Negat. Volt. Reg. with Passing transistor?

[Menticol]

Hi!

I got this schematic from an old textbook, a variable votage regulator with passing transistor, to handle more current. Nothing special for all of you.

**broken link removed**

I want to use the same transistor trick, on a fixed voltage PSU to power my boombox from a car battery. The requeriments are

+ 5V
0V
-5V

At 3 amps peak, 2 nominal

As far as my novice knowedge can see, the 0,6 ohm resistor (let's call it R1) triggers the transistor when too much current is beign drawn from the regulator.


So, three novice questions:

Can I replace the MJ2955 transistor with any other power NPN?

What should be the value of R1 if a LM7805 regulator is used? (I think it's determined by R = Operating Voltage / Maximum Regulator Current but I'm not sure)

Can I use the same transistor trick with a LM7905? (most critical question)

Thank you so much in advance

 

[ericgibbs]

So, three novice questions:

Can I replace the MJ2955 transistor with any other power NPN?
Note its a power PNP, not a NPN.
Use an equivalent rated transistor.

What should be the value of R1 if a LM7805 regulator is used? (I think it's determined by R = Operating Voltage / Maximum Regulator Current but I'm not sure)

Can I use the same transistor trick with a LM7905? (most critical question)
What voltage does the 'boom' box require.?

hi,
There are components missing from that bypass circuit.
It is possible to make the voltage output of a 7805 variable, but the regulation isnt ideal.

 

[Menticol]

Thank you by the fast answer, Ericgibbs.

The "boombox" (love that word haha) works with 6 D cell batteries, I think 4.5V positive and 4.5 negative. I don´t need variable regulators, so I believe 7805 and 7905 would be fine. What do you think?

PS: Sorry if I bother you, what components are missing? I copied the schematic "as is" (#$%&/@ textbook!)

 

[ericgibbs]

hi,
Look at this extract.

Note: to get +5V out, the 7805 requires at least +3V more, ie: +8Vin

 

[Menticol]

that's a real schematic!! thanks again Eric. I'm gonna use a 12V automotive battery, so the voltage is not a problem. One last question, can I use the 7905 with the same schematic? the datasheet doesn't show that

 

[ericgibbs]

that's a real schematic!! thanks again Eric. I'm gonna use a 12V automotive battery, so the voltage is not a problem. One last question, can I use the 7905 with the same schematic?

Hi,
Get the LM340 datasheet from DatasheetArchive :: Free Datasheet Search Engine :: Electronic Components

This will give you some help.

NOTE: with 7905 the bypass transistor will be a power NPN and the polarity of any polarised caps reversed ok.?
Depending upon the current thru the bypass it may require a heatsink.
The R1 should be 3Watt rating.

The attached dwg shows a two, 7805 dual, use a 7905 for the Vneg.

 

[ericgibbs]

hi,
You MUST use two separate batteries for the +V and -V regulators,
I expect you know that, but I thought I would highlight that point..

 

[Torben]

hi,
There are components missing from that bypass circuit.
It is possible to make the voltage output of a 7805 variable, but the regulation isnt ideal.

Hi Eric,

I think the OP was asking about whether an outboard high-current pass transistor could be used with a 78xx.

If I'm correct, the answer is "yes".

I agree (from what I've read, I've never built one) that there are parts missing.

Menticol, check out the Art of Electronics for more information. According to that, you need some current limiting to protect the pass transistor.

I'd recommend buying that book.


Torben

[Edit: Wow. I waited too long before hitting "Submit". When I started this post there were 2 posts on this thread. Oh well. ]

 

[Menticol]

I think the OP was asking about whether an outboard high-current pass transistor could be used with a 78xx.
Torben

Not really Torben, I asked if the transistor could be used fot a 79xx, negative voltage regulator. Thank you by the current limiter warning, I found a foldback protection circuit and I'm figuring out how does it works.

ps: Beautiful book! I'll look foward to import it.

Hey Eric very good schematics, I'm working with them. I didn't know that 2 separate batteries would be needed

How can a (i.e) car audio amplifier get negatve voltage from a single battery? or is just a single power supply amp? I hope there is some way to get the thing working!

