>=20
> Ok, so I have product that has a small 12V motor running off a 48V bus.
> It's exceptionally low duty cycle, about 5 seconds a day. And low power,
> about 500mA. Given that, I had been using a couple resistors and a zener
> diode to power it off the 48V bus. It's a little less than elegant, but i=
t's
> been working fine and fit within my space constraints.
> Recently I was asked to build a variation of it that needed a larger moto=
r.
> I'll spare you the gritty details, but eventually I found a suitable moto=
r
> and shoehorned it into position. It was also rated 12V so I gave no thoug=
ht
> to powering it.
>=20
> Now the face-palm moment - it needs more current (duh).
>=20
> The kicker is it needs at least 2A to start and drops back down to about =
1A
> when running. Attempting to alter the resistor values to accommodate the =
1A
> run current leaves it with a very anemic startup (won't start under load)
> and adjusting the resistors to allow 2A for startup results in excessive
> speed or very unhappy zeners.
>=20
> The simple fix would be bigger zeners, but I can't seem to find anything
> bigger than 5 watt rated parts.
>=20
> The elegant fix would be to lay out a small buck regulator, time is not m=
y
> friend. This needs to ship before I could have boards made.

I would 'just' add a plastic power transistor emitter follower (or TO-3 2N3=
055) to the zener regulator. Then you could increase the resistor value to =
allow the transistor to be capable of (say) around 3A which would give you =
some margin without the zener taking heaps of current. You could probably a=
llow the supply to sag to 10-11V at 3A.


--=20
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