Voltage Regulation issues - 78Lxx + zener?

[indecided]

Hi.. i'm trying to get a regulated 8v supply with the use of a ua78L08 at the moment.

My question is, will the quality of the supply improve if i keep the existing zener diode clamp to ground? will leaving the zener improve or deteroriate the supply?

also, can a 78l08 run stably without the 0.33 and 0.1uf caps required in the datasheet?

Thanks!

 

[gary_choo]

Hi, sorry bcos i can't really understand ur Q -->
"will the quality of the supply improve if i keep the existing zener diode clamp to ground? will leaving the zener improve or deteroriate the supply? "
ANyway, the capacitor used to filter out the unwanted high freq noise and to stabilize the Ripple generated by the IC or ur input voltage supply.
It is better to use the recommended capacitor. Especially if ur Regulator cct is use for Audio cct, bcos Audio cct will be very sensitive to the stability of it's supply voltage. The high frequency noise or Ripple from the supply might caused the distortion or noise interrupt to ur cct.
U can ignore the additional cap as suggested in the IC's application cct only if:
1. this reg cct is not use to supply to any voltage or current sensitive cct (the ripple contains at the reg output might damage ur cct bcos of the not stable output voltage and current will not be stable if the voltage is not stable)
2. Ur input voltage for this Reg IC is VERY STABLE.

With this Reg IC, u don't have to use any additional Zener Diode bcos the output of this IC will be regulated to 8V. Unless the output voltage u required here is lower than 8V. But in this case, u better change the IC to a lower rank part.

Sorry for my long explanation. hopefully it could help u..

 

[Oznog]

You cannot use a zener properly on a reg output. What "quality" issue are you trying to solve?

The reg may or may not work without caps. If it oscillates, it might oscillate a lot! I think I've seen a 5V reg swing between 0 and the 12V input. In any case, seeing the circuit stable once doesn't prove anything, changes in load, source voltage, temp, or stray capacitance may start an oscillation.

 

[indecided]

You cannot use a zener properly on a reg output. What "quality" issue are you trying to solve?

The reg may or may not work without caps. If it oscillates, it might oscillate a lot! I think I've seen a 5V reg swing between 0 and the 12V input. In any case, seeing the circuit stable once doesn't prove anything, changes in load, source voltage, temp, or stray capacitance may start an oscillation.

This regulated supply is part of a circut designed for use in a automobile, where the transients are quite persistent. the pertinent problem is because of the use of a voltage divider for a op-amp/comparator, i need a constant voltage so the circut's linearity is mantained. Hence, I want to do as much as i can to make sure that it does not stray, given 8v or 7.5v.

and i discovered that the 78L08 certainly does not work in a automobile without it's capacitors, the circut loses its linearity totally.

 

[Nigel Goodwin]

and i discovered that the 78L08 certainly does not work in a automobile without it's capacitors, the circut loses its linearity totally.

You should NEVER use a 78xxx regulator without the capacitors, they are there to stop it oscillating. As Oznog says, even if it doesn't oscillate when you test it, it's certainly likely to do so under varying conditions during use.

As it's an automobile application you want MORE components in the PSU, not less, as you are already aware the power supply can have high spikes and noise levels.

 

[Roff]

Get rid of the zener and the 470 ohm resistor (I'm assuming this is relating to LM2907 shift lamp problems). Like the other guys said, you absolutely need the capacitors.
BTW, why didn't you append this question to the old thread? It makes things a lot simpler when we have continuity and history.