78L05 voltage convverter is hot

[peterg]

Any general reasons for a voltage converter to heat up? The voltage readings look ok.

[gritz]

The usual reason is just science:

Voltage dropped between the input and output pins multiplied by current through the regulator = heat generated in Watts. Check the datasheet for information on limits.

How hot is "hot"? Too hot to touch? Hot enough to shut down? What's the input voltage and how much current are you sinking? What exact regulator is it?

Edit: ah, a 78L05. A Tiddler. I really should wear my spectacles more often! Is it a verified circuit you've built and maybe there's a problem somewhere, or is it that you're just asking a bit much of the regulator?

[peterg]

Gritz. I'm working on the Merlin Equinox Reverb. It was working but not now. Input voltage is 9V through a 1N4001 diode. Its hot to the touch. I hope my PT2399s aren't fried! 

[R.G.]

It was working but not now.

Was the regulator hot before the reverb quit?

[peterg]

It was working but not now.

Was the regulator hot before the reverb quit?

RG - not that I noticed. I checked the PT2399s on an other unit and they are still working. I eliminated  q1, r22, r23 and c29 and wired aa true bypass switch. I also removed c4 and its switch. I haven't done full readings since I didn't want to keep it running but was getting + readings on both sides of c33.

[Jdansti]

Without investigating the resistors you removed, I'll just say that if the resistance at pin 6 of the PT2399 is too low at power-up, it can lock up.

As for the regulator, if you have a meter, you can place it in series with the 78L05's output and see if the current is close to the limit (see the data sheet for the 78L05's output current limit). I had better luck with an LM7805 on the Solstice, which is similar to the Equinox. It runs cool enough that I don't need a heat sink. I can't recall if the pinouts are different for the 7805 and the 78L05, so check them if you decide to use the 7805.

[peterg]

Without investigating the resistors you removed, I'll just say that if the resistance at pin 6 of the PT2399 is too low at power-up, it can lock up.

As for the regulator, if you have a meter, you can place it in series with the 78L05's output and see if the current is close to the limit (see the data sheet for the 78L05's output current limit). I had better luck with an LM7805 on the Solstice, which is similar to the Equinox. It runs cool enough that I don't need a heat sink. I can't recall if the pinouts are different for the 7805 and the 78L05, so check them if you decide to use the 7805.

Thanks JDansti - The referenced resistors are in relation to Q1 which is for turning on and off the reverb effect and achieving tails.  I'll look into the LM7805. I notice on the topic started by Merlin that a number of builders have had the same issue.

Edit - Just saw your note re the same issue with the Solstice Reverb. What was the final solution? The LM7805? I also saw a note about grounding pins 4 of the PTs. I wired it with true bypass which is shy I removed Q1 and it's resistors as per Merlin's direction.

[Jdansti]

^Yes, I switched to the 7805 and haven't had any problems since.

Caveat:  I haven't done an A-B test to see if going back to the 78L05 would reproduce the problem, so consider my suggestion as anecdotal, but plausible.  

BTW-I just checked the pinouts, and both regs are the same- Input--Gnd--Output. It's a tight fit with the larger one, and it's on the edge of the board, which could present a problem with a small enclosure-I really don't know as I haven't built the Equinox. Fortunately, the tab on the 7805 is ground, so you could attach it to the enclosure if you want. If you left it "floating" near the wall of the enclosure, I'm not sure what the effect of intermittent contact with the wall would be.

[peterg]

Thanks Jdansti

I created by own layout on perf board and left space to fold down the 78L05 so I should have some space for the 7805.

[Mark Hammer]

FWIW, I think it is either Toshiba or Hitachi that make a slightly oversized 78L0x 3-pin regulator, that looks like the TO-92 case's big 6'5" brother.  It is a TO-92 footprint, just a tad fatter and taller.  It is spec'd for 150ma, rather than the more typical 100ma.  makes a nice compromise and margin of safety when the space available does not permit use of the larger 1A type.

[bluebunny]

BTW-I just checked the pinouts, and both regs are the same- Input--Gnd--Output.

Did you mean the 78L05 vs. 7805 pinouts are the same, John?  If you call the flat part the "front" of the TO-92 package (the 78L05), and if you call the tab on the TO-220 package (7805) the "back", then 78L05 goes OGI and 7805 goes IGO.  So yes, they're the same, but not in the same direction!  

78L05: (bottom view)

7805:

[Jdansti]

^ Yes. Thanks for pointing that out. I should have clarified that the relative order of the pins is the same, but you have to know which end is which (polarity).

Front and back on a TO-92 are confusing to my brain cell.   The "bottom" of a TO-92 is relative to how its mounted on a PCB, but the bottom or top of a TO-220 isn't. It's best to refer to the data sheet for each type (as you have provided).