Back Feeding Voltage problem
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If you all recall, I was using a ULN2803 as a driver for my project. The
2803 sinks the output to ground so relays, lamps, or other device can be
operated. Everything seems to work pretty good but I did find a bug.
I installed an LED from the 24 volt positive rail, thru a 2k resistor then
to the trace going to the output of the 2803.
When the 2803 turns on a channel, the LED lights and the relay or whatever
is activated.
The problem is, when I interfaced to another TTL Device, which has it's
input pulled up to 5vdc, it causes my LED to light. Other than that it
seems to work on the TTL Device but I sure don't like the looks of it. When
the 2803 sinks that output, the LED Lights Bright like it should and the
other device's TTL Input is pulled to ground.
I noticed when the 2803 does not have it's ouput sunk to ground there is 24
volts on it.....
24 VDC
TB1
|
2k
LED
|
Stamp --- 1k
2803
TB2
Freewheel Diodes tied to 24 +
Stamp Ground, 2803 Ground, and 24 Neg tied together.
I take it a diode would solve the problem like below....
24 VDC
TB1
|
2k
LED
|
Stamp --- 1k
2803
|<----TB2
Wouldn't the diode fix "Both" problems? The 24 volts leaking out TB2, and
the LED backfeeding thru TB2 with different voltage levels?
On this set of boards, I didn't leave a place for a diode so I may be forced
to use relays to interface to the other TTL devices to be safe. Since it's
only a couple it shouldn't be a problem. If the diode fixes it I will put
them in the next set of boards I make.
2803 sinks the output to ground so relays, lamps, or other device can be
operated. Everything seems to work pretty good but I did find a bug.
I installed an LED from the 24 volt positive rail, thru a 2k resistor then
to the trace going to the output of the 2803.
When the 2803 turns on a channel, the LED lights and the relay or whatever
is activated.
The problem is, when I interfaced to another TTL Device, which has it's
input pulled up to 5vdc, it causes my LED to light. Other than that it
seems to work on the TTL Device but I sure don't like the looks of it. When
the 2803 sinks that output, the LED Lights Bright like it should and the
other device's TTL Input is pulled to ground.
I noticed when the 2803 does not have it's ouput sunk to ground there is 24
volts on it.....
24 VDC
TB1
|
2k
LED
|
Stamp --- 1k
2803
TB2
Freewheel Diodes tied to 24 +
Stamp Ground, 2803 Ground, and 24 Neg tied together.
I take it a diode would solve the problem like below....
24 VDC
TB1
|
2k
LED
|
Stamp --- 1k
2803
|<----TB2
Wouldn't the diode fix "Both" problems? The 24 volts leaking out TB2, and
the LED backfeeding thru TB2 with different voltage levels?
On this set of boards, I didn't leave a place for a diode so I may be forced
to use relays to interface to the other TTL devices to be safe. Since it's
only a couple it shouldn't be a problem. If the diode fixes it I will put
them in the next set of boards I make.