What causes voltage regulators to fail?

[lti]

A few months ago I was asked to repair an Emerson boombox that would not play CDs. The CD player had only been used a few times. I found that a 7808 voltage regulator was bad. After replacing it, the CD player worked again, but I noticed that the output voltage dropped while changing tracks and that there is 0.5V on the input of the regulator when the CD player is not being used.

What could have caused the regulator to fail? If it fails again, what would be a better replacement? It was a pain to work on this POS and I don't want to keep replacing voltage regulators in something that is so cheap.

[Wizard]

Re: What causes voltage regulators to fail?

Is it a 78L08 in a TO92 package (blah!) or a real 7808 with metal tab? And is heatsinked?

Cheers, Wizard

[Jack Crow]

Re: What causes voltage regulators to fail?

I,
To answer your question...
"What causes voltage regulators to fail?"

Lots of things, not all of them are going to make a lot of sense.

A great many things can kill them.
Three terminal regulators are very tough parts, nearly impossible to kill. Yet I have changed a number of them over the years in projects and two way radios.

Most of us never think about what i call the Micro Environment our parts live in.

Look at another three terminal regulator chip.
See how the pins are molded into the epoxy case?
Now what happens if that seal, metal to epoxy is not 100%?

Air gets into the body of the chip and screws up the wafer, then the regulator fails.

What if a pin is over heated and all that thermal stress gets zapped straight into the regulator wafer?

A great many electronic issues are mechanical in nature. Some easy to spot and some subtle.

OEM's almost never use enough heat sink or thermal compound on their heat producing parts. The chips have thermal cut off circuits but if used too often they can fail as well.

Heat in general will trash solid state parts.

What is the Input to your 7808? The higher the voltage generally the hotter the chip will get.

What is the current loading? Did the OEM do something bone headed and try to over dissipate the chip? Measure the current, the average draw your meter will tell you about. Your peak draw will need to be measured with a floating ground scope. You said the change tracks command will cause the output to drop, might want to see what that current slug is. If it's over the rating of the chip that can cause an early failure.

Wild ideas.

The guys here know all too well that caps can and do go bad. In the fancy audio side of things did you know that there are places selling fake premium semiconductors? Your gizmo may have gotten a second or a test fall out part.

Hope this helps.
Jack Crow in VA

[lti]

Re: What causes voltage regulators to fail?

The input voltage is 14V. This is the regulator I purchased. I don't think it is fake.

The original voltage regulator was rated for 1.5A, which makes me think that the current is greater than 1A. It is attached to a heatsink, but it is just a flat piece of metal bent into a U shape.

[Jack Crow]

Re: What causes voltage regulators to fail?

ITI,
That's about standard.
Looks like you caught the 1 in 1K that was weak.
Much luck.
Jack Crow

[lti]

Re: What causes voltage regulators to fail?

I still think that the regulator is running close to its maximum current rating due to a poor design. One of the filter caps in the boombox was running at its maximum rated voltage and had failed.

The CD player was only used a few times before it stopped working, so the original regulator must have been a dud. It was manufactured in September 1999.

My username actually starts with an L. That mistake has been happening a lot.

[Wizard]

Re: What causes voltage regulators to fail?

Suggestion is as i echo is measure the current and trace out the circuits and split the circuits into two and put two seperate regulators for both. And other suggestion I echo lowering the input voltage.

And, NO matter what you DO, these linear regulators run HOT even on large heatsink, just merely takes longer time to heat up. What you can do is put a small quiet fan anywhere inside the player just to stir up the air fed from 5-7 voltage anywhere.

That is generic part. Preferred brands: ST, Motorola, National Semiconductor etc not clones or generic names from china.

Cheers, Wizard

[lti]

Re: What causes voltage regulators to fail?

There is no way to split the circuit since the regulator powers the servo controller IC in the CD player. The chip controls the spindle motor and moves the laser. I was thinking about measuring the current, but I put the boombox back together without measuring current and don't want to take it apart again. The heatsink wasn't very warm after playing an entire CD, which surprised me.

I don't know how I would decrease the input voltage. There is some kind of switch so the regulator is only powered when the boombox is switched to CD mode. However, there is a constant 0.5V on the input of the regulator in all other modes or with the power off.

I didn't want to spend much money on the repair, so I did not use a name brand part.

Is there a drop-in replacement for the 7808 that can supply more current?

[b700029]

Re: What causes voltage regulators to fail?

They don't make 3-terminal single package linear regulators for multi-amp ratings because they aren't very efficient and would require huge amounts of cooling. Dropping 14V to 8V at 1.5A already means 9W of waste heat. Switching regulators are more practical at high currents.

[Krankshaft]

Re: What causes voltage regulators to fail?

I have a Yorx Chinese stereo from that era made in 1998 and the voltage regulator for the 3 CD changer is on a heatsink. It gets scorching hot when it's in CD mode.

