Variable DC Power Supply

[shaiqbashir]

HI guys!

well! i just came across this circuit in the project section of this website:

http://www.electronics-lab.com/projects/power/011/index.html


here is its diagram:

now i want to ask you about its quality? will it work properly or not?

i want a voltage of around 12-15 V and a current of 245 mA. what do you think that this supply will be able to provide that or not? is there any technical mistake in its partlist or design. Plz do tell me about that!

Thanks a lot in advance!

Good Bye!

Regards!

 

[audioguru2]

Hi Shaiq,
All other regulated power supply circuits I've seen have a voltage reference like a zener diode. This circuit doesn't have one so I don't think its voltage regulation will be very good.

Your opamp mic preamp and LM380 power amp project doesn't need a regulated supply and doesn't need a supply with an adjustable output voltage. Just use an AC to 12VDC/0.5A adapter to power it.

 

[shaiqbashir]

THanks a lot Audioguru!

 

[MP1]

Actually, you do not need a zener. This is a very typical circuit for voltage regulation. Using a zener would be better, but you can use this circuit, trimmed to the voltage you want and then leave it alone. Believe it or not, there are a lot of places in the world where zeners, voltage regulator chips, and AC adapters are not easily or cheaply available. This circuit is completely adequate for such a situation.

MP

 

[audioguru2]

Believe it or not, there are a lot of places in the world where zeners, voltage regulator chips, and AC adapters are not easily or cheaply available.
MP

Believe it or not, an ordinary silicon transistor makes a pretty good zener diode.
A silicon transistor has avalanche breakdown of its reverse-biased emitter-base junction at about 6.6V. It has a slightly positive temperature response. If you add in series the transistor's forward-biased base-collector junction which has a slightly negative temperature response then the resulting 7.2V zener diode is nearly perfect.
Don't connect the base to anything. The emitter of an NPN transistor is the cathode and its collector is the anode of a DIY zener diode (the transistor is upside-down). Try it with a BC547 or 2N3904 NPN transistor. A PNP silicon transistor (BC557 or 2N3906) is the same, except with the opposite polarity.

In this power supply project I would add a zener diode in series with the emitter (to ground) of its Q1. Then the output voltage will be much better regulated because the voltage-setting pot won't attenuate the negative feedback so much. Of course, the minimum output voltage won't be less than the voltage of the zener diode. ;D

 

[indulis]

Are you SURE you want to go with “that” (collector – emitter) or stick with the base - emitter  junction?

 

[audioguru2]

Are you SURE you want to go with “that” (collector – emitter) or stick with the base - emitterView attachment 38186

 

[indulis]

Not always, the MPSA17 has a min V(br)EBO of 15V while a MPSH11 is 3V. The diode equation tells us that the "normal" P-N junction has a drift of ~2mV/

 

[audioguru2]

It looks like older silicon transistors have different Veb breakdown voltages than newer ones.
Looking at the only 3V rating of an MPSH11, I saw the 4.5V rating of some old 2N2369 transistors I still have.

As for reliability, I was taught that when an emitter-base junction avalanches, hot spots are burned in it and its small size is not able to dissipate heat nearly as well as the larger collector junction. The damage caused by the hot spots reduces the transistor's current gain if used later as an ordinary transistor.

Avalanching emitter-base junctions have been used for years as a white noise source.

 

[someshwar_ms]

the basic ckt from  shaiqbashir was of great help to me .....but some modifications were recomended by d senior members of the forum....any ways thanx a lot to all of u..........

 

[Muhammad Abu Bakar]

Audioguru, your electronics is vry strong.  What is ur major source of knowledge? :-\ Would u like to recommend a good book/study material to me to study.  I feel great defficiencies here (although I am BS Electronics engg).  Thanks!

 

[audioguru2]

The book called "The Art of Electronics" is pretty good. It is available in our Resources Forum.
I downloaded it and saw it for the 1st time recently.

 

[Muhammad Abu Bakar]

Thanks for the title.
But I am unable to download it from corresponding site.  When I do "Save As" it  gives no response..