Hello all, I am working on a problem in a relay which is used for application of braking system in a vehicle. I have a brake booster for assist of braking and to which a pressure sensor is connected. The pressure sensor input and supply voltage is given to a comparator circuit (VCU). and the output of the comparator circuit is given to a relay to turn on the vacuum pump to fill the pressure loss to the booster. Now the issue is the ground pin of pressure sensor is connected to the chassis of the vehicle and the moment all the Auxiliary systems are ON, the relay of the vacuum puump will start to flicker constantly. Here the Auxiliaries are Head-lamp, Radiator fan, Wiper, Horn etc and all of the grounds are given to the chassis which are 12V systems. The input supply of pressure sensor is 5V. Can someone help me find the root cause of the problem. Thanks in advance.

 

First thing I would suggest is creating a dedicated ground for the pump / controller circuit to the battery. Chassis grounds can fluctuate enough to cause problems with something like a comparator. It may be a weak ground strap that looks right, measures right, but with enough of a load starts to limit current. Most automotive stuff can handle it and ECUs have their own ground systems to combat any problems. Another thought would be to add some hysterises to the comparator.

Edit...
Is this an add on that you created or factory installed? If factory installed I would almost guarantee a weak ground strap somewhere.

 

Thanks for the reply, Hysterises is already present into the comparator circuit. Even then the problem persist. Now only the difference is that only when all other auxiliaries are continuously ON, only then relay starts to flicker.

 

Sounds like ground bounce, as recognised in post #2. Perhaps better regulation of the comparator power supply and/or smoothing of the sensor signal would help. How rapid a response is needed from the sensor? Can you post a schematic of your setup?

 

There are two possibilities to check...
You may have a sag in your supply voltage when everything is turned on and the load from that relay is the "straw that broke the camel's back" and causes the relay to release... then reconnect.. then release... Check voltage at the battery when all are on an if you have a meter with max/min over a period of time, try that to see if your system falls below 10.5v or so.

alternatively, you may have a high resistance connection at your battery, at a ground strap (a missing ground strap). Note That current x resistance = voltage. So as your current increases when the last relay is added, you may get too much of a voltage drop across the ground strap. In this case, measure your supply voltage across the relay coil when all accessories are on as it oscillates. You may have surprisingly low voltage.

also, check for oxidation/loose negative battery cable at each end, weak alternator (voltage should be charging battery at 14.5v when running with all accessories on). Check cable connections for oxidation to alternator.