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Series pass transistor, LM7812

Driving 3 LEDs at 1000mA. Is this circuit / driver ok?48V DC Power Supply Using LM7812 in SeriesHow to test and rate a home-built low dropout regulator?Better to regulate voltage by supplying or subtracting base current to BJT pass transistor?Single Transistor Voltage and Current RegulatorNeed alternative way to deal with LDOs minimum current requirementWhat causes a faulty Linear Voltage regulator to output wrong voltageHigh voltage linear regulator (with pre-regulator)MOSFET-switching between two power supplys: how to avoid transverse current?Lm7812 circuit help

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
  1 hour 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
  1 hour 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