A MOSFET question (Regarding isolated and unisolated Power converters)

Thread Starter

Newb Gaming Networks

Joined Apr 17, 2017
16
I had previously tested the circuit Fig.1 which uses a 5v regulator that is unisolated. I'm now thinking of changing the circuit over to Fig.2 and I was wondering if this new circuit works even though both sides of the circuit are now isolated. Let me know if more detail is required to answer this question.

Fig.1

Screenshot_3.png

Fig.2 (Will it still work?)
Screenshot_2.png
 
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#12

Joined Nov 30, 2010
18,167
Something is very wrong here. Pin 8 is connected to pin 15 which means your 5V converter is shorted. Meanwhile, Q1 doesn't know where ground is, so the gate signal doesn't mean anything.
 

Thread Starter

Newb Gaming Networks

Joined Apr 17, 2017
16
Something is very wrong here. Pin 8 is connected to pin 15 which means your 5V converter is shorted. Meanwhile, Q1 doesn't know where the ground is, so the gate signal doesn't mean anything.
Sorry, I didn't elaborate on this, but it's more of a simplified example. Both of those transformer symbols aren't technically transformers, they are isolated AC to DC converters where there's two input lines and two output lines. I've changed the post a tad so it makes more sense.
 
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#12

Joined Nov 30, 2010
18,167
You still have the problem that the 24 volt converter is isolated from the 5 volt converter. If pin one of the 24 volt converter has no connection to pin 1 of the 5 volt converter, the signal to the gate of Q1 creates zero voltage from gate to source. It won't work until you connect the grounds.
 

Dodgydave

Joined Jun 22, 2012
8,671
I had previously tested the circuit Fig.1 which uses a 5v regulator that is unisolated. I'm now thinking of changing the circuit over to Fig.2 and I was wondering if this new circuit works even though both sides of the circuit are now isolated. Let me know if more detail is required to answer this question.

Fig.1

View attachment 126087

Fig.2 (Will it still work?)
View attachment 126086
Don't understand why you need isolation, your first circuit in Fig 1 is ok, the microcontroller won't take any excess current so the 7805 regulator won't be getting warm, and saves on two converters, but if you want to use two separate supplies, you need to join both Negative outputs together.
 

Thread Starter

Newb Gaming Networks

Joined Apr 17, 2017
16
You still have the problem that the 24 volt converter is isolated from the 5 volt converter. If pin one of the 24 volt converter has no connection to pin 1 of the 5 volt converter, the signal to the gate of Q1 creates zero voltage from gate to source. It won't work until you connect the grounds.
Don't understand why you need isolation, your first circuit in Fig 1 is ok, the microcontroller won't take any excess current so the 7805 regulator won't be getting warm, and saves on two converters, but if you want to use two separate supplies, you need to join both Negative outputs together.
Exactly what I had thought. Isssue is that even though the first circuit works just fine, the LM7085 won't be able to handle higher voltage and or current. I do believe the 7805 has a recommended input voltage max of about 25 volts. I'm also trying to keep things compact without complicated D.C./D.C. Converter wiring and components.
 

Dodgydave

Joined Jun 22, 2012
8,671
Exactly what I had thought. Isssue is that even though the first circuit works just fine, the LM7085 won't be able to handle higher voltage and or current. I do believe the 7805 has a recommended input voltage max of about 25 volts. I'm also trying to keep things compact without complicated D.C./D.C. Converter wiring and components.
Then use a Lm317, it can take upto 37V input.


http://www.reuk.co.uk/wordpress/electric-circuit/lm317-voltage-calculator/

Use 120 ohms R1, and 360 ohms R2 resistors.
 
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