# Two little design questions

Thread Starter

#### metermannd

Joined Oct 25, 2020
191
I promise I won't do a 180 after posting... I still feel like a dunce for posting that other thread (and I'm not done sitting in the corner yet either).

Anyway, I've got two questions that have been rattling around in the vast cavern of my mind a while. Proper answers to both will permit me to finish out the power supply for my project (I have decided for simplicity's sake to use SMPS modules rather than a conventional transformer / bridge rectifiers etc.). I'm usually good at doing web searches for what I need, but...

First question - what is the proper formula for determining the rectified DC equivalent of an AC voltage? I have one section where the DC voltage is simply represented as TXVCC. Poking around in some collected notes, I find that the transformer originally used with this circuit has a separate winding for the TXVCC supply, and which is given as 35VAC, 250mA. I keep coming up with 12 volts for some reason, which is illogical in this case, as part of the circuit is already fed from the 12V bus.

Second question - proper circuit grounding. Do I run the DC ground to earth ground, or do I need to keep it isolated from the AC side, or ? I have seen a capacitor (picofarad capacitance with a voltage of 1kV or higher) used between the circuit and case grounds on SMPS units or similar in the past. Since I have mains voltage on the power supply board, I want to be sure the grounding is done right!

#### ericgibbs

Joined Jan 29, 2010
12,908
hi meter,
Check thru these PDF's
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Thread Starter

#### metermannd

Joined Oct 25, 2020
191
Thanks. Will review the attachments.

#### AnalogKid

Joined Aug 1, 2013
9,254
Without getting into transformer and rectifier losses, the energy available from a DC voltage source equals the energy available from a sinusoidal AC source at the same voltage, RMS. In terms of the heat produced in a 120 ohm resistor, 120 Vacrms and 120 Vdc (flat line, no ripple) produce the same amount of heat, 120 W. Note that 120 Vrms = 170 Vpeak = 339 Vp (peak-to-peak).

The DC voltage across a filter capacitor in a linear supply (transformer-bridge-capacitor) is theoretically 1.414 times the secondary RMS voltage. But the rull rated secondary current is not available at that voltage. This is indicated on the circuits page, right side, second from the bottom.

ak

Thread Starter

#### metermannd

Joined Oct 25, 2020
191
At this point I've worked out what voltage I need for the TXVCC supply, and that leaves the grounding question.

#### ericgibbs

Joined Jan 29, 2010
12,908
hi m,
I would not add an Earth Ground to my power supply circuitry..

Connect the Transformer body and the PSU metalwork to the incoming Mains Earth connection.
Keep the PSU circuits on their own Common internal connection, not connected to the PSU metalwork.
ie; so the PSU is 'floating' this will enable using either PSU Polarity with respect to a Common external powered project.

You could add on the front panel of the PSU an optional Mains Earthing strap, so that you could connect the PSU circuitry to Earth as required.
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Thread Starter

#### metermannd

Joined Oct 25, 2020
191
Part of why I was asking about grounding is that in my past experience with 'transformerless' power supplies, which the SMPS modules I plan to use may well be, leaves the 'ground' at some indeterminate voltage above ground potential, resulting in a VERY noticeable tingle when coming into contact.

#### ericgibbs

Joined Jan 29, 2010
12,908
hi,
Transformer-less power supplies are not applicable to the post I made, regarding Earthing.

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Thread Starter

#### metermannd

Joined Oct 25, 2020
191
I've brought the DC ground over to a pin on the incoming power connector, so if I choose, I can easily add a grounding lead to the power cord.