Does a voltage regulator run off a higher voltage one need smoothing caps?

[Chris Wilson]

I am building a device which is part 12 volt and a tiny bit needs 5 volts. I have mains transformer to full wave bridge rectifier giving 15 volts, then a 12 volt regulator with an electrolytic on input and output. I need ton then feed a 5 volt regulator from this one, does the 5 volt regulator also need smoothing caps? Thanks.

[Rerouter]

Its preferable to have a small one near to the regulators input, e.g. 10uF.

[danadak]

The datasheet will give you the recommended bypassing. Also
any considerations for cap ESR minimum.


Regards, Dana.

[Cliff Matthews]

Is that transformer+rectifier giving 15vdc with no load? and was that measured with or without filter cap? (important). With some loads, the voltage in/out differential may drop to the point where some regulators (yours unidentified as yet..) cannot function correctly.

[Chris Wilson]

Thanks for the replies, much appreciated. The 15V DC at the full bridge is conservative, probably closer to 17V and that's at the regulator input. The regulator is an L78S series and will run down to a 14V input, but thanks for considering that!

[mariush]

Like someone else said above, a bridge rectifier will convert your AC voltage to a DC voltage with a peak voltage equal to about  1.414 x Vac - ~ 1v (voltage drop in the diodes of the bridge rectifier)

That 15-17v is peak voltage, it's not all the time that much. After the bridge rectifier you need a capacitor to guarantee the voltage the regulator sees is always at least a minimum amount ( 12v + dropout voltage of the linear regulator)

You can estimate the value of the capacitor with  this   Capacitor size =  Current  / [ 2 x Mains AC Frequency x  (Vdc peak - Vdc min)] 

So for example for 12v 1A , if your linear regulator has a dropout voltage of 1v, it needs to always have at least 13v on the input. If you're in UK where mains frequency is 50 Hz and the Vdc peak is 17v, you'll need  C = 1 A / ( 2 x 50 Hz x (17v-13v) = 1 / 100*4 = 1/400 = 0.0025 Farads or  2500 uF ... so at least 2500uF of capacitance are required to always have at least 13v at the input of whatever.

The datasheets of linear regulators assume the input is fairly filtered, so whatever capacitance they recommend at the input doesn't take in account the capacitance required to keep the output of the transformer reasonably smooth and at certain minimum.

It's generally a good idea to at least use a 0.01uF ceramic capacitor for decoupling very close to the in and gnd pins of the regulator, and if the linear regulator is far away from the big capacitor right near the bridge rectifier, they often recommend an additional capacitor, usually around 10uF to 100uF in size.
Some linear regulators also require a tiny amount of capacitance at the output - usually any kind of capacitor 1uF or something like that would work, but there are some designs which require capacitors with a bit higher capacitance value (around 10uF..22uF) and a minimum amount of ESR which makes them work best with electrolytic capacitors (some won't work right with ceramic capacitors at the output). If that's the case, it's usually written in the datasheet.


Your 5v regulator could be connected directly to the output of the bridge rectifier + capacitor, or if the 12v linear regulator can provide more power than what your circuit will use, you can connect the 5v regulator at the 12v regulator output.

Keep in mind though that linear regulators regulate voltage by dissipating what's not needed as heat, so if your product uses 5v at 100mA, the 5v regulator will take the same amount - 100mA - from the 12v linear regulator. Don't forget that when deciding where to connect the regulator.

[Zero999]

Like someone else said above, a bridge rectifier will convert your AC voltage to a DC voltage with a peak voltage equal to about  1.414 x Vac - ~ 1v (voltage drop in the diodes of the bridge rectifier)

That 15-17v is peak voltage, it's not all the time that much. After the bridge rectifier you need a capacitor to guarantee the voltage the regulator sees is always at least a minimum amount ( 12v + dropout voltage of the linear regulator)

You can estimate the value of the capacitor with  this   Capacitor size =  Current  / [ 2 x Mains AC Frequency x  (Vdc peak - Vdc min)] 

So for example for 12v 1A , if your linear regulator has a dropout voltage of 1v, it needs to always have at least 13v on the input. If you're in UK where mains frequency is 50 Hz and the Vdc peak is 17v, you'll need  C = 1 A / ( 2 x 50 Hz x (17v-13v) = 1 / 100*4 = 1/400 = 0.0025 Farads or  2500 uF ... so at least 2500uF of capacitance are required to always have at least 13v at the input of whatever.

I agree, the drop out voltage is likely to be an issue. The L78S12 will only have a drop out voltage of 1V, when the output current is close to zero. To get the full output current with good regulation, the input-output differential needs to be at least 2.5V, with 3V being optimum.
http://pdf.datasheetcatalog.com/datasheets/270/9514_DS.pdf

A 12V transformer is not suitable for use with the LM78S12, unless the current draw is under 200mA, the transformer is overrated with a Schottky diode bridge rectifier and a 10,000µF capacitor but it's still a marginal design at best. A 15V transformer would be more suitable.

Why use a linear regulator for the 12V? The 12V could be supplied by a wall plug switched mode power supply, with the 5V linear regulator for the logic circuit.

[Chris Wilson]

Hi all, thanks again for the replies, let me check just what voltage the transformer gives, I built the bulk of the circuit years ago and the 12V supply has never caused any issue I am aware of, it may be 17V or more. The need for a 5V supply is very recent due to mods to the thing

I appreciate all the info on caps and why and where they are required, and I will act accordingly.

Excellent info, thanks again!

[Zero999]

It's possible that the device running off 12V doesn't need a regulated supply and the regulator might be dropping out without causing any problems.

[T3sl4co1l]

Yes, needed.

Probably the important thing you were wondering about: the caps can be shared common to both regulators.  Since the first one is going to have some capacitance, you don't need to put more in parallel with it.  As long as length limitations (usually stated in the datasheet) are observed.

Tim

[TimFox]

When in doubt with 78xx-series regulators, always put a suitable capacitor with short leads between the input pin and the common pin.  The original data sheets specified 0.33 uF minimum, and I usually use a 0.47 uF X7R ceramic capacitor with short leads.  When other people did not use a similar capacitor close to the device, there was oscillation at some undetermined frequency around 1 MHz.

[T3sl4co1l]

BTW, 79xx regulators are LDO style, so need ESR -- avoid using a preponderance of ceramics up close!

Tim

[Chris Wilson]

Great info, thanks for going into the detail, sorry for the late reply, been away most of the week and came back to an A4 list of things to be done before Christmas by she who must be obeyed!