Spike in power line relay

[FreshmanNewbie]

I have this circuit below.

I have an issue where, when the input turns on the transistors, I see a spike in the nearby power supply rail. I have other power supply rails such as 3.3V and 5V. Whenever, the transistor turns ON, I see a spike of say 15V peak to peak on the 3.3V rail. This causes my ICs connected to 3.3V malfunction.

However, the spike of 15V peak to peak is not happening always. The amplitude of the peak to peak spike varies by say +/-5V

Can someone let me know why this happens and what fix I can try?

 

[KlausST]

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

 

[andre_luis]

In addition...

I see a spike in the nearby power supply rail.

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.

 

[D.A.(Tony)Stewart]

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.