I have an STMicro L7805 that I breadboarded to check the output voltage. I need to regulate 28V down to 5V, with a max current requirement of about 250mA. When I tested the regulator (with the suggested .33uF input and .1uF output caps), it is outputting 7.6V at 28V input. It holds 5.06V output as long as the input is 20.5V or below. I tested with no load on it, and was using a bench power supply. The datasheet says max input is 35V. I tried a second regulator and got the same results.

My other question, now that I have learned a bit more, is ~6W a lot of power to dissipate [(28V - 5V) * .25A]? I was planning on screwing the regulator down to a 1/2" thick plate of aluminum which is acting as a base for this assembly I am putting together.

 

hi aero,
Welcome to AAC.
As you say 20.5v -5v = 15.5V * 0.25A = 3.8Watts, which is exceeding the heating limit of the 7805 when not fitted with a heat sink.
What size and shape is the alum plate.?
E

 

The datasheet says that the minimum load for regulation is 5mA.

 

Hi Eric - I had it screwed to a finned heatsink, but it's only 1" high x 5/8" square with 4 fins. I did not have thermal paste since it was a quick test. It immediately was outputting 7.6 volts @ 28V, not acting like it heated up and started doing that. The regulator was not warm at all. Also, I thought they shut down when getting too hot as opposed to acting abnormally like raising the voltage?

The aluminum plate is about 8x10x1/2" thick, although there are two 2" wide openings (about 5 inches long) cut out in the middle, end to end. Separating the two openings is a 1/2" wide section that the 7805 would be screwed down to.

Is there a better solution overall besides the 7805 I should be looking at? (And hopefully not too much larger).

 

It immediately was outputting 7.6 volts @ 28V

Per post #3, stick a 1k (or less) resistor on the output and try again.

 

hi,
Its important to realise its the 'die' temperature of the 7805 that governs the thermal shut down.
At 28V Vin with a 250mA load current, the die will heat up very quickly as the thermal conduction of heatsink is slower it will appear to still be cool.

Your heatsink is too small, do you have any low value power resistors in your bits box.?

E

EDIT:
I read your opening post as you have are drawing 250mA into a test load, please confirm.?

 

I would use something like this:
https://www.ebay.co.uk/itm/5V-PCB-M...h=item5b3f8585c6:g:deIAAOSwhMBZ5EgO:rk:1:pf:0

(Maximum voltage rating of the MP1584 is 30V)
Its hard to justify linear regulators these days, where there is more and a few volts voltage drop, or the current is more than a few mA!

 

The datasheet says that the minimum load for regulation is 5mA.

You nailed it. This thought crossed my mind yesterday, but never tried it. I don't see it in the datasheet, but there it is in real life... I shorted the output with a 50ohm 1W resistor and it held 5V really well. It also started getting warm right away. Now I need to figure out if I can deal with that heat...

Thank you.

 

hi,
Its important to realise its the 'die' temperature of the 7805 that governs the thermal shut down.
At 28V Vin with a 250mA load current, the die will heat up very quickly as the thermal conduction of heatsink is slower it will appear to still be cool.

Your heatsink is too small, do you have any low value power resistors in your bits box.?

E

EDIT:
I read your opening post as you have are drawing 250mA into a test load, please confirm.?

The 250mA is the intended maximum load. It will be 150mA constant (max - it may be a little lower, don't know yet), then intermittent +50mA and another +50mA possibly for short periods of time (switches turning on with series resistors to limit current through them).

 

OK,
Look thru the plots on post #2.
E

 

My circuit is very simple. 28V is supplied, I need to run a 5V oled display (datasheet says 150mA current draw), and 4 pairs of switches (who's outputs need to be 5V) where realistically maybe three can actuate at a time. I will have probably 100ohm series resistors for the switches to limit the current through them (they are small microswitches), so let's use 300mA maximum with 150mA as a constant load. If the 300mA is held for a while, the 7805 will be dissipating 7W. I can hope that the heat won't be a problem or I can use a different means of voltage regulation. Is something like this reasonable:

https://www.mouser.com/datasheet/2/670/vxo78-1000-1313360.pdf

 

The 8" x 10" aluminum plate should be sufficient to dissipate the maximum 5.75W from the regulator, but it would be better if you could mount in on a larger area of the plate, rather than the 1/2" strip in the middle.

 

hi a,
You could connect a low value power resistor in series with input to the 7805

 

You nailed it. This thought crossed my mind yesterday, but never tried it. I don't see it in the datasheet, but there it is in real life... I shorted the output with a 50ohm 1W resistor and it held 5V really well. It also started getting warm right away. Now I need to figure out if I can deal with that heat...

Thank you.

You need to be sure to read the notes -- they are often critically important.

 

You need to be sure to read the notes -- they are often critically important.

View attachment 171192

I looked again, and the STMicro datasheet I have (dated Nov 2004 - from either Mouser or DigiKey) doesn't mention that. It does list 5mA to 1A as test conditions for the output voltage rating, so I guess I can surmise the 5mA is a lower threshold.

 

I looked again, and the STMicro datasheet I have (dated Nov 2004 - from either Mouser or DigiKey) doesn't mention that. It does list 5mA to 1A as test conditions for the output voltage rating, so I guess I can surmise the 5mA is a lower threshold.

Yep, that's the correct way to interpret it.

And, as you can probably guess, it caused enough grief for enough people that they made it more explicit by adding that note in the later datasheet.

 

I don't like dropping more than a few volts using linear regulator. 7 watts is a lot of heat and wasted power. You should be looking at switching power supply. Or have you thought about using two linear regulators? One to drop 20 to 12, then 12 to 5. It's still a lot of heat! ugh.

 

I'll often incorporate a power-on LED in the circuit -really can be useful -that provides a minimum load for small circuits that draw in the microamps range.