# what's the difference with or without the resistor connected to ground

Discussion in 'Homework Help' started by kisnasin, Jul 14, 2014.

1. ### kisnasin Thread Starter New Member

Jul 14, 2014
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0
Hi, I am a new member here. I have come across a problem regarding a logic with a resistor connected to the ground, and one without this resistor and ground. It operates normally with this resistor connected to ground. However the one without has problem operating normally. Attached is the circuit diagram with the 100ohm resistor. May I ask why it is needed there so that it operates normally? Have been trying to figure this out but I am not sure why and the difference.

Many thanks!

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2. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,575
389
hi,
When the Control Panel switch is closed, 28V is switched either thru the Relay coil or the 100R.

Pins 4 and 5 of the Logic Unit will also have 28V applied when the Control switch is closed.

When switch [1,2,3 pins] is closed OR open there will still be 28V applied to pins 4,5,6/

IF the 100R was not in circuit pins 4,5,6 would be 'floating' ie: open circuit when the Control Panel switch is open and pins 1,2,3 switch is open.

Its possible that the Logic Unit will not tolerate a floating input pins, 4,5.

E

3. ### kisnasin Thread Starter New Member

Jul 14, 2014
3
0
Thanks ericgibbs! I understand the first few points. Could you explain more about "floating" you have mentioned? does it mean without the 100R & grounding, when the switch is ON, 28Vdc flows into Pin 2, then...... (sorry I am not sure here). can you explain further about floating? Many thanks!

4. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,575
389
hi,
On high impedance logic inputs eg: CMOS if an input is not connected, it can assume a high or low state or anywhere in between, so the following circuitry will malfuction.

The term 'floating' best describes an input that can change in state because its not connected.
On your circuit the switch contact is either to the relay coil or the 100R, so pins 4,5 are never left disconnected.

E

5. ### djsfantasi AAC Fanatic!

Apr 11, 2010
2,899
870
When a pin is not connected to anything, it is said to be floating. I visualize it as just "floating" in the air. When not connected, the input value is indeterminate and can potentially (pun intended) change on its own, causing circuit malfunction.

6. ### KJ6EAD Senior Member

Apr 30, 2011
1,426
363
Research the terms "pull up resistor" and "pull down resistor".

100Ω is a strong pull down. I'd suggest using 4.7kΩ or 10kΩ.

7. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
428
since the leads to the relay are close to other wires carrying 120 vac and 24 vac, if not loaded by the resistor, induced currents could produce enough voltage to fool a high impedance input into a high.

8. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,575
389
hi KJ,
Not knowing enough detail about the OP's circuitry, I am wondering if the relay coil is also around 100R.?
It maybe a case that the 28Vdc needs to have a 'minimum' load in order to hold it around 28Vdc, which the Logic see's??

Eric

9. ### JoeJester AAC Fanatic!

Apr 26, 2005
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1,211
There was a time in electronics when resistors were used on power supplies for "regulation" as well as discharging the power supply capacitors.

Eric is probably correct considering the 28 volt supply.

10. ### kisnasin Thread Starter New Member

Jul 14, 2014
3
0
Thanks guys! This is really good explanation, so that 100ohm resistor is a pull down resistor that makes sure signal at Pins 4, 5, 6 does not "float".

When it is switched on, without the pull down resistor, Pin 1 is not connected, so it could cause the unstable signal at Pins 4, 5, resulting malfunctioning of the logic unit. So does it have to do with making sure higher current to Pins 1, 2, 3 and lower current to pins 4, 5 because of this high impedance logic input? I am not an electric guy so really appreciate your explanations!