79XX datasheets show differing capacitors on input and output. What is correct values and types to use?

Question

I am making a power supply using a 7812 and 7912 regulator. It's 24 VAC in through diode bridge, resulting in about 18V positive and negative. I'm planning to use a 1000 uF capacitor on each side and then go into the voltage regulators.

The regulators I have are ON Semiconductor MC7812BTG and MC7912BTG. The datasheets for both of these regulators shows .33 uF on the input and .1 uF on the output. If I check other datasheets for LM7912, I see other values and specifically tantalum caps. They even recommend 10x the value if using electrolytic.

Can anyone tell me what is correct? Is there a difference between MC78XX / MC79XX and LM78XX / LM79XX (other than manufacturer)?

ON Semiconductor MC79XX

TI LM79XX

Answer 1

Your situation needs caps for three purposes.

1. Hold up. Since you are using rectified AC, you need to hold up the voltage until the next sine wave. An approximation with margin is to assume that you need to hold up until the next peak. This is also referred to as bulk capacitance to keep the ripple to an acceptable level (as Tony described in his comments).

Allowable drop (ripple) = Vpeak - Vout - VdropOut

For your case, Vpeak = 18V, Vout = 12V, VdropOut = 1.3V (from spec)

Allowable drop (ripple) = 18V - 12V - 1.3V = 4.7V

Next calculate how much capacitance is needed.

C = Current_Draw * Hold_Up_Time / Vdrop

Current_Draw = 0.5 A (from your comment)

Hold_Up_Time = 1/120 = 8.33 mS, assuming full-wave at 60 Hz

Cap = 0.5A * 0.00833sec / 4.7V = 882 uF

If all the assumptions are correct, 1000 uF seems to be in the ballpark.

You need to be confirm that your peak voltage is really 18 Vdc after the rectifier (with a load). If you want it to work if the line is 10% low (recommended), then recalculate with 18V - 10%.

2. Stability. Some regulators need a small cap with low ESR (equivalent series resistance) near the input and/or output for stability. Others may not absolutely need a cap, but the performance can be improved. The stated performance may assume that caps are present. Ceramic caps have low ESR.

3. Destination/decoupling. These should be near the circuits that are consuming the current.

Use the recommended caps for exact part that you plan to use. If you might use several possibilities, use the highest value recommended.

Answer 2

They differ because they are guidelines/recommendations and not requirements.

Generally up to application to determine what is actually needed. More output cap is better surely, but if you have a single low current load with no switching it might be ommitable, on others if you are taxing the regulator and you have switching loads the recommended amount might not be enough.

If your regulator is close to your intended 1000uF cap bank then the input cap can be omitted safely, if there is a long wire then maybe it is needed

Generally one starts with MFG recommended for the package and then test from there. There is usually no reason to outright omit them except very cost sensitive designs.