Capaciter voltages for a step down circuit

Hey

so learning about circuits etc and I need 3.3 to power my esp-01 to control a relay.

using a lm317 regulator i have built the circuit. I am using a 220uf to bridge vin to ground.

my vin is 13.2v at 1.2a. what voltage should the cap be? i would assume 220uf 16v would be fine?

the second cap is 10uf and that bridges vout (3.3) to ground. would a 10v cap work there?

would i be better using say a 36v cap on both?

yes i know i could go out and buy a buck converter but thats not going to teach me anything :slight_smile:

The voltage rating of the capacitor must be higher than the voltage you expect it to encounter.

Have you looked at the LM317 data sheet? Capacitor sizes are recommended in the copy I have: 0.1 uF on input, 1 uF on output.

The data sheet should have cap values based on a load as a reference design. If you use caps that are rated for at least 1.5 times the voltage they will see would work.

yep ive gone over the datasheet :slight_smile: i was just unsure about the voltage part :slight_smile: yeah i had the right caps just used the wrong voltage ones (i changed psu and forgot to up the cap)

that which does not kill you etc etc :wink:

ill swap out to a 36v 220uf.

thanks for your help guys.

I think 36V is overkill for a 3V3 supply, you could go down as low as 6V1 and still be fine.
As a rule derate the voltage of a capacitor by 80%

An ESP draws ~80mA on average, but several hundred mA during short transmit burst.
It is wise to use a >=470uF low ESR buffer cap on the 3.3volt power rail.

A linear regulator (LM317) will dissipate 0.8watt powering the ESP.
It will be hot without a heatsink.

Next time use a (Pololu) 3.3volt buck converter.

1/3 of the power draw from your 13.2volt power source, and no heat.
Leo..

Hi khios,

Two points of clarification:

When selecting the voltage rating of a capacitor you need to consider the voltage it will see (sounds obvious but read on). If you 13.2 volts is coming from a regulated supply, the chances are the capacitor will likely only see 13.2V give or take some small amount. Here a 16V cap is fine, I would however use a 25V part because I'm more comfortable with the extra headroom.
However if your 13.2 volts is coming from an automobile then 16V is woefully too low. An automotive circuit can see 50V transients.
If your 13.2 volts is coming from an automotive battery charger without a battery connected...... well you had better check.

The significance of using a lower value capacitor as suggested by jremington is because at high frequencies low value capacitors actually have a lower impedance than high value capacitors. This has to do with the physical construction of the larger capacitors. Longer leads results in poor high frequency performance. I think Grumpy_Mike had some information on his website.

A good reference is Understanding Destructive LC Voltage Spikes. This comes from Pololu. They used a "good" regulated power supply and got spikes more than three times the operating voltage just by switching their device on.

For the "downstream" side of the LM317, you can use capacitors with voltages as low as 1.5x the operating voltage. For the "upstream" side, you would usually need much more generous ratings.