78L05

--
Maybe; try it and see...

--- Tell us what else you've got hooked to the battery, where it's hooked up and how long the wires are, and how long the wires are going from the battery to the regulator. Also, do you have anything else hooked to the +5 side of the regulator other than the 2 chips you already mentioned?

-- John Fields

And does this motorbike battery have a charger? If it does, does the charger put out any ripple? Any vehicle is going to have horrendous spikes from the charging system. Ripple from these chargers will go straight past the battery and cause problems unless designed around.

Hi

Im currently using a 78L05 chip to power up my PIC 16C773 and my 231CPD driver/ reciver chip. The circuit functions ok when running, but the circuit is real hard to boot up. When the circuit is connected to Hyper-terminal when phalse boots occur junk comes through.

The 78L05 is fead with 12V DC on the input and has a 10uF polarised cap accross this input to ground. When i disconnect this cap the booting becomes impossible and the junk on hyper-terminal is worse. Therefore if i say put a

100uF cap in place of my 10uF, would this solve all my problems? If so what is the maths behind it, or is it just a plug and play theory?

After reading through a few of my monthly issues of practical electronics, i notice that most of the circuits that they build us 100uF! Is this the reason why they use them?

Cheers Turbo46

These three-terminal regulators oscillate, not really a surprise given the high gain inside.

If they aren't properly bypassed, they will oscillate.

Read the datasheet. They will only specify an input capacitor if the regulator is far enough from the filter capacitor in the power supply; after all there would be a large value there and a small capacitor will only come into play if there is significant distance.

But they will specify a capacitor on the output. You've mentioned the input, not the output.

And the type of capacitor affects the value. They will specify that if a tantalum is used, the value is different from an ordinary electrolytic. But this is because of the structure of the capacitor.

Larger is not necessarily better, in this case.

Michael

I think it has zero ripple because it comes from a 12V DC battery. It a small version of a motorbike battery.

Ur um, the thing to work please. lol

"Tuurbo46" wrote

Then put a .01uF cap on the output and maybe a 1uF as well. I wouldn't use more capacitance on the output than on the input.

Add seperate bypass caps to the PIC unless you like allot of mysterious resets. PIC chips are fussy about how fast Vdd ramps up too. If it comes up too slow (>100mS), then the PIC won't come out of reset.

BTW, please don't top post.

I think the 78L05 data sheet suggests a 330 nF capacitor on the input.

Leon

--
Leon Heller, G1HSM
http://www.geocities.com/leon_heller

"Tuurbo46" schreef in bericht news:cub2kr$j7q$ snipped-for-privacy@news8.svr.pol.co.uk...

Well,

First of all, get the datasheet. It is advised to use a 330nF capacitor on the input and a 100nF one on the output. Ceramic types are prefered and you have to mount them just beside the regulator. Then be sure to decouple every chip with a 10-100nF ceramic capacitor. Unless you have a very noisy environment this should do.

But, if not: Are you sure the current does not exceeds the 100mA rating during power up? Is it possible that powerdissipation is too high during boot? Do you have long lines between the 12V source and the regulator and/or between the regulator and the load? These can be a reason to add elco's for decoupling. Are other devices then the ones you mentioned connected to the output? Devices that can do weird things at power up?

petrus bitbyter

You're slamming the 78L05 with a helluva transient unnecessarily and this is interfering with the PIC power-on reset. Use a current surge limiting resistor: View in a fixed-width font such as Courier.

78L05 / 47 1W +-------+ BATT>-o o----/\/\--+----------+----|IN OUT|--+---- |+ | | | | === === | GND | === |10U |330n+-------+ |100n | | | | | | | | GND >--------------+----------+--------+------+-----

How about having a look at the waveforms with a scope ?

Rene

--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

Gee I've used PIC for years, and as long as I enabled POR (Power On Reset) I've never had a problem even with 'interesting' power supply configurations. I always have at least a 100mfd/25v cap on both in and out of the 78L05 as well as .1/25 disk caps. Jay

circuit

becomes

a

what

i

It's getting away from a simple 78L05 but this idea may be of general interest to others using PICs. As Anthony F rightly mentions, the PICs can be picky during power up resets. Had trouble with this aspect (esp the 12C' chips) so figured this 'Snap On'

5V supply. The circuit works fine at other input/output voltages and I've use it in designs where the turn-on/off characteristics of the external power supplies are unknown. +Supply o-----o-----------------------o--------. | | | .-. .-.15k | | | | | | | | | | | 10k'-' '-' | | | | | .--------o | | | | | | | .-.33k | | |< | | | o---o--------| BC557 | | | | | |\ '-' | | | | | | z | | | |/ zener A --- '---------------| BC546 5V1 | --- | |>

| | 10u | | | | | | .------. | | '--------o------o|+5V | | | | | | | | | | | | LOAD | | | | | 0V o-----o---o-----------------------------------o|0V | '------' (Transistors are just general purpose PNP NPN types)

The output voltage 'snaps on' from 0V to 5V when the supply voltage gets upto about 8V and will snap off when the supply falls to 7 volts. ON/OFF levels set by the ratio of the 15k and 33k resistors. Nice feature is that the output voltage holds within mV of whatever the zener voltage is. regards john

Yes, with fair supplies and POR set, the PICs seem to come up OK. For cheapness I'll usually stick a 10u on the 7805 in and out. The interesting supplies' I seem to get lumbered with are usually 'tat' (ie cost-effectively engineered :-) and can look to the PIC like 1/2 wave rectified sine waves while they are running up. Average DC level rises up to a point (10mS ripple) where the PIC gets it's knickers in a twist and locks up. Usually no problem if the RC on the boss 'MCLR' reset pin can be made to hold the PIC reset for say 5-10 seconds but if a 12C... type PIC is programmed to use MCLR as GPIO, then the internal power up timer/brownout/reset etc, just loses it. regards john

Reset)

out

(ie

to

locks

Interesting that bit about the internal MCLR being 'dodgey'. Could you explain further?

regards,

Reset)

You must mean PWRTE. ;-)

out

I didn't have any problems either until I started working with the "nanowatt" 16F88.

made to

I don't think he means that it's flaky. IIRC, it's just a simple pull-up resistor with no cap internally, consequently no time delay.

IANEE

I don't see that circuit snapping much in my simulations- and I don't think it will work well for switching a battery source versus an ac-supply. I believe the POR disruption is caused by ON/OFF switch bounce in addition to unlimited surge currents into the power entry 10u- so you can see how a 100u multiplies all these time constants x10 and is a brute force way of curing the problem. A slightly less crude way of debouncing the ON/OFF with zero quiescent overhead, and turn-on that is fast enough, should be something like this:

View in a fixed-width font such as Courier.

ON/OFF 78L05 / +-------+ BATT>---o o--+----------+-----47----+---|IN OUT|-------. (+) | | | | | | | 100K |+ | | +----+ === | === | GND | |CKT | 220n 2n7000| 10u +-------+ | | | ||---+---+ | | +----+ +-----| | | | | | ||---. | +-------+-----------. | s | | | +---. | | | | | | | ||---. | - | .--| 470K ^1n4148| ||---. | - | s | | | | 2n7000 | | | | | 0V ---------+---+------+-----------+