[time-nuts] Power supply for Thunderbolt

David McGaw david at endor.com
Mon Apr 21 09:10:48 EDT 2008

My Thunderbolt has a turn-on transient of as much as 750 mA on the 
+12V supply due to the OCXO oven.  Be careful not to design to the 
steady-state current.


At 06:48 AM 4/21/2008, you wrote:
>David C. Partridge wrote:
> > I'm in the process of buying a Thunderbolt, and will need to 
> provide it with
> > some volts.
> >
> > I sat down over the weekend to design a linear supply for it.  Initially
> > things looked OK using a 12-0-12 transformer, and a few IC voltage
> > regulators, but then I did some worst case design and dissipation analysis,
> > and found that I needed to use a 15-0-15 transformer to allow for a -10%
> > mains variation, and add pass transistors and lots of heat sinking to all
> > the regulators to allow for a +10% mains variation.
> >
> > Add to that transformer regulation issues (over-size transformer), the time
> > to design the PCB and the cost of getting it made, and things started to
> > look expensive :-(.
> >
> > I can probably source a linear supply on eBay, but these are typically
> > pretty large and heavy.  For example: eBay item number 330201077319
> >
> > So is it reasonable to use an SMPS, and add additional filtering to the
> > output?  Typically these talk in terms on 150mV ripple & noise which I need
> > to reduce by a factor of ten or better.
> >
> > If you have been there, done that, I'd be interested to hear what you did.
> >
> > Thanks
> > Dave Partridge
> >
>You may be able to find an OEM open frame linear supply with foldback
>current limiting that is suitable.
>Most of these (Condor, Calex,  etc) tend to use 723 regulators with
>2N3055 series pass elements.
>Alternatively since you only need 25mA @ +12V, 25mA @ -12V and 300mA
>@+5V (if I'm reading the right datasheet) you can use a much lower
>voltage output transformer suited to the 5V regulator and use a pair of
>voltage doublers or triplers to feed your 12V regulators.
>A 7.5- 0 - 7.5V transformer should suffice (if you use low dropout
>linear regulators), and the total linear regulator dissipation should
>only be a few watts in normal operation.
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