# Linear Power Supply Problem!!!

Discussion in 'General Electronics Chat' started by Guinness, Feb 19, 2012.

1. ### Guinness Thread Starter Member

Dec 31, 2009
81
1
Hi,

Attached is a circuit where I am trying to get 24V 1A output, the zener diodes are ment to put 36V to the base of the transister and 12V to the common of the voltage regulator. I am currently getting 39V to the base of the transister and 15.4V to the common of the regulator. I have tried puttin a resistor before the zeners and got the correct voltages but the final output current was then limited to 250ma. Basically I need the zeners to drop the correct voltages without limiting the current to below 1A.

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2. ### evilclem Active Member

Dec 20, 2011
118
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What is the voltage at V1? What is the purpose of Q1?

How are you limiting the current through the zener diodes?

3. ### R!f@@ AAC Fanatic!

Apr 2, 2009
9,647
1,111
Don't u have a better solution ?

That circuit has a problem, u will need Resistor in series with the zeners and the way u have connected the transistor is way wrong.

4. ### praondevou AAC Fanatic!

Jul 9, 2011
2,935
491
Is there any way you could lower the input voltage? You want to transform 48VAC to 12VDC with a linear regulator @1A.

You will be dissipating lots of power either the regulator or the circuitry to bring down the DC input voltage or both.

5. ### Guinness Thread Starter Member

Dec 31, 2009
81
1
The voltage at V1 is 48Volts. Q1 is there to regulate the voltage down to 36 volts. The zener diodes are what I need help with.

The circuit is not my choice, I just have to figure it out. Yea I know im missing a resistor or something with the zeners, thats what I need help with. I dont see what is wrong with the transister?

The input voltage is fixed, this circuit is from an actual working power supply, I am just missing one or two things. I know there was a resistor down at the common of the 12V regulator but cant remember how it goes down there. The value of R1 will prob not be correct as I have to figure that out.

6. ### Jony130 AAC Fanatic!

Feb 17, 2009
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I thinks that more correct diagram should look like

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7. ### Ron H AAC Fanatic!

Apr 14, 2005
7,012
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Did you pick 2N5210 at random, just for the schematic? 2N5210 is a 100mA max transistor.

8. ### Audioguru Expert

Dec 20, 2007
11,251
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There is a full-wave bridge rectifier so the input might be 48VAC which makes about 66VDC to blow up the little transistor and zener diodes.

9. ### Guinness Thread Starter Member

Dec 31, 2009
81
1
I agree, but it is for some reason behind the resistor, thats why I was wondering if there should be a second resistor before the zeners?
But if there is, what value could you put so the final output could go upto 1A?

The 2N5210 was picked from the list in LTSpice, I did forget to mention the actual transistor is a TIP31.

Also forgot to say the 12V regulator used is a 7812, but could not find that in the spice library.

For the circuit I have put a 48Vdc supply in Spice, the actual power supply is ac with about 48VDC after the bridge rectifier. Sorry I did not post that at start.

Last edited: Feb 19, 2012
10. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
513
This is one of those cases where you have to go back to your boss (or teacher) and tell them what they have asked you to do may not be impossible, but it certainly is absolutely ridiculous and then show them the right way to do it. If you become an engineer, you will be doing a lot of that.

FYI: if you start with a 66V DC voltage and "drop" it down to 24V @ 1A using linear devices, the laws of physics dictate that 42 Watts will have to be dissipated somewhere en route to the 24V point.

That is not practical or feasible. The solution? START WITH A LOWER INPUT VOLTAGE. You have the wrong transformer.

11. ### Ron H AAC Fanatic!

Apr 14, 2005
7,012
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Note that, in his schematic, the source is 48V DC. Still, it's too high.

12. ### Felo Member

Feb 20, 2012
91
13
Hi,

Instead of going linear, I would suggest a step down Buck converter, it would be a lot more efficient and if you don't need it to be too precisely regulated, you could even get your way with a simple 555 pwm implementation. Haven't tried it myself but I've seen it done in some web, ask Google a little more, it's wisdom is legendary.

Good luck

13. ### Guinness Thread Starter Member

Dec 31, 2009
81
1
Hi,

Thank you, I dont like the design of the power supply myself, would design it a lot different. But its not an option to change it, I have got it all working now though. So thanks everyone.

14. ### Felo Member

Feb 20, 2012
91
13
You could post the solution that worked for you, it may help others