7805 regulator heats up/fails

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cudajim

cudajim

cudajim
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I'm using a 7805 regulator as the dash regulator (69 Barracuda) and it heats up after a while and stops working properly. The fuel, temp and oil pressure all show a lot lower than they should until I turn the car off and then turn it back on. The fuel gauge points have been pulled apart so it's not in the circuit anymore. I do have a small heatsink on the regulator with the proper heatsink compound. If I hit the regulator with freeze spray it starts to work properly for a while. I'm guessing I'm pulling too much current but I didn't think I'd have a problem. Any suggestions?
 
The regulator will run hot to the touch . Have you checked the supply voltage? If you are pumping 14- 16 volts to it continuously it will overheat also poor or insufficient ground to the dash will do the same thing.
 
cudajim said:
I'm using a 7805 regulator as the dash regulator (69 Barracuda) and it heats up after a while and stops working properly. The fuel, temp and oil pressure all show a lot lower than they should until I turn the car off and then turn it back on. The fuel gauge points have been pulled apart so it's not in the circuit anymore. I do have a small heatsink on the regulator with the proper heatsink compound. If I hit the regulator with freeze spray it starts to work properly for a while. I'm guessing I'm pulling too much current but I didn't think I'd have a problem. Any suggestions?
Click to expand...

They are meant to run with a heat sink. Radio Shack used to sell them, be sure to get the silicon grease and insulator tab with it.
 
The more the voltage differential, the less current these will tolerate. That is, if your are running nominally 15V in, 5 out, that's 10V differential

I'd measure what the gauges all draw when they are up near full scale, and see what you have. If it's much over an amp, I'd say build one more regulator to run at least one gauge separately, or maybe even build three separate regulators
 
67Dart273 said:
The more the voltage differential, the less current these will tolerate. That is, if your are running nominally 15V in, 5 out, that's 10V differential

I'd measure what the gauges all draw when they are up near full scale, and see what you have. If it's much over an amp, I'd say build one more regulator to run at least one gauge separately, or maybe even build three separate regulators
Click to expand...

Yup, I'll put a meter on it this week to see what the total current is while the vehicle is running. I may just have to beef everything up a bit or just stack 3 regulators or just buy one from RTE and be done with it (I'd miss all this fun though).
 
Pretty sure those tolerate about 35v input. Heat sink and thermal paste is your solution. Long as the heat sink cannot touch ground or positive you do not need insulator separator between heat sink and vr (improving thermal efficiency of your heat sink dramatically). Like posted, radio shack has little metal heat sink and the thermal paste. Attach to VR with small nut and bolt apply dab of thermal past where heat sink and VR mate. You can tolerate much more load with good heat dissipation.

Another option is buy a good self regulating VR unit that will not overheat and rated above your wattage load.

This company has good electrical components. Below is my suggestion for a VR. PN JTL4024S05. Will tolerate 8 amps. Needs no heat sink.

http://www.newark.com/jsp/displayProduct.jsp?sku=79R9200

http://www.xppower.com/orderPriceList2.php?seriesid=100596&groupid=100062&catuid=2&lang=EN
 
Not so sure he needs a 40w regulator for that job - and 80 bucks too!

LM7805's come in several current ratings, and are basically an antique anymore. Good chance you got an 800ma rated part...

Use a more modern part like this:

http://www.fairchildsemi.com/ds/KA/KA378R05.pdf

Good for 3 amps, rated to 80c temps. 3 amps of output. You still need a heatsink, but the tab is fully insulated so you can just bolt it down to a heatsink.

A much better part.

B.
 
3 Amps @ 5 v. Input 9 - 18Vdc

http://www.newark.com/jsp/displayProduct.jsp?sku=52R2842

The above is smoothed out regulated reliable power. Not just a VR.

