hexibot43

Hello Everyone,
I am looking for some insite. I am using a Pic 16f, and a Pic 18f in two different projects that eventually will be used in an auto. I'm powering the 16f project from a 9 volt battery for now. It also encorporates a small 2x20 backlite lcd display. It is working fine for the moment. But I'd really like to ditch the 9 volt battery, and power it using the charging system of the car. I was also hoping to change it so that pressing any of the control buttons would wake it up. So it would always be plugged into power. I am hoping to do the same with my second project using the 18f452. I am powering it using a regulated 13.8 volt supply that is plugged into 120vac. It draws .6 amps total at 5 volts. I am actually using a step up device to power a vga screen driver at 9 volts from the 5 volt system. I just changed it to use two regulators, one 7809, and one 7805. The 7809 is power only the screen driver. And the 7805 is powering everything else. The 7809 is getting very hot while powering the circuit at about .5 amps. The 7805, now only having to power the pic, and a fpu co processor is running nice and warm. I worry for how hot the 7809 is getting.
Does anyone have any suggestions as to the best way to power my circuits.

Thanks,

Marcello

chemelec

Put Heat Sinks on the Regulators so they run cooler.

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Russlk

When powering from the auto electrical system, keep in mind that there can be 60 volt spikes, so protect your circuits with an LC filter.

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hexibot43

So, how much heat is normal? What should I consider a good working temperature; a safe temperature? I am wondering about longevity of my circuit. I am going to add a large piece of metal to use as a heat sink. The simple 6 fin type I am using is getting very hot. I hate to think what is going to happen when it is in a car that is already quite hot.

- Marcello

And thanks for the input. I really appreciate hearing what other people are thinking about this stuff. I don't have anyone in my daily life that has any idea what I am talking about, except for one person.

fredsm

Try to keep everything below 60 deg. C --
you may need to add a small fan.
It may help to add a pass transistor. See:
http://people.txucom.net/fsmyth/page...ogs1.html#5v25
Lower is obviously better; semiconductors
tend to degrade over time when run hot.
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Optikon

IMO, good but you should put more protection than that. Zener diodes & MOV's have been shown in reference designs. LC's can potentially ring with the right spectral content in the "spike", an LC should really be an LRC for damping & diodes for clamping.

eblc1388

Normal zener diodes are not fast enough and MOVs are generally for much higher voltage than 12V.

The proper protector to be used is "transorb" surge protector.

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Optikon

ah yes! Thanks! Thats the name I was trying to think of.. Transorb looks like two series back to back zeners.

I agree zeners maybe too slow.. depending. If they are behind the LCR network, it might be ok. The LCR network really slows things down and the zeners are just low frequency clamps.

MOV's can be used in 12V designs. There also exists ESD type of protection networks that would probably be well suited.

zevon8

I have had good results using Epcos MOV's in all my automotive applications. I use the S14K14 device, directly across the supply line, in front of the regulator. Most of my stuff contains CMOS and FET's and so far I have not had a failure of parts. I also place a suitable sized diode inline with the supply feed, incase of accidental reversal of polarity.

hexibot43

I am having problems with overheating of my 9v - MC7809 voltage regulator. I am drawing about 540ma through the regulator. Now I have a heat sink on it, but it sure doesn't seem to be doing the job. I am using this to power a board I purchased to display data sent over an rs232 to a vga screen. I just took a look at the board and it has an L7805c-V as a regulator. Is it acceptable to power a regulator with the output of another regulator? I was thinking of taking out the 9v regulator I just installed, and put a better 5volt regulator directly on this board. In the notes it was not advised to power with a 12volt or more source, because that would overheat the regulator. My primary power source is 12.6-14-2 volts from the charging system of an auto.
I've enclosed a picture of the heat sink I was using. The system runs for about 5 minutes before the 9volt regulator overheats and shuts off. Will simply using a bigger heat sink do the job? 13 volts input with a draw of .5amps at 9volts through the regulator.
I figure I've got to stop this thing from overheating before I can worry about anything else.
Thanks,

Marcello....

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img_8673.jpg (402.5 KB, 612 views)

Nigel Goodwin

Try putting a larger heatsink on it, I'm presuming you have the two capacitors fitted as close as possible to the chip?, I can't see them on the picture! - instability could be making it hotter than it should be?.

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hexibot43

So the Caps are that critical? Ok, I was told to give it try without them. Can I ask what other things I might expect to occur when I leave them out? I was using a Mean Well SDM30-12S5 as a power source before I switched out to the regulators I'm using now. The Mean well although it worked very well was too big to fit into the casing I finally decided on using for my project. I'm now noticing my AD conversions are jumping around a lot. It seems the closer the signal I'm converting to Digital is to ground the worse it gets.
I changed the heatsink to a nice piece of steel about 6 inches long and my running temperature is a constant 85 degrees F (~30 C). I left my 5v regulator with the original heatsink and it is running about 87 degrees F. I ran the system for about an hour and the temperature was very constant.
I'm wondering if I could see the problem with the regulator output with a DSO?
I'm looking into the other recommendations made regarding signal conditioning before the regulator to protect my circuit. I'm still learning the hardware side of all of this. The software is where I have a lot of experience. I've written a lot of Assembly over the years. But the hardware side of this has always been a bit of an enigma. I had never heard of a TransOrb, or a MOV before. I swear everytime I am think I'm in the know someone blows my mind again.

Thanks Everyone,

Marcello

Nigel Goodwin

Yes, the caps are critical!. They maintain the stability of the regulator, it can (and very probably will!) oscillate without them. This might not be all the time, and may only be under certain specific conditions, but it's foolhardy to leave them out.

Other possible symptoms are incorrect output voltage, either too high or too low - but no circuit is going to like high frequency AC as a supply instead of DC.

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hexibot43

Alright. In choosing the Caps to go with a 7805, and a 7809 where does one start? I've read a few peoples thoughts, and come to the conclusion I'd better get some more input. And the choice of Cap type? I've seen specs from 1000uF to 10uFs. Stating all electrolytic. Then someone else wrote that using strickly electrolytics could give rise to problems.
I am really trying to understand this, so any info would be helpful. Thanks, Marcello

Oznog

The LC filter can bring the reg's stability into question, and isn't guaranteed to protect against current spikes. In fact the L of and LC filter MAKES high voltage spikes when current consumption shuts off suddenly. If the C is poorly chosen it can make the reg blow itself with no fluctuations from the power supply.

A really good choice is a transorb combined with a self-resetting polymer fuse. If you have a reg spec'ed to handle up to 30v, just choose a 25v transorb and say a 3 amp polyfuse. Clamps very well. However if you still want to use a nonresetting fuse you will either need to put a larger one in front of this suppressor or a normally sized one after it; the only downside of the ladder being that it won't protect against transorb shorting failure (pretty unlikely I think).

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