In article ,
R.M wrote:
Well I built it and it seems work exactly like I want expect for one
thing. Please excuse the terminology error here, but I'm going to refer
to the part of the lm317 that you attach to a heat sink as the "top". I
built my transmitter inside an ammo can with a BNC-BNC bulkhead
connecter on the top of the can so I'm have an external antenna. I also
figured that the ammo can would be perfect to affix the lm317 to for
heat distribution. You may already see the problem. Basically when I
installed bulk head BNC connecter to the ammo can it grounded the entire
ammo can. The problem is, the "top" of the lm317 that you attach the
heat sink to has positive voltage on it. Its the same voltage as the
output pin on the lm317. Is this normal?
Yes, this is normal for an LM317 in a TO-220 package. The heatsink
tab is connected, internally, to the Vout pin.
So when I connected the lm317
to the ammo can(not knowing the problem) its created a short and the
lm317 really really hot.
Yup. The LM317 will have put itself into its internal
current-limiting mode, and might eventually go into thermal-limit as
well.
You're fortunate that the LM317 has such a self-protection circuit.
If you were using a regulator with a separate, external pass
transistor, and hadn't included current-limiting safe-operating-
area protection components, the pass transistor would probably have
blown out in a fraction of a second.
I just assumed that the "top" of the lm317
would work if it was grounded, but l was wrong. Am I missing something
or is this normal. As long as I either unhook the lm317 or the bulkhead
BNC connector from the can everything works great. Any comments.
Un-ground it. You can either use a separate heat-sink (one of the
little finned types would be best) which is isolated from ground, or
you can mount the LM317 to the ammo can using components which
transmit heat, but not electricity. A nylon screw, and a washer with
a raised inner rim are often used for the mounting; the electrical
insulation can be provided using a thin "Sil-pad" silicone pad, or the
traditional "thin mica insulator, with a very very thin coating of
silicone heat-sink grease on either side" arrangement.
--
Dave Platt AE6EO
Hosting the Jade Warrior home page:
http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!