[time-nuts] Thunderbolt Power Supply Question

Bob Camp lists at rtty.us
Mon Apr 26 17:34:32 EDT 2010


One gotcha on your proposed circuit:

The -12 supply is used for the RS-232 and the DAC that controls the OCXO. I
suspect you can indeed mess things up with negative supply when the supply
gets within some magic range of the DAC output. What that range is - no


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Ed Palmer
Sent: Monday, April 26, 2010 5:25 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Thunderbolt Power Supply Question

Hi Robert,

Robert Atkinson wrote:
> Hi Ed,The better quality "industral" power supplies normally have a
regulator for each rail. Some specifications will put a minimum load
requirement on the primary supply though. 
You're right that better quality power supplies have regulation on all 
outputs.  I don't think I'm comfortable using any other kind on a 
Thunderbolt.  I was just surprised to see some 'close enough' power 
supplies in use.
> I've been looking at a simple supply for my TBolt. I wanted to avoid
switchers or multiple transformers and think I have a solution. I propose a
15V 30VA toroidal transformer (if dual secondary windings, connect in
parallel) . This feeds a full wave bridge and capacitive input filter
followed by a 12V linear regulator (your choice, I was looking at a LM317T).
It also feeds a half wave rectifier and capacitive input filter followed by
a 5V linear regulator. For the -12V there is a capacitively coupled voltage
doubler feeding another regulator. I've attached a sketch of the circuit I
used to simulate this arrangement. R1, R3 and R5 represent the 12, 5 and -5V
loads for the simulation. I think this gives a good repeatable solution
using standard parts. I've plenty of odd
>  multi-winding transformers that I could have used, but that would not
have helped others.  I hope to get it built this weekend.
I was thinking of something similar, but I've never been a fan of 
half-wave rectification.  I was thinking of running the +5 supply off 
the output of the +12 supply.  Total current for +5 and +12 is less than 
1 amp at startup and less than 0.5 amp normally.  Power dissipation in 
the 5V regulator will only be about 2 watts.  For the -12 supply, you 
could just use a max232 and pull off the -V from the capacitor.  After 
all, it's just going to be used for RS-232.

> Robert G8RPI.
> --- On Mon, 26/4/10, Ed Palmer <ed_palmer at sasktel.net> wrote:
> From: Ed Palmer <ed_palmer at sasktel.net>
> Subject: [time-nuts] Thunderbolt Power Supply Question
> To: "Time Nuts Mailing List" <time-nuts at febo.com>
> Date: Monday, 26 April, 2010, 9:02
> Many low cost triple output power supplies are designed to power digital
circuits (e.g. +5V) with maybe some RS-232 or analog circuitry (+- 12V).
Since the only 'important'  voltage is +5, that's the only voltage that's
regulated.  The others are designed to be within maybe 5% up to the rated
load.  I see various power supplies on fleabay that are sold for use with
the Thunderbolt that seem to fit this model.
> In the Thunderbolt the +12 runs the oscillator.  Won't an unregulated, but
relatively steady, +12 supply degrade the performance of the oscillator or
does the Tbolt have a built-in regulator to deal with this?
> Ed

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