Re: Resistor in Power Supply Input
- From: ehsjr <ehsjr@xxxxxxxxxxxxxxxx>
- Date: Tue, 19 Dec 2006 00:12:31 GMT
Jan Panteltje wrote:
On a sunny day (18 Dec 2006 08:55:18 -0800) it happened "Studio271"
<studio271@xxxxxxxxx> wrote in
<1166460918.256295.296910@xxxxxxxxxxxxxxxxxxxxxxxxxxx>:
I (had) a broken Roland TR-707 drum machine, and found the problem to
be that the very first component in the power supply input had gone bad
(infinite resistance) and was keeping the input 12VDC from getting
anywhere else in the circuit.
clip of the power supply schematic:
http://users4.ev1.net/~jaaws/tr707ps.jpg
The physical resistor is labeled:
"FN192R7J"
"MICRON 53"
next to perfboard for size comparison:
http://users4.ev1.net/~jaaws/tr707res.jpg
It is a 2.7 Ohm wire wound resistor.
Its function is very likely to limit in rush current to C1 (470uF).
The 7805 has fold back current limiting.
No idea of the current, but it is no ta whole lot I think.
So if <1A (7805) and 2.7Ohm it should be able to handle 1 x 1 x 2.7 = 2.7 W.
(i x i x R).
So a 3 W 2.7 Ohm wire wound would do it I hope.
3 watts is too low - no safety margin to speak of.
Go for at least 5 watts.
Ed
.
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