I spent a couple years making 5 V, 12 V, 18 Volt and adjustable VRs in all kinds of variations for communications projects and you simply cannot beat the reliability and smooth power output of a production unit designed to suit the job. In the end we gave up on fabrication as it was neck in neck with purchase price of production units and started buying production units. For several years we were replacing the hand fabricated voltage regulators in the field about once a year (by the hand fulls).
 

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Again, that is overkill for this job.

The fairchild part has 55dB of ripple rejection already, and 0.1 % load regulation.

Remember this is a simple hot wire gauge, running on battery power. You could add a capacitor to the regulator output if the 55dB is not good enough.

That fairchild part is less than 2 bucks. And is all that is needed to replace the OEM regulator. No need to spend 80 bucks on a DC-DC convertor.

B.
 
Makes sense.





bohica2xo said:
Again, that is overkill for this job.

The fairchild part has 55dB of ripple rejection already, and 0.1 % load regulation.

Remember this is a simple hot wire gauge, running on battery power. You could add a capacitor to the regulator output if the 55dB is not good enough.

That fairchild part is less than 2 bucks. And is all that is needed to replace the OEM regulator. No need to spend 80 bucks on a DC-DC convertor.

B.
Click to expand...
 
I'll yank the dashboard out this weekend and measure the current draw and most likely replace the regulator with one capable of a bit higher current. I do have an output cap on it as well as a proper heatsink with the frequency grease (that's an old military term).
 
cudajim said:
I'll yank the dashboard out this weekend and measure the current draw and most likely replace the regulator with one capable of a bit higher current. I do have an output cap on it as well as a proper heatsink with the frequency grease (that's an old military term).
Click to expand...

cudajim said:
Yup, I'll put a meter on it this week to see what the total current is while the vehicle is running. I may just have to beef everything up a bit or just stack 3 regulators or just buy one from RTE and be done with it (I'd miss all this fun though).
Click to expand...

That's cool. When you decide R&R the instrument panel repeatedly is no longer fun , I'll be happy to tell you where the problem is. Wont cost you a dime. Have fun
 
RedFish said:
That's cool. When you decide R&R the instrument panel repeatedly is no longer fun , I'll be happy to tell you where the problem is. Wont cost you a dime. Have fun
Click to expand...

So Redfish, where do you think the real problem lies (other than with me)?

I'm thinking I'll just say "screw it" and go with the regulator board from RTE.
 
cudajim said:
I'm thinking I'll just say "screw it" and go with the regulator board from RTE.
Click to expand...

That will cost you quite a few dimes. It wont solve the problem though. Maybe they can tell you where the problem is.
Imagine all the "fun" in store.
 
I have not had time to get back into the cars dash just yet. I did order a fancy regulator from RTE/dash-worx but they're really slow at responding. I'll let you all know what I find.
 
Ok, so I did pull the dash and replaced the 7805 with a regulator unit from RTE. While I had it on the bench I hooked up a 14vdc input and checked the output with my trusty analog VTVM (that would be a Vacumn Tube Volt Meter). The needle deflected and pulsed as it should so I then checked all 3 gauges with a 20 ohm resistor in series to ground... they all worked just fine.

Their web-page instruction photo was no good because the defrost hose was in the way so I had to relocate the regulator from where they had it pictured. In any case it worked perfectly and we were able to make it to Reno for "Hot August Nights".
 
So, cudajim, the replacement volt reg from RTE pulses? Like does the output vary in voltage rapidly? I seem to remember people mentioning the factory one does this. Also why did you use a vtvm rather than a dvm, or would either suffice?
Thanks!
 
The RTE regulator pulses to emulate the original bi-metallic regulator which was built into the fuel gauge. You can see this better on an analog meter with a needle vs. a digital meter because the pulses are about 1/sec. I just happen to have an old VTVM from when I worked on TV's. The digital meter just kind of freaks out because it reads too fast.
 
I am going to have to look that factory regulator info up. Would you have any links to data on how it is made and built into the dash? Thanks!
 
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