need help of 5 V realy getting stuck.

[julybrownie]

This is the schematic for a adapter boards which I use to drive the two motor .
Design looks to be working but following 2 issues. For 1 I had workaround but for 2 I am not sure how to get out of it as it looks to noise or transistor not switching properly.
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1. When U is 120V and "OD" On sheet one is grounded U21 get triggered and LS1 is chattering.( looks like relay switch is closing and opening instead of
staying close)

Voltage across R140 is 4.9V ,Voltage across R125 is 3.4 V.

I checked both Q1 ,D1 VCC all looks correct bit still LS1 is continues

chattering. While it is doing that I connected Pin 1 of LS1 to ground and chattering

stops.

-===> To get of this issue i removed R171 and it started working.Looks Like relay is not getting required voltage.

Please suggest if this will have any impact on and what is the good way to solve it.

I guess we need to adjust the resistor values also to avoid any such conditions.

Relay is expecting 3.5 volt minimum and I think it is getting only 3.8 bolt.

I want to boost it up to 4 and above.

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2.After the above change it work but started failing after 24 hr of Run.

most of the time LS6 gets stuck and sometime LS9 get stuck.

I check the power and looks like board is using around 500ma of current when motor is running.

 

[BrianE22]

For 2) above, if there is not enough voltage across the coil there may not be enough force on the NO contacts when the relay is on. This could cause arcing on the contacts and eventual welding of the contacts. Look closely at the closed contacts when running and see if there is any sparking between the contacts.

Are you cycling the contacts on/off or just keeping them on for your 24 hour test? If you are cycling, then maybe there's an inductive spike across the contacts on opening. Is there a spark across the contacts when opening? If so you'll need some sort of transient suppression across the contacts.

The diode across the coil will cause a slow release of the contacts. If you are cycling the contacts and they are failing then you might need to alter your coil spike suppression. There are ways to protect against the coil spike and allow faster decaying of the coil current.

I don't know how many of the Electrical Engineers monitor this forum (Motion Control). You might get more responses if you post in the Circuit Design area.

 

[LiteYear]

Is U 120V AC or DC? If it's DC then the voltage divider provides at least 5V and 5mA to U21, so I can't see why it wouldn't be hard on.

The voltage across R125 is suspiciously low. It suggests the 74HCT14 is being pushed too hard and can't maintain a high output level. Maybe then the 3.4mA into the base of Q1 means it can't provide the necessary collector current to reliably operate LS1. What are the current specs of LS1?

Maybe you could increase R125 and use a Darlington transistor in place of Q1? That would take the load off the driving circuit.

As the 74HCT14 gets hot its sourcing current falls even further. Could be why it stops working. Or it might be something else entirely!

 

[julybrownie]

Thanks Brian and lite for looking into it.
Yes i am cycling the relay continuously.I had not observed any sparks but i will recheck it.
Lite, U is 120V AC.
All the relay in the ckt is V23105 series relay.-V23105-A5501-A201.
I will check 74HCT14 also and change on boards to see if it works or not.

 

[LiteYear]

120V AC? Well then isn't your relay chattering because the control voltage is cycling through zero? I'd expect U21 to be off for about 15% of the time with a 120Vac source. I'm confused. Might be best to ignore me.