lm7805

[Dr_Doggy]

I am using this chip to drive my led matrix from a 7.4v source, but I am noticing heat on it, i used it so that it would provide sufficient current weather 1 or 25 led's were on, 20ma to 500ma + supply for 8 IC's
theres not that much heat, but it has me considering installing heat sinks for long term runs

I think this problem is effecting the uC timing since the serial port uploading works 95% of all times, but I switched over to Bluetooth serial link which seems to work on the first few runs, but after a min or 2 my checksum byte returns 0 or double, MAYBE this is another problem that i am overlooking??

also i know the instructions said to use capacitors, to remove ac flaws?, which i didn't but so far it hasnt been a problem, could this contribute to the heat?

I am also thinking about putting 2-4ohm resistor between the source and lm7805 maybe it will reduce the stress on the IC?

then again maybe i should cut out the 7805 and just use a volt divider with resistors?

 

[alec_t]

Assuming a 500mA load then the 7805 is dissipating (7.4-5)*0.5 = 1.2W, so naturally it will get quite warm. Use a heat sink.

i know the instructions said to use capacitors

So why didn't you? Without them the 7805 may oscillate at high frequency, which would increase heat generated as well as affecting the regulation.

 

[audioguru]

The heating of a linear voltage regulator is simply the voltage from input to output times the current.
You have 2.4V x maybe 580mA= 1.4W which makes it hot. Add a heatsink to cool it if you want.

A linear regulator will oscillate at a high frequency if you do not add the input and output capacitors as close to its pins as possible.
The datasheet shows these capacitors.
If you add a series resistor to share the heat then you must make sure there is the minimum voltage (about 2.5V) from input to output of the regulator so that it regulates.

 

[duffy]

Yes, those capacitors are very important, even the small ones. Place them near the regulator pins AND the μC pins.

I am using this chip to drive my led matrix from a 7.4v source
...
I am also thinking about putting 2-4ohm resistor between the source and lm7805 maybe it will reduce the stress on the IC?

No, the 7.4V is getting close to the dropout voltage on a LM7805 - if you reduce the input voltage with a series resistance it won't regulate properly.

 

[tronitech]

With 7.4V in and 5V out that's 2.4V across the regulator @500mA = 1.2W. It will get hot, maybe not quite hot enough to cause damage short term but an heatsink is adviseable.

Your missing capacitors shouldn't contribute to the heat but it's good practice to fit them as per the manufacturers datasheet.

It's OK to put a resistor between the source and the LM7805 but you run the risk of dropping the voltage below the threshold required on the input for correct voltage regulation. The 1.2W of energy have to go somewhere and the resistor will get warm/hot depending on its wattage rating.

A voltage divider will not work because one set up for 1 LED at 20mA will be different to the divider needed for 25 LEDS at 500mA.

Fit an heatsink and capacitors and you'll be fine!

 

[crutschow]

.............................

also i know the instructions said to use capacitors, to remove ac flaws?, which i didn't but so far it hasnt been a problem, could this contribute to the heat?

.....................

Ah, another follower of the Earl "Madman Muntz" school of design. Earl was, among other things a builder of cheap black and white TVs in the '50s. His Muntzing design approach was to keep removing parts until the TV stopped working. Then he would say that all the removed parts except the last one removed were unnecessary and could be left out to save costs.

 

[Dr_Doggy]

lol, interesting & efficent, definitely a parallel, i just say if its not broken, dont fix, there are no protection resistors on any of my 125 LED's either (say vs a few parallel ones i had to add to "plug a leak") against all the threads iv read, but hey, the regulator keeps it all at 5v anyways, in fact i added an uln2003 to boost my currents, so why resist?(actually i did learn my lesson here when i started getting balancing issues on less conductive stages, just barely squeezed through without having to redo it all)

pic is limited at 20ma anyway, no? wouldn't i want my led's at max brightness? why add more limitations?

also i didnt really concern myself on the source side caps since i couldnt see too much ac getting in from my battery anyway..

turns out i did have a capacitor in there aswell, on the output side, unintentionally for the pic, all thats left is to make the heat sink outta washers an a bolt.

 

[audioguru]

No schematic??

 

[Dr_Doggy]

only if you dont laugh...!

it seems that i forgot to draw the uln2003 on the shiftregister output, to feed all cathodes
each 2n4401 on each of the multiplex pins, for each anode (5layers * 3colors/layer)
and the battery7.2v-power sw-lm7805 - and then the lm7803 running off of the lm7805 output
to feed the 3v circuits,

common cathodes on RGB led's were another learning curve

ohya, and the red led's transistors run off the lm7803 since RED are lower voltages


....a pull up resistor on mclr and vcc, and a 10uf capacitor across pic vcc&grnd (5v supply line)

 

[ronsimpson]

Oh I see....another share ware CAD program. lol

 

[Dr_Doggy]

which is nothing compared to my etching technique

 

[Dr_Doggy]

HMM..... maybe i should reask my question,,

considering that my circuit goes from 20-500ma @5v and my source is 7v +, what is the best way to supply this circuit, or did i do it right with the lm7805?

 

[alec_t]

Methinks you've already been told that a 7805 is ok providing you use supply decoupling caps correctly and use a heatsink.

 

[april]

Added because this is relevent to me now

Bolt and washers sounds fine how many washers and how do you put it together?
I realise this is a 1 amp device with a hole but I am still interested as the heat sink is the ground terminal too

I'm going from 24V to 5V and drawing about 80mA through a 100mA device
I am burning them out big time.
Thats about 1.5 watts isn't it?

I tried a 1 A device but it got pretty hot too
Perhaps I should do it in stages , 24V to 12V, then 12V to 5V but specs say 35 V max

 

[kubeek]

Why not start a new thread for your topic?

 

[april]

It relates directly to this one . I would like to know how he connects the bolt and washers. The metal heat sink is attached to ground so where and how you place it could be important in some applications

Why criticise ? If you don't like something just move on without posting
What is it that drives you to want to tell me what to do?

 

[jpanhalt]

Hi April,

Can you give a little more information about your heatsink? They have datasheets too that will show watts/°C. It then becomes simply a calculation of watts produced and acceptable temperature rise. What are you using to conduct the heat from the regulator to the heatsink?

Johm

 

[april]

Hi April,Can you give a little more information about your heatsink? They have datasheets too that will show watts/°C. It then becomes simply a calculation of watts produced and acceptable temperature rise. What are you using to conduct the heat from the regulator to the heatsink?Johm

I have had no heat sink thats why I am asking about this one . I might be able to fashion one up
I tried using the plastic and metal from a spent 1 A 7805 with the leads cut off and superglueing the 100mA 7805 to the metal backer . Its holding atm and is now quite cool

 

[Brevor]

Why criticise ? If you don't like something just move on without posting
What is it that drives you to want to tell me what to do?

He was not criticising you, It is common to start a new thread rather than pull up one that is a year old. One reason for this is that often the original poster is no longer around. This is the case with "Dr. Doggy" he rarely logs on anymore. You would be better off not using any of his ideas anyway most of his projects were very poorly done.

 

[april]

Hmm looking closely at your post, that obviously is your opinion. Fine , We don't all have similar opinions thankfully .

No doubt a bolt as a heat sink is a common idea . I'm simply interested in how he put it together mechanically.

The post is fine where it is in my opinion as it concerns the original poster be he here or not. If he's not then I won't get an answer from him will I ?