[time-nuts] Regulator choices

ed breya eb at telight.com
Sun Jun 30 14:47:13 EDT 2013


I believe the original problem was that the raw unregulated voltage 
may be marginally too high for a conventional three-terminal to take 
safely. I have often encountered this problem, which is due to the 
wide input range possible considering the worst-case line voltage 
tolerance, transformer regulation, transformer selection limits, and 
possible surge voltages. If you drop the voltage with extra stages or 
series devices, you may run out of headroom, but if you don't, then 
it may run dangerously close to the maximum input rating of the 
regulator. If adding to, or reusing existing power circuits, there's 
often already some degree of protection from MOVs or gas-tubes, but 
these are very coarse, so are unlikely to be effective.

Whenever I run a three-terminal regulator from raw DC, I put in a 
series fuse or PTC, and an over-voltage clamp ahead of it to assure 
that the input rating will never be exceeded. Also, the load may need 
to be protected - it depends on its characteristics. If the raw 
voltage is too high, the first step is to add some series diodes that 
can drop it some - you just have to make sure there's enough 
remaining headroom at lowest line and maximum load, etc conditions. 
Be sure to bypass the regulator input with as much extra C as 
possible to stabilize it, and provide more filtering. In the other 
extreme, when high-line occurs, there should be a comfortable 
distance from the raw voltage to the input rating. Then, only 
transient protection should be needed - heavy zeners or transorb type 
devices should be enough. If facilities are available, it's best to 
do this design part empirically, with actual parts, variac, and curve 
tracer - you can put real conditions on the real stuff. I also always 
add the usual reverse-protection diode across (O-I) the regulator, 
even if this fault seems unlikely - I can't count the number of times 
I've accidentally shorted out the regulator input nodes during design 
and construction - this would normally take out the regulator if it 
has lots of output C (almost always).

If the peak at high-line is too close to the input rating, then I use 
an amplified zener clamp, with a big bipolar transistor driven by a 
big zener of the proper voltage. Alternatively, a traditional SCR 
crowbar circuit could be used, but I prefer a more transparent, 
self-resetting solution. Depending on the particular application, my 
favorite, and the most durable version is the "substrate" style, 
where the supply is referenced to a chassis ground or board ground 
plane. For example, to protect a positive supply, a large PNP 
transistor is anchored directly to the chassis - I'm talking about 
using only power devices, like TO-220 and larger - with no isolation, 
to provide maximum cooling from the collector tab into the substrate 
(the collector lead should go to the supply common too). The emitter 
then goes to the +supply, and the zener (typically 1W size) goes from 
the base (K) to ground (A). A small resistance around a couple of 
hundred ohms from B-E will make sure it doesn't turn on unless 
needed. For a negative supply, an NPN would be used, etc. If the 
supply common is not the substrate, then the same circuit topology 
can be used, except that galvanic isolation from the transistor tab 
to the substrate will be needed. Cooling won't be quite as good, but 
since the circuit floats, either type transistor can be used for any 
polarity. This same type of circuit can of course be used to protect 
the load side - this is even easier since the voltage is usually 
fixed, and the regulator (even if failed) adds to the series 
impedance limiting the fault current. I also put a large 
reverse-protection clamp diode on any supply that is exposed to the 
outside world, or is within a multiple-supply environment, where a 
fault between any two is possible  - during design or testing too, as 
above, even if unlikely in operation.

The transistor SOA rating should be sufficient to trim the peaks 
during possible high-line and transient conditions, and clear the 
series fuse if necessary, depending on the situation. Bigger is 
better for this purpose.

The degree of protection and complexity, of course, depend on the 
criticality of failure, and value of the load. Not much is needed for 
routine or low power circuits, but for very important stuff, these 
things can make it nearly indestructible from the powering perspective.

Ed

 >>Bob Stewart wrote:

I also wanted to reduce the amount of power wasted through passive 
devices.  As it turned out, though, I had more tolerance for heat 
waste than I had thought.  But, the general discussion this has 
become is also good.

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