Prototype board

I put a socket for a 24vdc wall adaptor on my prototype board, with a 7805 5vdc regulator and 47uF capacitor between ground and output (5v). The center pin is ground, both on the adaptor & the plug. 24v only goes to the Input leg of the 7805, and nothing else. This simply sends 5v to the main path at the edge of the board.
The 7805 blew up in short order. I can't see anything wrong with the wiring. I see now that there's some kind of 7805 built in, with the TO-92 case - is this interfering with the 7805 I added? Is 24v too much to attach directly to the rail?


Senior Member
As a general rule the 78xx series input voltage should be limited to 2 to 3 volts above the output voltage. Hence 7805 'naturally" should suit a 7-9 V input.

Even though it should handle 24 V OK, the power difference between the input and output appears as heat. If the input voltage is unnecessarily high, the regulator will overheat,& unless sufficient heat dissipation is provided (through heat slinking),the regulator will shut down.

I'd suspect your "blow up" relates to a heavy current output as well. WHY 24V? ARE YOU SURE IT'S -duh- DC & not a AC ?! Suggest you provide a schematic, even hand drawn & uploaded to Tinypic.
Two possibilities. The 24DC from the reg may be a lot more when it is unloaded - maybe measure this and see what it is.

Also, if you draw 1 Amp from the reg, it will be dropping 24-5 = 19V at 1A = 19 watts, which will surely cook it. Some 7805s have internal overload shutdown and some don't and there is no way of really telling which are which.

24VDC to 5V is a bit wasteful of energy as well - maybe use a 9V wall wart instead.
I was hoping to use the same power supply that powers another project, which has to be 24v. Is there a way to drop the voltage without the extra heat/power loss?
Also, I've got another 7805 giving 5vdc to the Picaxe 08M and Sensirion SHT-11, plus 2 relays. The 7805 has been running 24/7 for a few weeks, with no undue overheating. The spec sheet says the 7805 will handle over 30v...
You can build a 'pre-regulator' - eg a 7815 to drop it to 15V then drop that to 5. The higher voltage regs tend to be more tolerant of higher input voltages as well. Heatsinks also help - maybe 5x5cm of aluminium to start with. The heatsinks can be warm but not burning hot (if they are, make them bigger).

Re the 30V spec - that would apply if you have the same brand reg as the one in the spec sheet - and also it isn't a fake (I got stung once with some fakes that had the national semiconductor symbol on them - but mirrored in reverse. Cost 20c which should have been a clue...)

I presume the 24V supply has a big capacitor inside it? 9V 1A wall warts have 4700uF and while that is a big value really you do need all of that to prevent ripple. If there is AC present it could peak well over 30V.

Edited by - Dr_Acula on 20/08/2007 02:46:19
The 7805 really requires a heatsink once you are drawing more than about 1/4 amp from it. Your existing setup may only have a small current draw and that is why it's not heating up much. The 19V drop across the 7805 is quite large and will heat it considerable, depending on total current draw from it. If you check the datasheets for a 7805, it will state the heatsinking requirements for it and whilst the sheets say it can handle 30V input, bear in mind that is a maximum, with proper heatsinking under best case conditions.
If you must use the 24V supply, it might be an idea to regulate down to 12V via a 7812, then down to 5V off the 12V regulator. At first glance it seems a waste but it will prevent undue heating of the regulators and minimise the amount of heatsinking required on each regulator.
Don't forget to use plenty of caps for the usual reasons.



Senior Member
Just a suggestion. I would consider a switching buck-mode regulator. Such as:<A href=';tier2=Integrated+Circuits&amp;tier3=Power+Supply+ICs&amp;tier4=LM2575T+1A+Buck+voltage+regulators&amp;moduleno=34450' Target=_Blank>External Web Link</a>

Just google 'switching regulators'. They are much more efficient, especially with high input voltages.

Edited by - boriz on 20/08/2007 03:07:38
Switching regs are great as they don't get hot. However, they need special diodes and the datasheets for the potcores never seem to be all that clear. Potcores are a bit of a mystery unless you have the right equipment to measure them, but essentially they need to have the right inductance and the right resistance. I suppose one could buy pre-made ones though the part numbers never seem to match with local part numbers. Sometimes you get a clear explanation - eg wind this many turns of this diameter wire on this potcore.

