# Newbie trying to understand a circuit

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#### Richardcavell

##### Member
Hello. I am an electronics beginner. I would like to understand a circuit that is in a book from the 1980s entitled "Computer Controlled Robots", published by Usborne. May I ask here?

1. The book does not specify the exact model of relay to be used, merely that it should be within certain specifications. The designer appears to have assumed that no resistor is necessary in series with the relay coil. Is this a valid assumption?
2. Is there a special name given to the diode?
3. My understanding is that the diode wastes any current that is induced by the relay coil after the transistor is switched off. Is that correct?
4. Is there a special name for the 6.8 kohm resistor? What is its function? Is it necessary?
5. If I read the book "The Art of Electronics", will all this become clear to me? (Heh, heh. I've had that book recommended, but the investment is not trivial.)

Thanks,

Richard

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#### ronsimpson

##### Well-Known Member
no resistor is necessary in series with the relay coil.
true....assuming a 12V supply and a 12V relay.....no resistor
name given to the diode?
Flyback diode or snubber.
diode wastes any current that is induced by the relay coil after the transistor is switched off.
Yes. I would not use the word "wastes" but ok. The inductance of the relay is like a rubber band. Pulling back on the rubber band (pulling the relay across 12 volts) stores energy. Releasing the relay causes the voltage to jump up just like the rubber band wants to shoot across the room. The diode clamps the voltage to +0.6 volts above the supply. Much like shooting the rubber band into the wall, (not across the room).
Is there a special name for the 6.8 kohm resistor?
Name? Well the computer has a output of 0 or 4 volts. (could be 0 and 5V or 0 and 3.3V) depends on what computer.
The transistor operates at 0 and 0.6V. At 0 volts and right up to about 0.55 on the base the transistor is off. There is no base current there fore there is no collector current. At about 0.6V the base-emitter junction turns on (just like a diode). The voltage is clamped to about 0.6V. Maybe 0.7 at high current. Base current causes collector to happen.
The computer wants to pull up to 4V. The transistor will try to hold the B-E to 0.6V. (conflict) A current limiting resistor (2.2k) limits the base current and allows the computer to pull to 4V.
The 6.8k resistor is not absolutely necessary but..... If there is a long wire from transistor to computer noise will get on the wire. Maybe enough to turn on the transistor. This resistor will eat up that noise energy. Also if the computer is unplugged and the base wire is left unconnected the noise might turn on the transistor. (long wire = antenna)

#### OBW0549

##### Active Member
1. The book does not specify the exact model of relay to be used, merely that it should be within certain specifications. The designer appears to have assumed that no resistor is necessary in series with the relay coil. Is this a valid assumption?

A relay coil has its own internal resistance due to the size and length of the wire in the coil; the relay manufacturer designs the relay so this resistance provides the proper operating current at the relay's intended coil operating voltage (5V, 12V, 24V, etc.). So yes, it's a valid assumption-- unless your supply voltage is higher than the relay's coil rating, in which case you'll have to add external resistance.

2. Is there a special name given to the diode?

I've always referred to it as a "catch diode."

3. My understanding is that the diode wastes any current that is induced by the relay coil after the transistor is switched off. Is that correct?

More or less, although it doesn't "waste" the current but rather provides a path for it after the transistor is switched off and until the energy stored in the relay coil's inductance is eventually dissipated.

4. Is there a special name for the 6.8 kohm resistor? What is its function? Is it necessary?

I've never seen a name for it, but it's mainly there to ensure the transistor can turn off by providing a path for any collector-base leakage current. It may be necessary (for instance, if the device is being operated in a high-temperature environment) or it may not; it's there as cheap insurance.

5. If I read the book "The Art of Electronics", will all this become clear to me? (Heh, heh. I've had that book recommended, but the investment is not trivial.)

Oh, yes. Plunk down the hundred bucks for that book and read it, and you'll understand everything-- electronics, the meaning of life, the ultimate fate of the Universe, and probably even how to mix a first-class Martini.

Seriously, it's a good book and in my opinion well worth the \$ if you're really serious about learning electronics.

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