18v power supply measures 16.7v

[JebemMajke]

I.ve found an 22v transformer and with 78l18, 78l09 and a heafty heatsink i've made a 18 and 9 v power supply. It was working flawleslly for over 6 months. Last night i was so tired so i've forgot to turn it of. It was hot. But not burning hot. Mildly. Anyway it wasn't working so i've measured with dmm. 16.7v. 9v part is intact. Should i put in a new 7818?

Ps i've used geofex spyder design for both.

pss iam using my phone, so iam sorry for lots of typeos.

[knutolai]

What does the input voltage to the 78l18 measure?

[JebemMajke]

20.01v

It was 20.01v before, not 22 as i've wrote

[petemoore]

 A once burning hot chip in any circuit puts the chip [and the circuit] under suspicion.
Considering the cost of the board/chip...the chip probably has auto cutoff when voltage drops under 20 that would probably survive repeated under-x-voltage at the input.
Very hot sounds more like a short or thermal runaway [not enough heat sink. Thermal runaway of course would tend to be more problem when dropping to 9v of course.
In any case the difference between 20v and 20.1v strikes me as small enough to suggest testing at 16v regulation instead of 18v regulator [IIRC 2v minimum 'headroom' is required], or finding the 24v transformer.
You've not needed more' heat sink for good long running time, but thermal dissipation rates can vary over time also, I would guess the 7818 barely got warm anyway though.
So either something failed or shorted, throwing another chip might cause it to work or even fix it, but without knowing the cause the risk of blowing the new chip is higher, examining and probing the board can be easy if it's obvious, difficult something it isn't right. 
 

[armdnrdy]

Does the transformer label say 22 volts?

What is the output current of the transformer? (How many milliamps?)

[JebemMajke]

Chinese transformer, so it has nothing on itself.

2,7 A was measured by me.

[armdnrdy]

Most transformers are not very precision devises. The manufacturer puts a rating on it..which is sort of a promise. They "promise" that the adapter will deliver for example, 12 volts at 200ma. (full load)
It is very common for a 9 volt adapter to actually read 12-13 volts. I have a 12VDC 500ma adapter that reads over 16 volts.

So, without knowing the rated current, and taking measurements using a constant current load (dummy load) to find the actual working voltage, I don't know how you could be sure of the voltage/current rating of this transformer.

Wall transformers are fairly inexpensive. It might be a better idea to use one that has the ratings printed on it so that you know what you are dealing with.

Edit:
How did you measure the 2.7A?

[JebemMajke]

When i was making this power supply i've had opened another thread. And there one guy told me to load the exit. Can not remember now how to do it. Like 1 kohm resistor ... Anyway i;'ve measured 2,7 A than.

Ps

I've tried another transformer with 24v output, after the regulator i've measured 17 v. So the regulator needs to be replaced. I ll try this 24 v transformer in that one's place

[armdnrdy]

Post a schematic of your power supply circuit.

Do you have a diode across the input and out put of the regulator?

How many pedals (what type) are you supplying with this P.S.?

The 78LXX series has a 100ma limitation. I wouldn't go anywhere near that...even with a heat sink.
That is what the 78XX series is for. (1 amp rating with heat sink)

[JebemMajke]

Well, as i;ve said in my first post, i was using geofex spyder as basic,
With a bit of a change in values, so after diode bridge the first thing is a 2200 u cap ( rated 35v), than the regulator ( 7818 and 7809, i am very bad at typing over the phone, so that l was accidental ), than 200n cap, than out for 18 volts, regulator for 9 v, 200 n cap, out for 9 v. No diode there at all.

Lets go with the list of pedals

18v:
All are diy
ibanez wh10, Zendrive, surh riot ( modified tonestack, and some caps for more bass control ), boiling point, and some which are currently being built, pigtronix philisophers tone, ts type of od, modified, fet based od.

9v:
all are diy except that tuner
Big muff, pharaoh fuzz, fuzz face with 1mosfet and 1 germanium inside, deep blue delay, echo base delay ( soon another delay ), foxx tone machine, tremulus lune, couple more fuzz faces with mods, some boosters, like lovepedal COT, SHO, some cheap Chinese tuner, cry baby type wah being made.

I guess that is it for now.

[armdnrdy]

This might be a silly question but...I have to ask.

The output of the transformer that you are using is AC correct?

How many of these pedals did you have powered when the regulator failed?

[JebemMajke]

1 - yes.

2 - 4 pedals, 2 per each

[armdnrdy]

Read R.G.s reply #12.

http://www.diystompboxes.com/smfforum/index.php?topic=107656.0

I would imagine that you have local caps on the power supply inputs of your stompboxes...that is like having capacitors on the output of the power supply.

[JebemMajke]

Well i guess on at least half of them i do have a big 100-220u caps for power filtering ....

Ok replacing regulator and transformer checked.

Should I put that diode that you've mentioned, on regulator, and how?

[armdnrdy]

I'm not sure why your regulator failed so..you should cover all bases.

The info you're looking for is in the link that I posted. Reply #10, sparkfun 7800pdf, Figure 7.

That will show the diode placement.

[JebemMajke]

Ok, thnx.

Well, i am going to replace transformer with the 22v one, i think that i've got 7818 at hand, i am really sorry that haven't got a 30v 5A transformer, than i would gladly put even bigger heat sink and 24v output, 18v output, 9v output.

[JebemMajke]

So yeah, changed regulator and transformer fixed the issue. 17,9 v and 8,9 v.

Also i've added two diodes as described in that link.

Thnx

[armdnrdy]

Good. good, good!