Supply 12V auto V to a 7805 regulator

[davew]

The 7805 is supposed to shutdown when it gets too hot. Maybe yours is bad or is a counterfeit one.

I think it was at the hairy edge of shutting down.

But in this case what5 is meant by shutting down, I don't think they mean it has fuse like properties i.e it'll never work again.
I assumed it was shutting down, as I reduced the input voltage back to nearer 12 from 20V the 7805 'recovered' as it cooled and I got back to 4.6
at an input of about 12.5V. I'm sure if I'd would have left it off to cool
I would have got it back up to 5V.

Maybe I should test one to destruction.
aka let a student use it.

She canne take it any more captain, it'll blow the warp core.

 

[audioguru]

It will not be destructed. It shuts down until it cools a little then starts working again. If it thermal cycles like that a few hundred times then it might fail with thermal fatigue.

 

[Hero999]

The 7805 is supposed to shutdown when it gets too hot. Maybe yours is bad or is a counterfeit one.

I think it was at the hairy edge of shutting down.

His temperature probe probably wasn't calibrated.

Also, many of the characteristics on the datasheet are just typical and have a wide tolerance and some voltage regulators are rated up to 150°C

 

[davew]

His temperature probe probably wasn't calibrated.

Also, many of the characteristics on the datasheet are just typical and have a wide tolerance and some voltage regulators are rated up to 150°C

The temperatrue probe does't need to be calibrated, I wasn't aiming to find the 'shuting down' temerature to a particular value , which wouldn't be much use anyway as I was measuring the case temperature rather than the junction temperature which was stated as TJ 125C.
The regulator I was using was a standard DE7805 nothing special and the operating range stated was TJ 0 to +125C.
Perhaps you were looking at the storage temperature which is -55 to +150 ?

 

[chrisjpitt]

But in this case what5 is meant by shutting down, I don't think they mean it has fuse like properties i.e it'll never work again.
I assumed it was shutting down, as I reduced the input voltage back to nearer 12 from 20V the 7805 'recovered' as it cooled and I got back to 4.6
at an input of about 12.5V. I'm sure if I'd would have left it off to cool
I would have got it back up to 5V.

Good question – as you noticed a reduction of voltage supplied to the load, it looks like the thermal protection possibly just pulls on the internal current limit circuit to reduce the current (thus dissipation) - to achieve a kind of temperature regulation around the protection point?

If it actually “switched” off at the thermal limit (with some hysteresis), you should have noticed the regulator turning off, then on again as it cooled – cycling, a bit like a mechanical thermostat would.

Not sure why the designers favoured the former method, it may have been just simpler to implement, but it may also be better for its long-tem reliability - as obviously, it does not thermal cycle the device?

 

[davew]

Good question – as you noticed a reduction of voltage supplied to the load, it looks like the thermal protection possibly just pulls on the internal current limit circuit to reduce the current (thus dissipation) - to achieve a kind of temperature regulation around the protection point?

That's what I was thinking.

If it actually “switched” off at the thermal limit (with some hysteresis), you should have noticed the regulator turning off, then on again as it cooled – cycling, a bit like a mechanical thermostat would.

Not sure why the designers favoured the former method, it may have been just simpler to implement, but it may also be better for its long-tem reliability - as obviously, it does not thermal cycle the device?

Well I didn't niotice at the time, although measuring current and voltage with a DMM might not show fast switching.
Time to experiment again

 

[audioguru]

I have never overheated a regulator IC so I don't know what it does to try to protect itself.
Somebody should try it and report about what happens. I wonder if all manufacturers do the same trick.

 

[3v0]

As said earlier, if you need to put a regulator in a car use one designed for use in a car. The are produced in such numbers that they are no more expensive then non automotive regulators.

 

[davew]

Use an LM2937. Its basically a 7805 made for automotive purposes.

Yes, I'd agree but they seem to be about twice the price of a standard 7805 and only rated at 500ma althpugh that seems OK for this use and mine which was an iPod charger on a boat running of fthe 12V generator.

 

[proffski]

This is good to know. I cant imagine this thing drawing hardly anything. It is a 1-3/4"x1" x 1/2" thick 3-digit display LED/LCD digital readout. It can read 0-99v DC. It states that it requires 100% DC for it to operate. Whatever this means......I wonder how sensitive it is for the supply V. Hopefullly +/- 0.5v I would think.

If LED rather than LCD the current drawn could be 20mA per segment per character, soon adds up.

 

[audioguru]

An LCD display draws a very low current. But it can't be seen in the dark without a backlight that draws a fairly high current.

The current per segment of an ICL7107 is 8mA. 20mA per segment would light a room and would be too bright at night.

