www.elektronik.si

[mi4je]

Pozdravljeni!
Stvar je taka:
Pod roke mi je prišla mešalna miza, ki ji je odpovedal napajalnik.Na izhodu je bila precej večja napetost, kot bi lahko bila.Napetost na izhodu mora biti +18, -18 in +48v(za napajanje kond. mikrofonov).Najprej sem zamenjal diodo-100v(obkrožena), ki je bila prebita.Ko sem stvar priključil nazaj, pa je odletel še FET.Zakaj in še huje, kje dobiti tak FET.Nikakor mi ne uspe najti kakšnega data sheet ali tip s katerega bi razbral za kakšen FET sploh gre in s kakšnim bi ga lahko zamenjal.
Če ima kdo kakšno idejo, bom zelo vesel..

[PPG78]

Tranzistor je STP7NB60FP dobil pa ga boš v Media Trade (ali pa alternativo).

LP, Peter

[mi4je]

Peter, hvala za odgovor. Bom poskusil v Media Trade...
Upam, da ko ga zamenjam, da bo napajalnik O.K.
Hvala še enkrat

[PPG78]

Tranzistor ponavadi ne pregori brez razloga. Preveri elemente okrog FET-a, povratno vezavo ( če je narejena), krmilni čip. Vezje na sliki izgleda nekoliko ožgano na spodnjem delu?

LP, Peter

[surla]

Prvo zlato pravilo pri popravljanju switcher napajalnikov:
ko napajalnik ne dela in si zamenjal kak del v njem ,priklopi namesto njegove varovalke 60W 230V žarnico.Pri vklopu v omrežje mora žarnica za hip zagoreti potem pa ugasniti.Če stalno sveti imaš nekaj v stiku.To bi ti moral narediti po menjavi diode in bi ti transistor ostal cel.
Potem ,ko žarnica ugasne pa pomeri napetosti.Ko bodo napetosti OK, priklopi breme preko varovalke.

[PPG78]

Kar je surla povedal je res, en angleški tekst o testiranju z žarnico:

The series light bulb trick:

When powering up a monitor (or any other modern electronic devices with expensive power semiconductors) that has had work done on any power circuits, it is desirable to minimize the chance of blowing your newly installed parts should there still be a fault. There are two ways of doing this: use of a Variac to bring up the AC line voltage gradually and the use of a series load to limit current to power semiconductors.

Actually using a series load - a light bulb is just a readily available cheap load - is better than a Variac (well both might be better still) since it will limit current to (hopefully) non-destructive levels.

What you want to do is limit current to the critical parts - usually the switchmode (chopper) power transistor of an SMPS or horizontal output transistor (HOT) of a TV or monitor. Most of the time you will get away with putting it in series with the AC line. However, sometimes, putting a light bulb directly in the B+ circuit will provide better protection as it will limit the current out of the main filter capacitors. Actually, an actual power resistor is probably better as its resistance is constant as opposed to a light bulb which will vary by 1:10 from cold to hot. The light bulb, however, provides a nice visual indication of the current drawn by the circuit under test. For example:

* Full brightness: short circuit or extremely heavy load - a major fault probably is still present.

* Initially bright but then settles at reduced brightness: filter capacitors charge, then lower current to rest of circuit. This is what is expected when the equipment is operating normally. There could still be a problem with the power circuits but it will probably not result in an immediate catastrophic failure.

* Pulsating: power supply is trying to come up but shutting down due to overcurrent or overvoltage condition. This could be due to a continuing fault or the light bulb may be too small for the equipment.

The following are suggested starting wattages:

* 40 W bulb for VCR or laptop computer switching power supplies.
* 100 W bulb for small (i.e., B/W or 13 inch color) monitors or TVs.
* 150-200 W bulb for large color monitors or projection TVs.

Depending on the power rating of the equipment, these wattages may need to be increased. However, start low. If the bulb lights at full brightness, you know there is still a major fault. If it flickers or the TV (or other device) does not quite come fully up, then it should be safe to go to a larger bulb. Resist the temptation to immediately remove the bulb at this point - I have been screwed by doing this. Try a larger one first. The behavior should improve. If it does not, there is still a fault present.

[podboj]

Kakšne pa so bile napetosti, ko že omenjaš da so previsoke? Kakor je razvidno s slike switcher nima nobene regulacije, tako da napetosti lahko nihajo glede na obremenitev. Če si switcher zagnal brez bremena se ne bi čudil, če bi bile napetosti tudi za 1/2 previsoke.

Ko si zamenjal tisto diodo upam da si našel tako z enako oznako. Predvidevam da je bila UF4007 ali kaj podobnega. Navadne 1N4007 ne bodo dobre, ker so prepočasne za 30kHz oz. frekvenco na kateri dela tvoj switcher.

Zelo verjetno je da imam v delavnici kakšen tak mosfet, se bom potrudil čimprej pogledat.

Preveri še diode na sekundarni strani, elemente na zažganem delu tiskanine (R11, R13, R16, R3, R4, D1, D2, odlotaj vsakemu elementu en pin in ga zmeri). Če je prebilo mosfet, preveri če ni zraven odneslo še greatza, NTC upora in varovalke. Morda ne bi bilo slabo pogledat še elektrolitov.

[majstr]

Predvsem elkote in ic!!

[jur]

Ko na switcherju zamenjaš pregorelo varovalko (ali diodo ali kaj drugega) in ga priključiš na elektriko, ponavadi crkne nova varovalna še nekaj več. Pri popravilu je vedno potrebno zamenjati VSE pokvarjene komponente, preden napravo priključiš na elektriko. Precej switcherjev uporablja varovalke za ščitenje žic, ne pa za zaščito elektronike. Hitri prenapetostni impulzi so tudi hitrejši od varovalk.

Nekateri switcherji (pravzaprav večina) za delovanje potrebujejo neko minimalno obremenitev, sicer se ne zaženejo. Za PCjev napajalnik je povsem primerno breme star hard disk.

Jur

[mi4je]

[jur]

Če ni prostora za ventilator, povečaj ali dodaj hladilna telesa na elemente, ki se grejejo. Obstojajo tudi majhni ventilatorji, na primer 4 centimeterski (debeli okoli pol centimetra ali manj), dobavljivi v vseh trgovinah z elektroniko. Če se ti stvar močno greje, mogoče nisi povsem odpravil napake. Če je napaka odpravljena in se močno greje, obstoja verjetnost, da bo zaradi pregrevanja spet kaj mrknilo.

Jur

[podboj]