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Series pass transistor, LM7812

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Series pass transistor, LM7812

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3

$begingroup$

I am making a 12 volt regulator for a solar panel to power an audio amplifier. I already have a switching regulator for most application but it is very noisy for amplifier even with a lot of filtering.
The amplifier draws over 3 amps max.

I want to build a regulator using 7812 with series pass transistor for higher current as shown in the datasheet, page 20.

http://www.mouser.com/ds/2/149/LM7812-461970.pdf

I made the same circuit as shown in the datasheet and used the 2SD1047 as Q1

https://www.st.com/resource/en/datasheet/2sd1047.pdf

^{simulate this circuit – Schematic created using CircuitLab}

But as I connected it to my bench power supply set to 18V I was not able to get a regulated output. It just showed ~2V less then the input voltage.

At 18V no load I got 16V output, 0.5A load 15V and 3A load 12V.
I tried different values of R1 calculated by the formula shown in the data sheet.

Is the output voltage affected by the the value of R1?
Please Help.

While calculating R1 I assumed the value for Ireg to be 0.1A, Beta of transistor to be ~80, and IQ1 to be 3A, when that didn't work I tried a bunch of different values of resistors with the same results.

The LM7812 works great when no series pass transistor is attached.
And the transistor is good too.

I already have a switching regulator and I also have a higher current linear regulator so don't suggest them please. I want this circuit to work, and I also want to understand this circuit further so if you could point me to other learning material concerning this circuit then that would be great. The only thing I had to work with was the datasheet. Thanks

PS: SOLVED!! My mistake, I connected a NPN BJT instead of PNP.

high-current linear-regulator lm78xx

share|improve this question

edited 1 hour ago

SamGibson

11.2k41737

asked 2 hours ago

Hamza KhanHamza Khan

227

$endgroup$

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  $begingroup$
  Glad you could work out some issues yourself. Yes, with a 'boost' transistor a minimum load is needed so the Vbe is high enough. Major mistake though. The boost transistor MUST be a pnp, with the 12 volt current boost coming from the collector. Try a MJE172.
  $endgroup$
  – Sparky256
  2 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Oh Sorry!! I made such an obvious mistake and Thanks for not criticizing.
  $endgroup$
  – Hamza Khan
  1 hour ago
  

  
  
  
  

add a comment | 

3

$begingroup$

I am making a 12 volt regulator for a solar panel to power an audio amplifier. I already have a switching regulator for most application but it is very noisy for amplifier even with a lot of filtering.
The amplifier draws over 3 amps max.

I want to build a regulator using 7812 with series pass transistor for higher current as shown in the datasheet, page 20.

http://www.mouser.com/ds/2/149/LM7812-461970.pdf

I made the same circuit as shown in the datasheet and used the 2SD1047 as Q1

https://www.st.com/resource/en/datasheet/2sd1047.pdf

^{simulate this circuit – Schematic created using CircuitLab}

But as I connected it to my bench power supply set to 18V I was not able to get a regulated output. It just showed ~2V less then the input voltage.

At 18V no load I got 16V output, 0.5A load 15V and 3A load 12V.
I tried different values of R1 calculated by the formula shown in the data sheet.

Is the output voltage affected by the the value of R1?
Please Help.

While calculating R1 I assumed the value for Ireg to be 0.1A, Beta of transistor to be ~80, and IQ1 to be 3A, when that didn't work I tried a bunch of different values of resistors with the same results.

The LM7812 works great when no series pass transistor is attached.
And the transistor is good too.

I already have a switching regulator and I also have a higher current linear regulator so don't suggest them please. I want this circuit to work, and I also want to understand this circuit further so if you could point me to other learning material concerning this circuit then that would be great. The only thing I had to work with was the datasheet. Thanks

PS: SOLVED!! My mistake, I connected a NPN BJT instead of PNP.

high-current linear-regulator lm78xx

share|improve this question

edited 1 hour ago

SamGibson

11.2k41737

asked 2 hours ago

Hamza KhanHamza Khan

227

$endgroup$

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  $begingroup$
  Glad you could work out some issues yourself. Yes, with a 'boost' transistor a minimum load is needed so the Vbe is high enough. Major mistake though. The boost transistor MUST be a pnp, with the 12 volt current boost coming from the collector. Try a MJE172.
  $endgroup$
  – Sparky256
  2 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Oh Sorry!! I made such an obvious mistake and Thanks for not criticizing.
  $endgroup$
  – Hamza Khan
  1 hour ago
  

  
  
  
  

add a comment | 

$begingroup$

I am making a 12 volt regulator for a solar panel to power an audio amplifier. I already have a switching regulator for most application but it is very noisy for amplifier even with a lot of filtering.
The amplifier draws over 3 amps max.

