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electrical problems (still)
https://www.slantsix.org/forum/viewtopic.php?t=22276		 Page 2 of 2
Author:  Jeffc [ Sun Mar 18, 2007 11:26 pm ]
Post subject: 
Quote:
Wouldn't a 7809 or 7810 split the voltage drop better with the 7805 since you're running closer to 14v than 12v? (4 and 5 volts each compared to 3 and 6 volts)


And how many amps do the gauges need anyway?
Maybe, have not try it.
The stock gauges were kind of a left over from the 50s 6 volt systems..
so anything over 6 or 7 volts starts the gauge to a early death.....

As Don said, (notes below the photo) he felt it made too much heat just using a single
7805, even if it will do the job by itself, and looked for a way to
reduce that heat.
Author:  supton [ Mon Mar 19, 2007 5:31 am ]
Post subject: 
Darn, I'm at work now and can't view those new links. "Pre-regulating" the 12-14V down to say 9V and then down to 5V is a good method of dealing with heat dissapation without getting into larger heatsinks and/or using fans on the heatsinks, as it effectively doubles the heat that can be rejected. Use 10uF caps (or more) on each input still.

The cast heatsinks are typically better at rejecting heat than stampings. If given a chance, look for the rating "C/W" or similar. Look for smaller numbers. Also, when mounting the heatsink(s), orient the heatsink such that, when the instrument panel is installed, the fins are pointing such that heat can properly rise off the heatsink. You do not want the fins point down unless if you have a fan blowing on the heatsink. These regulators are quite hardy, and I don't think I've seen them go bad, at least not yet.

Lastly, I wouldn't bother with thermal compound on the heatsink; if too much is applied, it will act as an insulator. If you are going to use it, put some on and then wipe it off.
Author:  KBB_of_TMC [ Mon Mar 19, 2007 1:05 pm ]
Post subject:  Lm7805
The output you want to be 5V, so use the LM7805 - it will accept quite an input range.

I put a LM7805 and a tantalum cap inside an old limiter case and made it a direct plugin for a friend; worked fine for years, but he said it took longer for the gauges to warm up. It also has the advantage of being current limited.

I used the case itself as the heatsink and a thin layer of heatsink compound, and it worked just fine. In retrospect, I should maybe have added an external heatsink, and if I did it again, maybe I would add a small one.
Author:  emsvitil [ Mon Mar 19, 2007 1:05 pm ]
Post subject: 
Assuming 1amp draw, we're talking 9watts of heat............... (14v-5v)*1amp

How hot is that?
Author:  supton [ Mon Mar 19, 2007 3:13 pm ]
Post subject: 
A quick look at a couple reveal around 12-13C/W thermal resistance. That translates into more than 100C rise at 1A. If ambient temp was 25C, then the regulator would be well over boiling hot. The regulator should be fine, just really really warm.

But that's at 1A draw.
Author:  emsvitil [ Mon Mar 19, 2007 3:21 pm ]
Post subject: 
Just use one of these for virtually no heat.......

http://www.dimensionengineering.com/DE-SW0XX.htm
Author:  Jeffc [ Tue Mar 20, 2007 2:35 pm ]
Post subject: 
A bit more $, but good option :)
Author:  Andy's GT [ Tue Mar 20, 2007 5:16 pm ]
Post subject: 
thanks to all for your help, but what about the ammeter? i think thats my problem, my gauges work better then my school cars' ;)
thanks!
Author:  Jeffc [ Tue Mar 20, 2007 10:57 pm ]
Post subject: 
Quote:
thanks to all for your help, but what about the ammeter? i think thats my problem, my gauges work better then my school cars' ;)
thanks!
Does the alt gauge work- does it move?
if it moves what does it do with key 'on' engine not
running?
what does it do with engine running at low rpm
and what does it do with engine running at high rpm??
Does it read +/- and at what times.

Does your battery ever run low so you need a jump?

Have your cleaned all your ground wires and are they
in good shape?- battery to engine,
engine to firewall, battery to radiator core support??

Power from the alt comes though the fire wall to the alt
gauge, then though the gauge, out and though the fire wall
to the (often to the starter relay frist, depending on year) battery (if the system is stock).
Author:  Andy's GT [ Wed Mar 21, 2007 10:28 pm ]
Post subject: 
its not starting, so the first list isnt really applicable, sorry to be so confusing, the ground wires are all new with the exception of the radiator one, ill check my wiring diagrams when i get home and try to if i have power after the alt gauge, i have recently replaced most of the ignition system, ill be back on here during spring break probably, trying to figure this out with everthing handy, thanks guys!
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