Center tapped full wave rectifier help needed

Thread Starter

Barrakkuda

Joined Mar 29, 2018
4
Hello, I'm trying to build a power supply for a TRS-80 Model 1 computer. I'm a novice in electronics so I'm using the schematics I found on a TRS-80 website. I'm confused about the output DC voltage the schematics show compared to what I'm actually getting.

The schematics show that with a 25.2V CT @2.0A transformer and a full wave rectifier I should be getting 18.5 VDC across pins 2 and 4.
What I'm actually getting is around 12.6 VDC. I'm using the exact same components listed on the parts list.

Please help me understand why I'm not getting the 18.5VDC. Maybe the schematics are wrong?

Thanks!

 

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LesJones

Joined Jan 8, 2017
3,523
There are no smoothing capacitors on the output of the rectifier so you only see the average voltage rather than close to the peak voltage. If you connect it to the computer you will probably find that the voltage goes up to close to 18 volts.

Les.
 

Thread Starter

Barrakkuda

Joined Mar 29, 2018
4
There are no smoothing capacitors on the output of the rectifier so you only see the average voltage rather than close to the peak voltage. If you connect it to the computer you will probably find that the voltage goes up to close to 18 volts.

Les.
Great!
I confirmed this is true by putting a capacitor across pins 2 and 4 for testing, now I get 19 VDC, which is fine.
Thanks for your reply.
 
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