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Topic: how does 2N3055 work and why need a switch here
Joined: Jul 25, 2016
Posts: 10
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Hell~ all
I have some question about my project.How does 2N3055 work and why need a switch here

In a lot of power supplies the main transformer has several outputs, which are switched via a relay when the supply output exceeds a certain voltage.

Below is a Mastech HY1803 psu schematic - the SK1 relay switches the transformer outputs, while the schematic around the U2A opamp handles the switching after the output voltage exceeds 5V.

The question is - why is that switch needed? Am I understanding it right that it is just to lower the transformer output voltage when the voltage on the psu output is lower than 5V and lower the 2N3055( dropout voltage and, therefore, the power dissipation? Are there any other reasons to do that?
[Edited by Bartowski on Aug 9, 2018 at 5:12 AM]


2N3055 circuit

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Joined: Mar 28, 2005
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This is really not likely to be the right spot for this level of technical conversation. I thought most switching power supplies were used to provide different voltages to different electronic components within a piece of technology. So, 5v and 12v could be provided at the same time. There is sometimes something inside as well which can help protect the power supply in case of a short, but mostly I thought it was all about the different voltages provided. So, a standard 5v power supply would not be 'switching'......

Or, I'm completely wrong, which could be. I would ask in a more engineering specific forum.

There's also a lot on Google, with more appropriate forums...
[Edited by AV_Integrated on Aug 9, 2018 at 10:23 AM]
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