EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: gatoazul on November 21, 2016, 10:03:33 am
-
Hi,
can someone explain me how to understand this schematic.
What did the transistors T3 and T4 do? can someone explain to me ? Did they do some kind of feedback to control the gate of the triac?
Is T1 transistor circuit a zero crossing detector?
Thanks for the Help
-
First, welcome on this forum.
Here, some explanations....
This circuit operates in a different manner from the traditional phase control and this is permitted since an universal motor does not need necessarily to have a symmetrical supply voltage.
But you can never use this kit to control a transformer.
T1 generates a pulse every 20ms (50Hz) and the control of the triac is therefore done once every 20ms and not every 10ms as in a conventional phase control circuit.
The waveform applied to the load will therefore be asymmetric.
The pulse generated every 20 ms serves to discharge the capacitor C2.
This capacitor then slowly recharges according to the speed setting.
The supply voltage on C1 is about 8V.
T3 is used to produce a bias voltage of 2.6V on R11.
When the voltage of the capacitor C2 exceeds 2.6V + 1.2V = 3.8V, T4 becomes conductive and applies a negative pulse to the triac gate.
This pulse is maintained until the next reset of the capacitor C2.
R8 is used to produce an hysteresis.
R14 and D7 are used to produce positive feedback that increases the motor torque.
I hope it help.
-
Oldway has pretty much summed it up.
T3 and T4 are a schmitt trigger, in days gone by we would have used unijunction to achieve the same effect as two transistors and 4 of the resistors!. (But today a PUT costs $1 , and transistors 10c each!)
I think D7 / R14 provides some speed feedback, as the motor back emf varies with motor speed, so it will adjust the charge rate of C2 to maintain an approximation to speed regulation. (If you used a triac based light dimmer, you wouldn't get good speed control, hence the need for this style of controller.)
The other feature to note is that while a triac can trigger in all 4 quadrants [+ and - gate] with [+ and - on MT2] , it triggers worst with positive gate and negative MT2, therefore most of the transistor type triac drivers are made "positive earth" (the AC neutral connects to + of the DC supply) and they pull the gate negative to trigger.
-
thanks for the help.
its much more clear now.
T1 generate the pulse in what position of the sinewave? its alleatory? or it detects the zero crossing?
-
I have made many circuits based on unijunction transistors 2N2646 in the past.
But these circuits have many disadvantages:
For example, it is impossible to obtain full conduction of the triac or thyristor.
Also, there are trigger failures with highly inductive loads due to pulse width which is far too short (1 or 2µs)
Regarding the regulation of speed of universal motors, it is necessary to explain what happens: when the motor is supplied with alternating current, even when the electromotive force decreases, the current does not increase much because it is limited by the inductance of the motor.
The best way to remedy this is to power the motor with rectified current ... so, just add a rectifier bridge.
NB: The pulse is generated by charging C5 through R2 when T1 is blocked, ie when its base falls below + 0.6V
NB2: If you want to look at waveforms with an oscilloscope, you must use a differential probe or an isolating transformer...it is very dangerous, be careful.
-
I vaguely remember a drill speed controller kit I built 30+ years ago.
It had a bridge rectifier, an SCR , 2 diacs?, and assorted pots , resistors and caps.
The pot/diac/resistor capacitor was arranged in a different way to exploit the back emf, and had pretty good speed regulation.
Some of these old circuits are still useful, I have an active project where I am using the old fashioned 20v zener + SCR to make a 20v preregulator, it works really well due to the high leakage inductance of the small Chinese transformer it is used with.
-
I think it is interesting to note that there is a new generation of triacs named three quadrants or snubberless triacs who are a great improvement to control inductive loads.
More details here:
http://educypedia.karadimov.info/library/AN_3Q_TRIACS.pdf (http://educypedia.karadimov.info/library/AN_3Q_TRIACS.pdf)