So it all gets a bit complicated. The best option is to get a kit of complete parts if one is available.


Senior Member
I always use some kind of &quot;phone chargers&quot; to avoid complications, mostly run 5.5 to 12v out 200Ma to 400ma without problems.
The 7805 can handle it.

The 7805 is a wonderful dinosaur which should easily be able to handle 24V - but noting the afforementoined points re Power dissipation which is ESPECIALLY important with large voltage diffs.
I take it you remembered that the metal tab/case is not isolated?
Popping one suggests bad wiring or bad luck.

Switched Mode is the best way to go and look at the Simple Switcher range from National Semiconductor.
However, they are more expensive AND you will have to READ the Data Sheet. Sorry I swore there, but the peripheral components are crucial in getting good performance.
You don't need to measure inductors and don't bother with DIY - I doubt if you can measure core saturation at home anyway.
Just read up on specs. and get the right bits.

Be aware that some other S/Mode devices have a limited input voltage and will not like 24V input.

If you choose the correct components and have a good PCB layout it is unlikely you will have to smother it in ferrite beads. You will also find that going OTT with capacitance is a waste of time too.

Have a look at these:-
<A href=';jsessionid=P2YJTQGOLWRPXQFIAFAZLTQ?N=411+1000010+106636&amp;Ntk=gensearch_001&amp;Ntt=simple+switcher&amp;Ntx=&amp;_requestid=26941' Target=_Blank>External Web Link</a>

Also, take a copy of this which gives some great circuits for different apps for the 7805:
<A href='' Target=_Blank>External Web Link</a>

So, a lot of this is dependent on your wallet - but you get what you pay for.
Remember Matel and cheap Chinese toys?
Or, if you have plenty of dosh, get yourself a 78S05 regulator.
It is a switchmode pin-for-pin replacement for a regular 7805 giving 2A @ 5v with virtually no heat for a 24v input.
Last time I bought one (circa 1986) they were about 20GBP ea. Might be much cheaper now, have a search.


Senior Member
There is a possible home-brew/cheapskate solution you could try, And it should be quite efficient, but I only recommend it for currents up to around 100mA MAX.

Use a 555 to generate a roughly 30% duty cycle at a few hundred HZ (not critical). Have this charge an electrolytic capacitor through a diode. The cap gets charged on the positive 30% duty. The 555 can source 200mA. Have a resistor across the cap to prevent the voltage climbing. Put this voltage into your 5v regulator. I recommend a 78L05 when using &lt;100mA. Adjust the duty cycle until you get about 8v across the cap, giving a nice 3v overhead for the regulator. The regulator will be able to handle a moderate ripple on it&#8217;s input. Component values are non critical. Experiment.


Senior Member
Seems the price on the 78S05 (or somebody's knockoff) has come down a bit - 2 for 1 GBP
<A href='' Target=_Blank>External Web Link</a>

From my own experience, I have a 24 VDC project and needed 5 volts DC for the PICAXE. I used a LM7812 and a LM7805 VDC 1 amp regulators in series with some electrolytic capacitors on each side of the regulators. (standard practice) I have also fused this circuit at 1 amp even though the 5 volt rail only draws about 50mA? I could probably fuse down into the mA region if I could find some size 3AG fuses rated in milliamps.

I experienced the same meltdown as you a couple years ago. I tried to go from 24 volts to 5 volts with only 1 regulator and unfused!. All worked OK for about 2-3 hours then it started to leak smoke from everywhere! I lost a 2 X 16 serial LCD, a PICAXE 28X, a 5 VDC 1 amp regulator and a few LED&#8217;s. The only component to survive was the MOSFET rated at 55VDC!

I rebuilt the regulator circuit again but with 2 regulators described above and this time I load tested it on the 5 volt side for days one end at about 100mA before plugging in the new PICAXE chip. I fused this part of the over all circuit at 1 amp! I decided not to be so fancy and did not buy another LCD. Also, I used a PICAXE 08-M as I didn&#8217;t really need all the extra I/O the 28X provided. It&#8217;s just what I had on hand at the time.

The dual regulator circuit has worked flawlessly for about a year now.

