MTF of a PIC / 78L05 - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: July 11, 2010, 07:25:10 pm
content: Hi guys, I've been reviewing a circuit design which was going to run on dual power supply (power side and MCU control side) and now intend to put the control circuitry which is essentially a pic powered on a 78L05 (or equivalent) on the main supply that is on 24/7.I'm now going to have the 78L05 reg and associate caps powered off 12 V and the pic off 5 V 24/7, what sort of life expectancy can I be looking at ?. I suppose I'm concerned as I still treat the pic as a computer versus a simple digital chip

author: EEVblog
author_rank: Unknown
timestamp: July 11, 2010, 10:21:06 pm
content: How are you getting your 12V?, that might be the weakest link.Dave.

author: DJPhil
author_rank: Frequent Contributor
timestamp: July 12, 2010, 04:59:20 am
content: Quote from: Simon on July 11, 2010, 07:25:10 pmI'm just guessing from before, but is this an automotive application?It's almost impossible to be sure without testing the whole design itself in quantity. You might be able to get part specific data from each manufacturer, but any given collection of components would have to be tested. Assuming no major electrical trauma and good build quality all you're left with is environment. Heat and moisture are the big enemies. If there's not a lot of heat generated by the circuit itself you might be able to pot the board (dunk it in black goop) to fight against moisture problems. Shelling out for parts with a military temp range could help as well.In the end, consider how many microprocessors (from a few in the 80s to several dozen nowadays) are in cars and how often they die. Computers (desktop) are a bit different in that they have to dissipate relatively huge amounts of heat just to function.Hope that helps some.

author: Simon
author_rank: Unknown
timestamp: July 12, 2010, 06:05:29 am
content: Quote from: EEVblog on July 11, 2010, 10:21:06 pmCar battery which is charged by a dynamo in a vintage car, yea it could be a rough supply I'll be trying to filter it out as much as possible, at least when left standing it will just be smooth battery power. Yes it's the same old project, me moving house and my friend having his test car still in bits gives me lots of time to rethink and refine the design

author: Zero999
author_rank: Super Contributor
timestamp: July 12, 2010, 03:05:03 pm
content: It might be a good idea to add some spike suppression before the the LM7805, a 18V zener from 0V to Vin in series with a 22R resistor (to limit the current spikes) should do.

author: Simon
author_rank: Unknown
timestamp: July 12, 2010, 06:05:49 pm
content: oh I'm planning better than that a 100+ ohm resistor in series with a 100uF cap on the 78L05 input, I think because I'm doing a smother regulation I'm getting smoother power out of the dynamo, apparently the dynamo alone and control box used to create some of it's own due to the crudity of it

author: TheDirty
author_rank: Frequent Contributor
timestamp: July 12, 2010, 06:54:54 pm
content: If this is an automotive application, use a proper automotive spec'ed regulator. They aren't that expensive and they protect against reverse voltage and high voltage spikes. If it's a non-critical systems and you really don't care, then just use the 7805 and whatever protection you add on. I've had gadgets running with non-automotive spec'ed regulators for a long time now, but any new design would have one for me.

author: jahonen
author_rank: Super Contributor
timestamp: July 12, 2010, 06:55:23 pm
content: Put a (schottky) diode before the input cap (make it large enough) so that if the input supply momentarily drops below 7-8 volts your pic won't suffer a brownout situation (possible reset) so easily. Large input cap also helps to absorb transients.For transient protection, TVS is more robust than zener, if you want to play safe.Regards,Janne

author: Zero999
author_rank: Super Contributor
timestamp: July 12, 2010, 08:18:06 pm
content: Is the leakage current important?What about using a Vbe multiplier? It's not very accurate but simple and it should be effective, if the correct component values are chosen.

author: Simon
author_rank: Unknown
timestamp: July 12, 2010, 08:41:14 pm
content: not sure what your referring to, some sort of pre-regulation ? I'm aiming for low consumption so that I don't have to worry about draining the battery, 1mA really, the reg will not take much and neither will the PIC until it detects an engine startup and the whole thing kicks off.

author: Zero999
author_rank: Super Contributor
timestamp: July 12, 2010, 09:31:35 pm
content: No just transient suppression.Look at the transistor and the resistors, Google for Vbe multiplier and you'll see what I mean.The leakage current is about 1.4mA at 12V but can be reduced by increasing the resistor values slightly.

author: Simon
author_rank: Unknown
timestamp: July 13, 2010, 06:45:45 am
content: so basically a high voltage (spike) will turn on the transistor which will divert the spike to ground, sort of zenner diode style.

author: Simon
author_rank: Unknown
timestamp: July 13, 2010, 06:49:14 am
content: Quote from: TheDirty on July 12, 2010, 06:54:54 pmany suggestions of parts ? yes this is highly critical, it is controlling the car dynamo and so is supplying power to the whole car and needless to say to itself ultimately, if it fails the only way to get the car going again (assuming it don't blow 50V through the system and wreck everything) is to replace it fairly quickly (ie before the battery does so flat you can't get it to self start again).

author: RayJones
author_rank: Frequent Contributor
timestamp: July 13, 2010, 10:05:04 am
content: It has to be asked, WTF is MTF?Or was it meant to be MTBF?

author: joelby
author_rank: Frequent Contributor
timestamp: July 13, 2010, 11:02:04 am
content: Quote from: RayJones on July 13, 2010, 10:05:04 amIt's MTF if it only fails once

author: Simon
author_rank: Unknown
timestamp: July 13, 2010, 11:47:21 am
content: basically "how long can i expect it to last ?"

author: Zero999
author_rank: Super Contributor
timestamp: July 13, 2010, 02:11:48 pm
content: Quote from: Simon on July 13, 2010, 06:45:45 amYes, that's the idea.Bear in mind that it's a very crude solution with a temperature dependant and current voltage characteristic but it should be fine for fairly low currents.

author: Simon
author_rank: Unknown
timestamp: July 13, 2010, 05:29:36 pm
content: well it has the resistor in front of it so current is limited

author: jahonen
author_rank: Super Contributor
timestamp: July 13, 2010, 06:10:41 pm
content: Why not simply the TVS, it is just robust zener-like component made to absorb surges, and leaks only microamps (if chosen correctly)? Probably it is faster too, although it probably doesn't matter. For example 1V5KE22A TVS would have standoff voltage of 18.8 volts, leakage 5 µA, and clamping voltage 30.6 volts @ 49 A.BTW, if idle current matters, then 78L05 is wrong choice, LP2951 is much better (albeit a bit more expensive). With LP2951 you also get reset to pic (it has open-drain error-output which is low when output voltage is out of tolerance), if the output voltage drops too low. Also LP2951 has lower drop-out (~450 mV @ 100 mA), so your PIC won't get reset so easily, for example when starting the car.Regards,Janne