SCR Latching current differences, why?

Thread Starter

AymanKhuzundar

Joined Apr 2, 2017
60
Hi every one,
I've build the circuit in graph 1 to test the holding and latching current of a BT151 SCR, and found them around 10mA.
then, I wanted to build a latching circuit that has ON/OFF push-buttons (just to clearly understand the SCR) with a DC motor (load) which draws about 40 to 50 mA which is higher than both the holding and latching currents, but the circuit (in graph 2) faced a problem that is the SCR doesn't keep latching while the input signal is not applied, then I tried another motor drawing about 200 mA and everything went well.
before changing the motor I've tried to change the gate resistors to increase the gate trigger current but with no benefit.
my question is that why the latching current differs in both circuits? does the inductive load play a role in changing the value of the Latching current?
Thanks.
 

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Thread Starter

AymanKhuzundar

Joined Apr 2, 2017
60
Always put the load in the Anode , and put a 1K resistor in parallel with the motor, to keep the current flowing, this prevents drop out from back emf.
I connect it in the Cathode because I want to draw the waveform on the oscilloscope.
I've tried to connect a free-wheeling diode across the motor and also failed, do you mean the same thing by using a 1K resistor ?
 

Thread Starter

AymanKhuzundar

Joined Apr 2, 2017
60
Try connecting the bottom of R4 to the SCR cathode instead of ground.
OK that may solve the problem, I have connect a 100 ohm resistor in parallel with the motor to draw more current and that solved my issue, but my question is that why the latching current changed ? just to understand the idea
 

AlbertHall

Joined Jun 4, 2014
8,381
The motor current will not be a nice steady DC. It will vary depending on the brush contact with the commutator. If the brushes, even temporarily, make poor contact then the SCR will turn off. The resistor across the motor will prevent the current falling too low.
 

TeeKay6

Joined Apr 20, 2019
394
OK that may solve the problem, I have connect a 100 ohm resistor in parallel with the motor to draw more current and that solved my issue, but my question is that why the latching current changed ? just to understand the idea
Your original circuit misapplies the gate signal as others have noted. Also, depending on the motor you chose as load, the load may be intermittent. If the motor uses brushes for commutation, there will be brief periods (usec to msec) when the brush loses contact with the armature and the current drops. This is not seen in all motors but certainly occurs in some. A cap or resistor in parallel with the motor can keep the current above the SCR's latching/holding current values.
 

Thread Starter

AymanKhuzundar

Joined Apr 2, 2017
60
The motor current will not be a nice steady DC. It will vary depending on the brush contact with the commutator. If the brushes, even temporarily, make poor contact then the SCR will turn off. The resistor across the motor will prevent the current falling too low.
can I see these drops on the oscilloscope if I connect the motor directly to 12VDC source?
 

Thread Starter

AymanKhuzundar

Joined Apr 2, 2017
60
Your original circuit misapplies the gate signal as others have noted. Also, depending on the motor you chose as load, the load may be intermittent. If the motor uses brushes for commutation, there will be brief periods (usec to msec) when the brush loses contact with the armature and the current drops. This is not seen in all motors but certainly occurs in some. A cap or resistor in parallel with the motor can keep the current above the SCR's latching/holding current values.
thanks for your brief description, MR AlbertHall says the same thing.
but another question, what would the other motor be if it's also a DC motor? I know that all brushed DC motors consists of that brushes, but would the distance be different or what's the difference between them?
 

TeeKay6

Joined Apr 20, 2019
394
can I see these drops on the oscilloscope if I connect the motor directly to 12VDC source?
You will see little looking at the motor voltage. However, if you put a small resistor (e.g. 1 - 5 ohms) in series with the motor and connect your oscilloscope across the resistor, you can usually see how noisy the current actually is...assuming your motor has brushes. (If you are connecting only the motor and not some driver circuitry, then--if your motor turns--your motor surely has brushes.)
 

AlbertHall

Joined Jun 4, 2014
8,381
If it was a shunt wound motor the field winding would maintain significant current despite whatever the brushes were doing but a small DC motor is much more likely to use permanent magnets for the field.
 

TeeKay6

Joined Apr 20, 2019
394
You will see little looking at the motor voltage. However, if you put a small resistor (e.g. 1 - 5 ohms) in series with the motor and connect your oscilloscope across the resistor, you can usually see how noisy the current actually is...assuming your motor has brushes. (If you are connecting only the motor and not some driver circuitry, then--if your motor turns--your motor surely has brushes.)
I assume you refer to one motor that draws 40mA vs another that draws 200mA. The higher current motor may differ in how many commutator segments it has, in how good the brush contacts are, and how much capacitance--if any--is connected internally across the motor leads.
 

Thread Starter

AymanKhuzundar

Joined Apr 2, 2017
60
You will see little looking at the motor voltage. However, if you put a small resistor (e.g. 1 - 5 ohms) in series with the motor and connect your oscilloscope across the resistor, you can usually see how noisy the current actually is...assuming your motor has brushes. (If you are connecting only the motor and not some driver circuitry, then--if your motor turns--your motor surely has brushes.)
and the other motor which I've used (draws 200mA) doesn't drops below the holding current so that it stills on, right ?
 

Thread Starter

AymanKhuzundar

Joined Apr 2, 2017
60
I assume you refer to one motor that draws 40mA vs another that draws 200mA. The higher current motor may differ in how many commutator segments it has, in how good the brush contacts are, and how much capacitance--if any--is connected internally across the motor leads.
ahaaa, I've got the idea.
Thanks alot for all <3 <3
 
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