&#8220;The Addict&#8221;



Senior Member
BB - <i>&quot;Or, if you have plenty of dosh, get yourself a 78S05 regulator.
It is a switchmode pin-for-pin replacement for a regular 7805 giving 2A @ 5v with virtually no heat for a 24v input.&quot; </i>

I have searched for info on the 'S' version and only see other manufacturers details on what appears to be an equivalent to the non-S verion - sure there is more current capacity but no mention of switchmode!

The block diagrams appear the same for the 7805 and 78S05

Would you please provide more info

Edited by - BCJKiwi on 21/08/2007 01:20:51


Senior Member

Doh! Forgot that. :(

Maybe a zener from the 555 0v pin to the circuit ground? No significant current should flow through the zener. The duty cycle would need to be shorter though.


Senior Member
Block diagrams are the same, but S part are more protected

7805c datasheet =&gt;<A href='' Target=_Blank>External Web Link</a>
78S05 datasheet =&gt;<A href='' Target=_Blank>External Web Link</a>

read the specs.

No matter what, I recomend you to use two regs (7812 &amp; 78S05) in cascade to avoid
problems, or use one small transformer (Like truck radio adaptor) 24 to 12v between
the wall wart and 7805 (They're really small, allow put on PCB)

Well, I just tried to find a source of 78S05 switching regulator and was somewhat dismayed to find that many LINEAR regulators are now using the nomenclature.

Put simply, even with todays low cost electonics you simply won't get a switching regulator for 50p. The clue is in the price if you are wondering what type they are.
The type to which I refer are these.

<A href='' Target=_Blank>External Web Link</a>

Available from RS at about &#163;15 ea.
T220 pin-for-pin replacement for 7805.
Unless you use a switchmode supply any voltage drop will be dissipated as heat through the regulator/s, the bigger the drop required the more heat you need to get rid of. 24v DC is not a good starting point and 24v AC is very nasty.
You could try 1 or 2 x 5 Watt low value resistors, they are cheaper than regulators.
<b>But </b> there are drawbacks to using this method e.g. if you have a heavy current demand or spike the regulator can drop out. This can be overcome by using large capacitors 1000&#181;F + on the regulator input and 10&#181;F to 47&#181;F on the output as well as 0.01&#181;F to 0.1&#181;F caps on the in/outputs.
30 to 50 Ohms divided over 1 or 2 5W resistors may be a good starting point.
When setting up you need to run the regulator at full load and ensure you still get at least the +2v on the input side the regulator needs to maimtain regulation.
It's all a balancing act.
Any heat coming off the resistors is heat that the regulator/s do not have to dissipate.

Edited by - Michael 2727 on 21/08/2007 14:14:38


Senior Member
Thanks BB

Yes the 78SR105's are around NZ$85+ compared to 7805's at NZ$1.50.

Just had another look at the MC7805 spec sheet which shows interesting data

Output Voltage of 4.75 to 5.25 (nominal 5.0)
when current is in the range
5.0mA &lt;= Io &lt;= 1.0A, P &lt;= 15W
BUT only as long as the junction temp is +25 C

Any one like to hazard a guess at how you might get 3A (15W) out of a device passing &lt;= 1.0A, and, what sort of a heat sink you would need to maintain the junction temperature at +25C while working this hard?




Senior Member
It's called a thermoelectric cooler and requires its own power supply :)

Seriously you usually run the heat sink up to 100 deg C and look at the derating curves in the spc sheet... One problem is failures from thermal cycling - a component fails sooner if repeatedly heated and cooled - that's life... It's about 90 degF and I'm about to go out on my bike - hope my personal components survive the cycling [ha!].
If you purchase a heatsink rather than bolt on a piece of aluminium, then the calculations are easy. The heatsink will have a specified Degrees per Watt.
ie a 3C/W heatsink will rise in temperature by 3C for every watt dissipated. It's that easy.
For home brew heatsinks, have a look at a similar sized comercial one and it will have a similar characteristic at least to the nearest order of magnitude.
One of the crucial issues is thermal resistance between junction and heatsink. This will be specified in the component specsheet. Even if you keep the casing cold, the junction can still melt.
Amazing what all those funny little numbers in datasheets are for and how they can suddenly become important when things melt!