I want to build a regulator using 7812 with series pass transistor for higher current as shown in the datasheet, page 20.

http://www.mouser.com/ds/2/149/LM7812-461970.pdf

I made the same circuit as shown in the datasheet and used the 2SD1047 as Q1

https://www.st.com/resource/en/datasheet/2sd1047.pdf

^{simulate this circuit – Schematic created using CircuitLab}

But as I connected it to my bench power supply set to 18V I was not able to get a regulated output. It just showed ~2V less then the input voltage.

At 18V no load I got 16V output, 0.5A load 15V and 3A load 12V.
I tried different values of R1 calculated by the formula shown in the data sheet.

Is the output voltage affected by the the value of R1?
Please Help.

While calculating R1 I assumed the value for Ireg to be 0.1A, Beta of transistor to be ~80, and IQ1 to be 3A, when that didn't work I tried a bunch of different values of resistors with the same results.

The LM7812 works great when no series pass transistor is attached.
And the transistor is good too.

I already have a switching regulator and I also have a higher current linear regulator so don't suggest them please. I want this circuit to work, and I also want to understand this circuit further so if you could point me to other learning material concerning this circuit then that would be great. The only thing I had to work with was the datasheet. Thanks

PS: SOLVED!! My mistake, I connected a NPN BJT instead of PNP.

high-current linear-regulator lm78xx

share|improve this question

edited 1 hour ago

SamGibson

11.2k41737

asked 2 hours ago

Hamza KhanHamza Khan

227

$endgroup$

share|improve this question

edited 1 hour ago

SamGibson

11.2k41737

asked 2 hours ago

Hamza KhanHamza Khan

227

share|improve this question

edited 1 hour ago

SamGibson

11.2k41737

asked 2 hours ago

Hamza KhanHamza Khan

227

edited 1 hour ago

SamGibson

11.2k41737

edited 1 hour ago

SamGibson

11.2k41737

asked 2 hours ago

Hamza KhanHamza Khan

227

asked 2 hours ago

Hamza KhanHamza Khan

227

1 Answer
1

active

oldest

votes

3

$begingroup$

Here is the diagram from page 20 of the datasheet you linked:

_{Image source: Fairchild LM78xx datasheet, Figure 13}

Notice that the pass transistor is PNP. The mistake in your design, is that you are trying to use a 2SD1047 NPN pass transistor - which won't work, as you have seen.

share|improve this answer

answered 1 hour ago

SamGibsonSamGibson

11.2k41737

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Thank you for the fast reply and not making fun of me.
  $endgroup$
  – Hamza Khan
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @HamzaKhan - No problem, and well done for writing a clear question and trying to solve the problem yourself :-) I see that you got the same reply in a comment, while I was typing this answer...
  $endgroup$
  – SamGibson
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Keep in mind Q1 needs a heat sink of ~ 2'C/W for 40'C rise @18Vin*3A so efficiency is 67%
  $endgroup$
  – Sunnyskyguy EE75
  1 hour ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @Hamza - Since you won't automatically be notified about the comment made by Sunnyskyguy EE75, I'm writing this comment, which will notify you. As he points out, one disadvantage of a linear regulator is that you have to dissipate all the heat from that series pass transistor. You say that your requirement is "over 3 amps max", so you will have to consider the heatsinking for that transistor and the regulator. If you need help with that topic, then feel free to ask a new question about it.
  $endgroup$
  – SamGibson
  1 hour ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  I have BIG heatsinks laying around, no prob. But is it OK if I place both of them on the same heatsink, of course the 7812 will have an insulator. I mean one wont get super hot and heat up another for no reason.
  $endgroup$
  – Hamza Khan
  1 hour ago
  
