I have four 6V, ~80mA Relays mounted on a Perfboard which are being controlled by the 2N2222 Transistors.
The basic problem is that after mounting everything the diodes can't fit in easily.
Is the Flyback Diode really necessary?
Electronic – Relay’s Flyback Diode – Is it necessary
diodesrelay
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From your question, I can't really figure out how your circuit works. But I can answer the question if you can use transistors as diodes with "yes".
One example where this is often done in discrete (non-integrated) circuits is when you want to have a diode with a very low leakage current: A diode-connected transistor (C and B terminals shorted together in order to use its B-E-diode) has much lower leakage than most regular diodes.
(Source: Linear Technology App'Note 43 by Jim Williams)
Just make sure you pay attention to the absolute maximum ratings of your transistors for the "blocking direction": The maximum allowed negative B-E-voltage is quite low compared to diodes.
Consider the operation of the circuit.
When the transistor is on current is flowing in the coil from top to bottom as the circuit is drawn we now switch the transistor off. The current in the coil still wants to flow.
For the circuit on the left this current can now flow back to Vcc via the diode the voltage across the coil has reversed direction and is limited by the diode the current can decay to zero safely.
For the circuit on the right the diode does not help. The current flowing in the coil will force the voltage on the collector to rise to the point where the transistor (or possibly the diode) breaks down and starts to conduct. At this point the current can start to decay in the coil but the energy in the broke down transistor (or less likely diode) will be excessive and may well result in the transistors death. Note a zener diode here will work because you allow the voltage on the coil to reverse so the current can decay to zero while limiting the voltage across the transistor to a safe value.
It should be noted the allowing the voltage across the coil to reverse to an higher voltage means the current can decay more quickly which is why you sometimes see a zener in the right hand circuit or more than one diode in series in the left hand one.
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Best Answer
-- EDIT --
If you can figure out a way to ramp the inductor's current down bit by bit without using PWM, with a MOSFET with a sufficiently high
currentblocking voltage rating you may be able to get away without the flyback diode. However this would require you to control this ramping precise enough not to ramp its current down too quickly and the turn down time can be very unbearable.-- ORIGINAL POST --
TL; DR: It is necessary - it is critical you can say. It makes your circuit last longer.
Theory: Flyback diode.
Extended ELI5 answer:
Relays are switches controlled by an electromagnet, and electromagnet is essentially a BFI - big f***ing inductor. Inductors do not allow the current through it change rapidly. The bigger the inductance it have, the harder it will fight to keep its current constant, drawing energy it stored in the form of its magnetic field.
simulate this circuit – Schematic created using CircuitLab
Consider we are using a mechanical switch to control this relay. When it is constantly kept powered, it is saturated and act simply as a small resistor, passing, let's say, 500mA of current.
simulate this circuit
Now switch it off. An opened switch poses several GΩ of resistance in the circuit, being essentially an air gap. However the current across the inductor cannot drop immediately, and to keep this several amps of current flowing the inductor from changing the inductor induces enough voltage to break the air down into plasma across the open switch contacts and force the current to arc across. That is several kV (!!) of voltage and a fire hazard. (!!!)
simulate this circuit
Now let's say we have a transistor instead of a mechanical switch. I am using my personal favourite
2N7000N-MOSFET here. With the control driven high, the coil is in a similar saturated state acting as a small drain resistor.simulate this circuit
When the gate is suddenly grounded the similar situation will happen across the MOSFET's drain and source pins, easily break the transistor down to conduct that induced current. Such a voltage pike will take out a signal transistor after several rounds of switch-off spikes if not outright. For BJT's like
2N2222the situation is even worse as one single Zener zap and your transistor is gone.simulate this circuit
Now we have a diode reverse parallel with the coil there. I used
1N5819Schottky which allows faster switching and better suited with motors that need PWM control, for relays slower1N4007can be used but it have a higher forward voltage. The same start situation, coil saturated. In this state, the diode is reverse biased and do not affect the circuit.simulate this circuit
The instant the transistor is off, the current begin to flow through the diode. The transistor will only see a drain-source voltage of at most a few volts higher than the supply voltage.
This is an extended explanation why you need a diode antiparallel across the coil. It keeps your circuit from being fried by an inductive voltage spike from the coil.