[time-nuts] Power supply for Thunderbolt
bruce.griffiths at xtra.co.nz
Mon Apr 21 06:48:42 EDT 2008
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.
> 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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