigger than 0.33uF without any issues. A 10uF electrolytic is not uncommon, although probably not necessary. The datasheet will tell you that it can be omitted if the regulator is close to other filter capacitors. The output capacitor should be small and preferably either ceramic or film. If the output capacitor or the capacitance of the load is greater than 10uF, you may need a supplemental protection diode across the regulator to prevent the load capacitance from backfeeding and damaging the regulator.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 05, 2022, 03:23:02 pm
content: https://www.ti.com/product/LM7800page 1!Add a single protection diode, say a 4001 to protect against input shorts.

author: glinjik
author_rank: Regular Contributor
timestamp: March 05, 2022, 03:56:08 pm
content: thank you all here is what i came up with

author: bdunham7
author_rank: Super Contributor
timestamp: March 05, 2022, 04:50:27 pm
content: Quote from: glinjik on March 05, 2022, 03:56:08 pmI'd say you should swap the capacitors, if you even need 100uF for any reason--like high source impedance. The regulator will respond reasonably quickly to input and load variations and you only need to use capacitors to filter out higher frequency variations that the regulator can't respond to. The purpose of that protection diode is to protect against large output capacitance in the load that happens to already be there, but there's typically no reason to expressly add it yourself.

author: TimFox
author_rank: Super Contributor
timestamp: March 05, 2022, 05:19:54 pm
content: Since the introduction of the 78xx series, the datasheets have always required a 0.33 uF or so capacitor directly at the input to prevent oscillation, unless the leads from the previous capacitor are short.The output capacitor is optional, but is often a good idea. Note that the rising output impedance with frequency essentially defines an inductance, which is important to stability questions.

author: David Hess
author_rank: Super Contributor
timestamp: March 06, 2022, 11:28:11 pm
content: The input decoupling capacitor is only required if the regulator is far from the input filter capacitors; a small ceramic or film capacitor is typical but an electrolytic capacitor can be used. The output capacitor should be a small bulk decoupling electrolytic capacitor of like 10 to 100 microfarads per amp. Ceramic or film decoupling capacitors are placed at the loads; they do not do anything useful directly at the output of the regulator.