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-   -   Zener Follower - what did i do wrong? (http://www.diyaudio.com/forums/power-supplies/252114-zener-follower-what-did-i-do-wrong.html)

ballpencil 2nd March 2014 12:31 AM

Zener Follower - what did i do wrong?
 
I'm making a regulated 150v power supply for an OTL headphone amplifier. Supply current at idle is around 100mA and can go up to 200mA (class A).

I'm making a zener follower circuit and it works but not for long. Somewhere along, the TIP122 Darlington died causing unregulated 180V supply to show up at the emittor (collector-emittor shorted).

Schematic is based on this but i made some changes i described below:
http://i.minus.com/izKDd2wHpZa47.jpg

Voltage drop resistor value is 2K2. TIP122 darlington. Zener is series of 36v and 120V zener. Input cap is 220uF/200v and the only difference with the schematic above is i put 100uF capacitor across the zener (so if you remove the zener, it's a capacitance multiplier circuit).

1. Why did my TIP122 die? It could be that i fiddled and somehow shorted the transistor by mistake (so nothing wrong with the schematic).. i just want to make sure before i put a new darlington. Here's the datasheet.

2. Does zener really have bad regulation? 36 + 120 = 156 v but i'm really getting around 168-170V at the resistor-zener junction. This is why i fiddled with the circuit and may have killed the transistor.

esgigt 2nd March 2014 12:44 AM

Hi,
In theory the schematic ought to work. But you need to take in account, choosing the transistor, that the load might have a capacitive component. This might cause the Vce to rise above the specified value for the TIP122 at startup (damaging the transistor more and more until it fails). I'd try a 200V type...

That also gives you the opportunity to bypass the zenerdiode with a capacitor, reducing noise.

Cheers

DUG 2nd March 2014 12:45 AM

With a 100uF cap across the zener you will have all 180V from collector to base when you first turn it on.

No surprise it blew up.

ballpencil 2nd March 2014 01:10 AM

Quote:

Originally Posted by DUG (Post 3839101)
With a 100uF cap across the zener you will have all 180V from collector to base when you first turn it on.

No surprise it blew up.

I see.. this is because it takes time to charge the capacitor? Meaning at start up, base voltage will be close to zero and rising until it reaches the zener breakdown? So.. my options would be to remove the capacitor or change the darlington with higher Vcbo, right??

Vcbo TIP122 is only 100V.. i guess this is the rating that i stepped over?

paulb 2nd March 2014 01:14 AM

36V and 120V zeners probably have different current requirements. You may need to use a string of identical zeners instead.

Fast Eddie D 2nd March 2014 07:59 AM

Quote:

Originally Posted by ballpencil (Post 3839126)
I see.. this is because it takes time to charge the capacitor? Meaning at start up, base voltage will be close to zero and rising until it reaches the zener breakdown? So.. my options would be to remove the capacitor or change the darlington with higher Vcbo, right?

Keep the capacitor. Go with higher Vcbo.

Quote:

Vcbo TIP122 is only 100V.. i guess this is the rating that i stepped over?
You got it.

A smaller capacitor will rise in voltage a lot faster too. Think about that. You can still put a filter capacitor on the output; but take its surge into consideration too.

Fast Eddie D 2nd March 2014 08:00 AM

Quote:

Originally Posted by paulb (Post 3839129)
36V and 120V zeners probably have different current requirements. You may need to use a string of identical zeners instead.

And heed this advice too. Shoot for the datasheet current when you calculate R if you want to get close to the rated voltage.

Mooly 2nd March 2014 08:51 AM

Why not use a power FET instead of a bjt ? More rugged, the gate drive draws essentially no current.

Mooly 2nd March 2014 09:30 AM

Or something like this,
 
3 Attachment(s)
a true regulator with error correction. The addition of three components, the extra transistor and two feedback resistors create a proper regulator. The zener becomes a 12 volt one. The two resistors and caps to the left reduce the ripple component. The bits to the right are just for testing.

You can see how much ripple I've added to the unregulated input and how well the circuit reduces this under load.

Merlinb 2nd March 2014 10:07 AM

Quote:

Originally Posted by ballpencil (Post 3839093)
1. Why did my TIP122 die?

As Dug said, you need a transistor rated for the full supply voltage. Also, because when you switch off, the collector voltage may fall faster than the base voltage, thereby reverse biasing the base-collector.
You need to connect a protection diode between base and collector (e.g. 1N4007). Another diode between emitter and collector is also good practice (some transistors already have this built in).


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