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Old 11-10-2019, 09:41 PM   #1
EasyGoing1
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Default Substituting PNP with NPN (schematics provided)

Hello,

Because I only have NPN power transistors, I want to replace the PNP power transistors in this circuit



Like so ...



Will this work?
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Old 11-10-2019, 11:53 PM   #2
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Default Re: Substituting PNP with NPN (schematics provided)

Not going to work. What does your simulation show as you vary the load or the feeding Voltage?
Best is to build it and find out what you will get.
Try this and see if it will work: http://www.reuk.co.uk/wordpress/elec...-power-supply/
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Old 11-10-2019, 11:58 PM   #3
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Default Re: Substituting PNP with NPN (schematics provided)

No, it won't work. You will need to use a different structure which introduces some regulation error or another structure that requires a smaller low current PNP transistor.

Using standard emitter follower structure:
http://www.bristolwatch.com/ele/basi...r_supplies.htm

Using small PNP along with NPN:
http://www.rason.org/Projects/vreg/vreg.pdf

If you only have those TIP120s and insist on using them, you're best off using a LM723 regulator instead of a LM78xx as the main regulator.
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Old 11-11-2019, 01:00 AM   #4
EasyGoing1
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Default Re: Substituting PNP with NPN (schematics provided)

Quote:
Originally Posted by budm View Post
Not going to work. What does your simulation show as you vary the load or the feeding Voltage?
Best is to build it and find out what you will get.
Try this and see if it will work: http://www.reuk.co.uk/wordpress/elec...-power-supply/
I need 12 volts with a consistent 2 amps ...
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Old 11-11-2019, 01:24 AM   #5
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Default Re: Substituting PNP with NPN (schematics provided)

How tight regulation do you need is another question...

I recently found out that my netbook's charger needs to be 12V. If it gets to 13.8V, it will appear to charge but not...
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Old 11-14-2019, 01:13 PM   #6
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Default Re: Substituting PNP with NPN (schematics provided)

Quote:
Originally Posted by EasyGoing1 View Post
I need 12 volts with a consistent 2 amps ...
Is there a reason you're not just "outsourcing" the design, manufacture and UL certification of the power supply to someone who makes 12V 2A wall warts?

(There's a reason SO MANY devices are sold with wall warts and bricks -- actually, SEVERAL reasons... )

For example, I just rescued a power supply for CCTV cameras (feeds 24 devices). Cracked it open to find a COTS "brick" wired to a distribution board (fuses and connectors) that ultimately feeds the 24 cameras. When/if the "power supply" sh*ts the bed, I won't be running around looking for a replacement of some proprietary circuit board made by who-knows-who; nor will I worry about troubleshooting the fault! I'll just pick up another similar capacity "brick", cut off the barrel connector on the output, wire it to the distribution board and put it in the case in place of the "dead" brick!

Last edited by Curious.George; 11-14-2019 at 01:19 PM..
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