 

[ericgibbs]

Hey Eric very good schematics, I'm working with them. I didn't know that 2 separate batteries would be needed What is the reason for that?

hi,

Its because of the COMMON 0V line, if you link the 78 and 79, 0v's together, you short out the 12V car battery.

Let me know when you do that, so that I can put my fingers in my ears before the bang...

Sketch it out on a piece of paper, you will see the problem.

 

[Torben]

Not really Torben, I asked if the transistor could be used fot a 79xx, negative voltage regulator.

The principle is the same; it's the same thing but negative.

Thank you by the current limiter warning, I found a foldback protection circuit and I'm figuring out how does it works.

ps: Beautiful book! I'll look foward import it.

It's a really good book. It's worth the money.


Toirben

 

[ericgibbs]

How can a (i.e) car audio amplifier get negatve voltage from a single battery? or is just a single power supply amp? I hope there is some way to get the thing working!

Hi
You need a circuit like this.

**broken link removed**

 

[erosennin]

It's a really good book. It's worth the money.

And it APPRECIATES tooo! Since I bought mine, it's gone up $40 ahahhaa....
In all seriousness though, I recommend it also.

 

[audioguru]

I think you are just guessing that the boom box has a positive and a negative supply.
6 D cells give 9V when new and if the amplifiers are bridged like in most car radios then the output power into 4 ohm speakers is only about 8W per channel which is not much. The peak current is 2A per channel so it is 4A total.

Doesn't the boom box have a jack for an AC adapter? With +9V and 0V terminals?

 

[Menticol]

I think you are just guessing that the boom box has a positive and a negative supply.
6 D cells give 9V when new and if the amplifiers are bridged like in most car radios then the output power into 4 ohm speakers is only about 8W per channel which is not much. The peak current is 2A per channel so it is 4A total.

Doesn't the boom box have a jack for an AC adapter? With +9V and 0V terminals?

No Audioguru, I'm sure it's 4.5. When I saw the 6 cells I though the same thing, "9V, very easy!" But it didn't worked.

My suspicions where confirmed when I dismantled the boom box, and saw a wire between positive of the third cell, and negative of fouth. Even more confirmed, on the power PCB one wire is labeled -5.

But you are right on the IC thing, it's a BA5417, doesn't require dual power supply.

https://www.ortodoxism.ro/datasheets/rohm/ba5417.pdf

The amperemeter of my homemade PSU is wired as follows, hope I didn't make a mistake

**broken link removed**

(It's made using a PC's AT PSU, so the 0 line is also shared by +3, +12, -12 not shown)

I'm reading the morphology pdf It's overheating my single neuron!

 

[audioguru]

The BA5417 has an output power of only about 2W per channel into 3 ohm speakers, less power into higher impedance speakers when the supply is 9V. Maybe your boom box uses a split supply to avoid adding the large output coupling capacitors.

 

[Menticol]

Thanks Audioguru, I didn't knew about that.
I found this diagram

Split Supply Generator

I was thinking aout using it to solve my problem, but the amperage and voltage are insuficient (+5V -5V at 3 amperes). Do you know any suitable circuit?

 

[audioguru]

There are circuits that generate a "middle" voltage from 12V. They are a powerful car audio amplifier IC. The IC might not work when its power supply voltage is only 9V.

I think you need a modern car radio and 4 speakers for it instead of the boom box.

 

[Menticol]

Hahaha yes, your solution follows the common sense, but I'm not trying to replace my car stereo with the boombox, instead I'm making a portable music player for outdoor parties.

Check my blog below, is in spanish but google translatin can make the trick. Is the entry of the SONY Boombox, with blue speaker caps.

Offcourse there are a lot of better IC's and custom PCB's for portable amplifiers, but as you may see, the objective of my site is reusing existing equipment, and adding new functions to the old one.

I'm gonna use the Eric's solution, with two 12 volt 7 amp/hour batteries and LM7805 - LM7905 regulators. Small but very heavy, I see no other solution.

Thank you so much guys for your answers! Very clear and professional, even for my simple problem.