If you really wanted a regulator with more output current at that odd voltage.

You'd need an adjustable regulator like an LM350 just set the resistive divider on the adjustment pin to give you the 8 volts.

Switching regs are great but they require an energy storage inductor and then there's EMI and high frequency noise to contend with.

[Pyr0Beast]

Re: What causes voltage regulators to fail?

Get a switching supply and run 8V from there,

[lti]

Re: What causes voltage regulators to fail?

Those are good ideas. I would have to try that if the regulator fails again.

The circuit is strange. From the output of the regulator, there is a 0.4uF ceramic cap, a tiny air core inductor, two or three 220uF caps (I don't remember the exact amount), and a 470uF cap. I don't know the reason for this, but there should be enough filtering there for a switching regulator.

[Pyr0Beast]

Re: What causes voltage regulators to fail?

Hmm. This circuit might be very noise sensitive. Also 8V is a bit of a strange voltage.

[Longbow]

Re: What causes voltage regulators to fail?

After replacing it, the CD player worked again, but I noticed that the output voltage dropped while changing tracks and that there is 0.5V on the input of the regulator when the CD player is not being used.

The 3 terminal 78 series regulators have current foldback protection to protect from output short circuits or low impedance. The big clue is that the output voltage drops under load - that won't happen unless your input voltage is inadequate and falls below, in this case, around 10 volts. Most likely the input to the regulator has become unfiltered due to a failing filter cap and you will get another failed regulator again. So it is likely that the regulator was not at fault. Check the regulator input voltage under load.

No input voltage when the device is not in CD mode would make sense since the 8v is probably used only for the CD function. So, there must be a transistor that switches the input voltage to the 7808. If that switch does not saturate, it will get hot and won't be able to provide the full input to voltage regulator. There is usually more to the story than one failed part.

LB

[lti]

Re: What causes voltage regulators to fail?

The input voltage is constant and the filter caps on the input were replaced. Also, the input voltage does not drop to 0V when the boombox is not in CD mode. There is 0.5V on the input in all other modes, but the input voltage should be zero.
I didn't look at the input to see how power is switched, but I did discover that one of the two caps after the bridge rectifier was bad and the cap on the input of the regulator was starting to fail. I checked every filter cap and those were the only caps that were bad.

[Wizard]

Re: What causes voltage regulators to fail?

Another method when needing to do massive current carrying is use a TO-3P or TO-3 package of 2N3055 on big heatsink with the base pin controlled by a 7808 IC. A quick google will turn up circuit samples for this method.

http://apowersupply.com/voltage-regu...rcuit-152.html

Once I rebuilt a regulator circuit because one IC voltage feedback was NLA in one TV using a zener instead to control a transistor for stand by power supply. Works a treat.

Cheers, Wizard

[Longbow]

Re: What causes voltage regulators to fail?

The input voltage is constant and the filter caps on the input were replaced. Also, the input voltage does not drop to 0V when the boombox is not in CD mode. There is 0.5V on the input in all other modes, but the input voltage should be zero.

Well, whether it is .5 volts or 0 volts is the same - .5 volts at high impedance can easily sneak around a transistor that is turned off. So, you have a constant input voltage to the regulator? What is that voltage? Also be aware that you might see that the input to the regulator has significant ripple. I am looking for something besides a short circuit on the 8 volt side of the regulator.

Another thought is ...what does the 8 volts do? If it runs one of the motors, I would want to look there for a possible problem. Finally, it is not uncommon to have one faulty diode in a diode bridge. That situation lowers the available voltage to everything, but especially to circuits that are drawing some power. You didn't say if the input voltage to the regulator comes from a switching supply or a standard diode/filter arrangement.

I make these comments to suggest that in my experience it is rather unusual for a 7800 type device to fail, unless it has been designed right on the edge of its current abilities. Yes they can fail, but if so, replacing the part will fix it forever.

[lti]

Re: What causes voltage regulators to fail?

The input voltage is a constant 0.5V until the CD mode is selected. The voltage then increases to a constant 14.30V.

The power supply is a transformer, bridge rectifier, and filter caps. After the bridge rectifier, there is a 2200uF cap and a 1000uF cap. The input of the voltage regulator is filtered by a second 1000uF cap.

The regulator powers the motor, laser transport, and the IC that controls them. While I was troubleshooting this circuit, I connected a 9V battery across the output of the regulator and the CD player worked fine, but the battery was warm.

[Wizard]

Re: What causes voltage regulators to fail?

Iti, may need to look at spindle motor to see it is not leaky. Have you looked at the suggested sample circuit in that links?

Cheers, Wizard

[lti]

Re: What causes voltage regulators to fail?

I looked at that circuit. Would any changes need to be made to get 8V at the output, aside from using a 7808 regulator instead of the 7805 in the circuit?

How would I find a leaky motor?