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Topic: 1117 3.3v voltage regulator (Read 252 times) previous topic - next topic

surepic

I build a voltage regulator circuit for esp8266-01 based on 1117 regulator. As per datasheet for fixed voltage output 0.1uf capacitor should be at Vin and 10uf on Vout.

My question is why on 1117 is oposite of other regulator circuits that 0.1uf capacitor is always at Vout and bigger capacitor is always at Vin?

Im getting very constant 3.23-3.3v on esp.

Thanks

wvmarle

#1
Jun 14, 2018, 02:18 pm Last Edit: Jun 14, 2018, 02:19 pm by wvmarle
My question is why on 1117 is oposite of other regulator circuits that 0.1uf capacitor is always at Vout and bigger capacitor is always at Vin?
Most likely because it's a different regulator.

That said, I normlly put 0.1µ ceramic + 10-100µF electrolytic or tantalum on both sides.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

MarkT

#2
Jun 15, 2018, 11:46 am Last Edit: Jun 15, 2018, 11:51 am by MarkT
The output capacitor is crucial for stability and high frequency regulation, the input capacitor protects the
regulator against voltage spikes, it normally doesn't have to be large as its not acting as a reservoir.
Spikes/noise on the input can directly affect the amplifier circuitry in the regulator, ie the input capacitor is
decoupling for this amplifier (linear regulators are basically error amplifiers).

My intuition is that if the input supply to the regulator is not stiff (ie comes in on long wires perhaps,
or is a PV panel), then the input cap should be large too.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

surepic

@MarkT now it makes sense to me. Thanks for explanation.

TomGeorge

#4
Jun 15, 2018, 04:14 pm Last Edit: Jun 15, 2018, 04:16 pm by TomGeorge
Hi,
LM1117 is a low drop out regulator, which is different to a normal  lm78xx series reg.
The higher input capacitor is to help keep the input voltage ripple free and any input volt drop due to load surges low.

A normal reg needs the input voltage to be at least 3V higher than the regualated output.
A 3.3V would need an input of at least 6.3V input.

A low dropout 3.3V needs its input voltage to be 1.2V higher than 3.3V, that is 4.5V.
This means it can be used to get 3.3V reg from a 5V supply.

Tom.. :)
Everything runs on smoke, let the smoke out, it stops running....

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