Spike in power line relay

Hi,

There are many issues:
* I see junction dots ... but is the "non junction dot" between the two diodes intentionally?
* what is the function of the bottom diode?
* did you calculate base current and collector current for "switching conditions"?
* My old saying: A circuit without a capacitor is no relibly working circuit.
* if you experience a "spike" on the power supply rail .. then why don´t you just add the "recommended" power supply decoupling capacitor.
* you say you see the spike also on other power supplies. You should be aware that these power supplies are not used at all in your circuit. So it´s impossible that the power supplies can be modified. You don´t ask yourself "why do I see it though?" .... then you could investigate "What do they have in common?"
You should be aware of the answer: "They all have a common GROUND".

So the problem - most probably - is NOT the power supplies, but a crappy GND.
--> I bet your circuit is not on a PCB with a solid GND plane (that´s what I have recommended very often).

So rather old and continous problems:
Not following the very basics of circuit design:
* using a slind GND
* using bypass capacitors on power supplies
...
* and still not doing basic calculations (I expect the BJT getting hot)

****
Prove me wrong.

Klaus

In addition...

I see a spike in the nearby power supply rail.

Click to expand...

You did not mention the load on the relay contact side; whether it is inductive ( motor, solenoid ) or not. If so, and depending on how close and long are both these traks crosstalking ( power bus / relay contact ), there will be indeed a huge interference. You could reduce the effect of this, at least partially, by adding a snubber in parallel with the relay coil.

Turning on the transistor is not this problem but it is weak. CHANGE 2.2k to 100k.
Turning on the coil is not a problem.
The only problem is the contracts switching a high C / low ESR load for conductive and radiated interference.