Does anyone here know where I can find details on resistors marked with a single color band?
I have a board in here for repair. The board is the front control panel for a CNC-router type of machine. The manufacturer provides a very basic schematic only -- no part numbers or component values identified. The schematic matches the board in terms of components present and their types, but I am somewhat confused by several resistors used on this board.
These resistors are identified on the schematic and on the board as "R" components and on the schematic with the standard (US) zig-zag symbol for resistors. The component reference numbers for them are R1, R3, and R10. I can't seem to get a realistic reisistance reading through any of these resistors. All of these three are identical in appearance, light tan in color, cylindrical (like many precision resistors -- no "bumps" on ends) with axial leads, and the size of a 1/4-watt carbon resistor. Each of these has a single black band around its longitudinal center.
In one instance -- R3 -- the resistor is connected between Pin 16 (1Y2) of a DIP 74LS244 and the cathode side of an LED -- the anode of the LED is connected to the +5V rail through a 220-ohm resistor (actually through one leg of a bussed SIP resistor pack). In another instance -- R1 -- the resistor is connected to the +5V rail on one end, and to one pin of a board interconnect at the other end.
The 74LS244 is used to control indicator LED's based on input from the controller mainboard. Pin 18 (1Y1) goes directly to the cathode of one of the (other) LED's with no series resistor between the IC and the LED. That LED's anode is also tied back to +5V via one leg of the resistor pack mentioned above.
I've described the above connections so as to help make some sense of the weird resistance measurements that I'm getting. As this board is on the bench and is not connected to any power supply when measuring, for all intents and purposes, I should be able to get a usable measurement of the resistance through R1 -- one leg of it is open with respect to the remainder of this board! However, when measuring these resistors with either a digital or analog meter, they act like thermistors! Measured resistance through these resistors will start out at about 0.9 ohms, and then rapidly drops to zero.
Based on the information presented here, is it likely that these are actually some low-value thermistors? And if so, why would they be used in such a circuit? Does any of this make any sense?
I would ask my boss, but he just left on vacation...
I have a board in here for repair. The board is the front control panel for a CNC-router type of machine. The manufacturer provides a very basic schematic only -- no part numbers or component values identified. The schematic matches the board in terms of components present and their types, but I am somewhat confused by several resistors used on this board.
These resistors are identified on the schematic and on the board as "R" components and on the schematic with the standard (US) zig-zag symbol for resistors. The component reference numbers for them are R1, R3, and R10. I can't seem to get a realistic reisistance reading through any of these resistors. All of these three are identical in appearance, light tan in color, cylindrical (like many precision resistors -- no "bumps" on ends) with axial leads, and the size of a 1/4-watt carbon resistor. Each of these has a single black band around its longitudinal center.
In one instance -- R3 -- the resistor is connected between Pin 16 (1Y2) of a DIP 74LS244 and the cathode side of an LED -- the anode of the LED is connected to the +5V rail through a 220-ohm resistor (actually through one leg of a bussed SIP resistor pack). In another instance -- R1 -- the resistor is connected to the +5V rail on one end, and to one pin of a board interconnect at the other end.
The 74LS244 is used to control indicator LED's based on input from the controller mainboard. Pin 18 (1Y1) goes directly to the cathode of one of the (other) LED's with no series resistor between the IC and the LED. That LED's anode is also tied back to +5V via one leg of the resistor pack mentioned above.
I've described the above connections so as to help make some sense of the weird resistance measurements that I'm getting. As this board is on the bench and is not connected to any power supply when measuring, for all intents and purposes, I should be able to get a usable measurement of the resistance through R1 -- one leg of it is open with respect to the remainder of this board! However, when measuring these resistors with either a digital or analog meter, they act like thermistors! Measured resistance through these resistors will start out at about 0.9 ohms, and then rapidly drops to zero.
Based on the information presented here, is it likely that these are actually some low-value thermistors? And if so, why would they be used in such a circuit? Does any of this make any sense?
I would ask my boss, but he just left on vacation...