  
  

  
  
  
  

add a comment | 

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3

$begingroup$

Here is the diagram from page 20 of the datasheet you linked:

_{Image source: Fairchild LM78xx datasheet, Figure 13}

Notice that the pass transistor is PNP. The mistake in your design, is that you are trying to use a 2SD1047 NPN pass transistor - which won't work, as you have seen.

share|improve this answer

answered 1 hour ago

SamGibsonSamGibson

11.2k41737

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Thank you for the fast reply and not making fun of me.
  $endgroup$
  – Hamza Khan
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @HamzaKhan - No problem, and well done for writing a clear question and trying to solve the problem yourself :-) I see that you got the same reply in a comment, while I was typing this answer...
  $endgroup$
  – SamGibson
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Keep in mind Q1 needs a heat sink of ~ 2'C/W for 40'C rise @18Vin*3A so efficiency is 67%
  $endgroup$
  – Sunnyskyguy EE75
  1 hour ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @Hamza - Since you won't automatically be notified about the comment made by Sunnyskyguy EE75, I'm writing this comment, which will notify you. As he points out, one disadvantage of a linear regulator is that you have to dissipate all the heat from that series pass transistor. You say that your requirement is "over 3 amps max", so you will have to consider the heatsinking for that transistor and the regulator. If you need help with that topic, then feel free to ask a new question about it.
  $endgroup$
  – SamGibson
  1 hour ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  I have BIG heatsinks laying around, no prob. But is it OK if I place both of them on the same heatsink, of course the 7812 will have an insulator. I mean one wont get super hot and heat up another for no reason.
  $endgroup$
  – Hamza Khan
  1 hour ago
  
  
  

  
  
  
  

add a comment | 

3

$begingroup$

Here is the diagram from page 20 of the datasheet you linked:

_{Image source: Fairchild LM78xx datasheet, Figure 13}

Notice that the pass transistor is PNP. The mistake in your design, is that you are trying to use a 2SD1047 NPN pass transistor - which won't work, as you have seen.

share|improve this answer

answered 1 hour ago

SamGibsonSamGibson

11.2k41737

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Thank you for the fast reply and not making fun of me.
  $endgroup$
  – Hamza Khan
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @HamzaKhan - No problem, and well done for writing a clear question and trying to solve the problem yourself :-) I see that you got the same reply in a comment, while I was typing this answer...
  $endgroup$
  – SamGibson
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Keep in mind Q1 needs a heat sink of ~ 2'C/W for 40'C rise @18Vin*3A so efficiency is 67%
  $endgroup$
  – Sunnyskyguy EE75
  1 hour ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @Hamza - Since you won't automatically be notified about the comment made by Sunnyskyguy EE75, I'm writing this comment, which will notify you. As he points out, one disadvantage of a linear regulator is that you have to dissipate all the heat from that series pass transistor. You say that your requirement is "over 3 amps max", so you will have to consider the heatsinking for that transistor and the regulator. If you need help with that topic, then feel free to ask a new question about it.
  $endgroup$
  – SamGibson
  1 hour ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  I have BIG heatsinks laying around, no prob. But is it OK if I place both of them on the same heatsink, of course the 7812 will have an insulator. I mean one wont get super hot and heat up another for no reason.
  $endgroup$
  – Hamza Khan
  1 hour ago
  
  
  

  
  
  
  

add a comment | 

$begingroup$

Here is the diagram from page 20 of the datasheet you linked:

_{Image source: Fairchild LM78xx datasheet, Figure 13}

Notice that the pass transistor is PNP. The mistake in your design, is that you are trying to use a 2SD1047 NPN pass transistor - which won't work, as you have seen.

share|improve this answer

answered 1 hour ago

SamGibsonSamGibson

11.2k41737

$endgroup$

share|improve this answer

answered 1 hour ago

SamGibsonSamGibson

11.2k41737

answered 1 hour ago

SamGibsonSamGibson

11.2k41737

answered 1 hour ago

SamGibsonSamGibson

11.2k41737