 

[chrisjpitt]

I have never overheated a regulator IC so I don't know what it does to try to protect itself.
Somebody should try it and report about what happens. I wonder if all manufacturers do the same trick.

Generally they do – as without a thermal limit, they could be a fire hazard under certain conditions. In high input voltage, low current applications (where little or no heatsinking is used), the short-circuit current limit could see tens of watts being dissipated if it were not thermally limited.

 

[Hero999]

Perhaps you were looking at the storage temperature which is -55 to +150 ?

I haven't looked at the datasheet for your device but TI's datasheet gives a maximum recommended operating temperature of 125°C and an absolute maximum operating temperature of 150°C. Some military and aerospace variants have higher absolute/recomended maximum temperature ratings.

https://www.electro-tech-online.com/custompdfs/2009/01/LM7805.pdf

It your temperature probe wasn't calibrated then the actual temperature could be much different to what you measured.

I agree, there is some hysteresis. I don't know what temperature it actually shuts down at, does it wait for the temperature to reach exactly 150 before shutting down or is there a risk of shutdown >125°C?

Back to the origional question: if it's <50mA then I would recommend the LM2936-5 which is very tough and is designed for this kind of thing.

 

[davew]

I have never overheated a regulator IC so I don't know what it does to try to protect itself.
Somebody should try it and report about what happens. I wonder if all manufacturers do the same trick.

I've run a few tests :-

7805 without heatsink (apart from the crocclip holding the thermocouple sensor to the earth tag)
There was a 100nf at the input and output.

Using a 100R load and a 10V input, I got 5.06V at 49ma and after 10mins
the temperature went from 17V to 24C and stayed there.

Using a 100R load and a 15V input, I got 5.06V at 49ma and after 10mins
the temperature went up to 32C and stayed there.

Using a 100R load and a 20V input, I got 5.05V at 49ma and after 10mins
the temperature went up to 40C and stayed there.

Using a 100R load and a 30V input, I got 5.05V at 49ma
and after 10mins I got 5.04V at 49ma at 55C

-----------------------------------------
Using a 18R load and a 10V input, I got 5.06V at 278ma and after 10mins
......................................................5.04 at 277ma and 56C.

Using a 18R load and a 15V input, I got 5.06V at 278ma and after 10mins
......................................................5.00V at 274ma and 94C.

Using a 18R load and a 30V input, I got 5.06V at 278ma and after 10mins
......................................................2.64V at 142ma and 131C.

I never got it above 140C and the regulator recovered when cooled.


I also had an oscilloscope across the load but didn't detect any switching or increased ripple or noise, although while using the freezer on the 7805 I did notice some oscillations but they only lasted a few seconds and could have been the liquid freezer shorting the pins.

So my conclusion is the temperature and input voltage can be quite significant and for most purposes I wouldn't use a 'naked' 7805 for anything more than 250ma.

 

[stuhagen]

Man, this topic has grown since i last visited.

I received these minature V meters, with no datasheet. Typical I suppose when coming from China. So I have no idea on there specs. I am sure it is a LCD. I will input 5v to it tonight and I should be able to detect what they are. If I was smart, I could figure away to get the current draw, but I am too dumb for this ha..ha..

A couple of things that I have changed around. I had been using a generic 7805, but decided to switch to a 10V regulator for running the LM311. After reading up on In/Out V swings, I figured going down from 14.4 to 10v is better than 14.4 to 5v. This has to reduce the heat significantly. Still, the issue of needing a 5v source for the V Meter is needing attention. I know this is stupid idea, but what if you use another 10v regulator, the use a voltage divider on the output to generate its 5V requirement? Or could I use this single 10v regulator to supply the LM311 "and" the V Meter via a divider?

I attached a PDF of the source of regulators. They range from 3-15v or so. They are 1A low drop out for a $1. I am going to mess with this.

Stu

 

[audioguru]

Don't buy cheap junk that has no datasheet.

A voltage divider works fine if it has no load current. Your project has lots of load current.

You don't even know if the cheap Chinese digital meter is high current LED or low current LCD??

 

[stuhagen]

OK, this unit is a LCD, no back light. It draws 40ma based on 13.8v DC bench supply. Readings /sec are 2-3. Accuracy is +/- 0.2%. Resolution 0.1V. Requires 5VDC to operate.
It measures 3/8"x1"x1-3/4"...cute little unit~!

 

[gerritv]

You can use Microchip MCP1700 regulators as well, specifically designed for automotive use. And less than US$1 in single quantities from 'the usual sources'. There is also the LM1117 style of LDO regulator.