[time-nuts] Parallel voltage regulators

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Oct 26 09:40:59 EDT 2007

Don Collie wrote:
>> Bruce
>> I don`t think the higher current created when a fuse is used instead of 
>> near-instantaneous current limiting is "alleged", but rather a real problem 
>> that can cause damage further down the line. Fast acting current limiting 
>> is preferable to all but the fastest fuses that are designed to protect 
>> semiconductors. Current limiting plus thermal shutdown in the regulator
> will protect both load, and regulator [and resovior capacitor, and diodes, 
> and transformer] Commonly available fuses won`t give much protection to the 
> load - especially the delay types often necessary with large filter 
> capacitors.
>     A precision, proven, high performance, low noise regulator like the 723 
> using an external pass transistor [or preferably a darlington], to avoid 
> chip heating, and a well bypassed reference would be a lovely solution.
> Cheers!,............................................Don C.
One can always use more than one fuse (one in the mains wiring and
another between the reservoir caps and the regulator input), only one of
which needs to be a slow blow type.
An LM350 has a more tightly specified current limit than earlier 3
terminal regulators.

Powering a load from an unregulated supply as suggested also offers
little load protection, however if the load isnt faulty then close
current limiting wont be necessary.
However it is always useful when testing to have close control of the
short circuit load current if at all possible.
If this is required then a 3 terminal regulator by itself will not suffice.
However a low power small signal npn transistor to sense the voltage
drop across a current sensing resistor in the load return lead can be
used to pull down the regulator adjust terminal when the transistor
turns on limiting the maximum load voltage to around 1.2V in this
situation which will limit the current in most loads but not in a short
circuit. The only way to add control of the regulator short circuit
current being to use an electronic current limiter in series with the
regulator input, however this increases the regulator dropout voltage
significantly as well as the circuit complexity.

The only problem with the 723 for a beginner is they are much more
complicated to wire up and too easy to destroy when probing if care isnt
Also with modern high gain transistors a darlington pass element isnt
really necessary for a load current of a couple of amps.
The commercial open frame linear supplies  dont use them even with a 3A
load current.


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