LDR Motor Circuit

[BobK]

You need a resistor between the '741 output and the base of the transistor. Try a 1K.

Bob

 

[pizoman]

I made some changes to the circuit. Added in a 1K resister in between the output of the 741 and base of the transister as per BobK's tip. Also added in a diode in parralel with the motor and switched the POT from pin 3 to pin 2 of the 741 thus moving the LDR part of the circuit to pin 2.

Now the circuit seems to be affected by the LDR when I cover it up. It doesn't stop entirely but just slows down the motor noticeably. I'm wondering if my POT (5K) resistance is not high enough to completely cut off the circuit. Also the transister seems to heat up as well, I think I need to switch it out for one with a higher current tolerance. Suggestions?

 

[john monks]

Sounds like the transistor and op-amp are destroyed. The transistor is only rated for 100mA. So I suggest replacing the op-amp and the transistor that is much more substantial. And you might try placing a 1kohm resistor between pin 6 of the op-amp and the base of the transistor and another 1kohm resistor between the base and emitter of the transistor. Pin 6 is probably two close to the negative rail. Most motors draw lots of current at start-up. That may be your problem. LED's normally have a series resistor with them. You should try a 330 ohm resistor in series with the LED.

 

[pizoman]

Hey guys,

Tweeked around with the circuit again. Finally got it to work. The final problem was that the motor wasn't hooked up properly, I had positive to negative.

The only problem now is the transister. I replaced the old BC108 with a BD139. The BD139 still heats up after 10 seconds of operation. The motor is still drawing too much current. I'm looking for another NPN transister that can handle more current. Maybe one of the TIP (132) series?

(The motor has a stall current of 4.6A and nomimal current at 0.4A)

 

[jackorocko]

The only problem now is the transister. I replaced the old BC108 with a BD139. The BD139 still heats up after 10 seconds of operation.

the collector current for a BD139 is 1.5A, that should be plenty for that motor. It could be heating up because you are running the transistor in its linear region. What exactly is the motor running?

 

[pizoman]

the collector current for a BD139 is 1.5A, that should be plenty for that motor. It could be heating up because you are running the transistor in its linear region. What exactly is the motor running?

The motor is just running alone by itself right now. I plan to use it later in a model toy train.

You said that the transistor (BD139) is running "in its linear region" which may be causing it to overheat, what exactly do you mean by that and how could I adjust the circuit to get the transistor to stop overheating?

 

[Rleo6965]

You should connect base resistor to limit current of base. This is to avoid to much current on collector to emitter. Try place 4.7K resistor.

 

[pizoman]

You should connect base resistor to limit current of base. This is to avoid to much current on collector to emitter. Try place 4.7K resistor.

I already added in a base resistor from the transistor to the 741 output. It was a 1K resistor. I'll try a higher resistance.

EDIT: Tried changing the base resistance from 1K to 4.7K. Results, motor didn't run, resistance was probably too high.

 

[Rleo6965]

Please try to measure current draw to dc motor. Apply dc voltage to the motor and place ammeter in series to ground. This will be our current reference for selecting transistor and base resistor.

 

[pizoman]

Please try to measure current draw to dc motor. Apply dc voltage to the motor and place ammeter in series to ground. This will be our current reference for selecting transistor and base resistor.

I get a fluctuating value from 2.8-4Amps so I'd say an average of 3.5Amps.

 

[Rleo6965]

I get a fluctuating value from 2.8-4Amps so I'd say an average of 3.5Amps.

That current reading was without load. You need around 10A of npn transistor to drive the motor.

This was reason BD139 got hot. BD139 have only 1.5A collector current.

Let's wait for *steve* to recommend required transistor.

 

[pizoman]

That current reading was without load. You need around 10A of npn transistor to drive the motor.

This was reason BD139 got hot. BD139 have only 1.5A collector current.

Let's wait for *steve* to recommend required transistor.

Who's "steve" by the way? Does he come on the forum often?

 

[Rleo6965]

Who's "steve" by the way? Does he come on the forum often?

He is a "super moderator" here in Electronics Points.

 

[Rleo6965]

The reason I recommend steve because I'm not really an electronic engineer. I'm only a Radio TV technician with 40 yrs experience in electronics and computer repair.

I'm not expert in interpreting datasheets.

 

[davenn]

I get a fluctuating value from 2.8-4Amps so I'd say an average of 3.5Amps.

Thats a long long way from the 0.4A you originally stated!!!
the transistor I suggested was ideal for that, but definately not for ~ 3 Amps

exactly how did you measure the current drawn by the motor ?

you should be doing it like this .....

The "A" in the circle is your multimeter in Amps range ... to be safe 10A range

Dave

 

[davenn]

Realistically if its drawing ~ 4 amps or so, you are probably better off using a FET rather than a bipolar transistor

Dave

 

[jackorocko]

Realistically if its drawing ~ 4 amps or so, you are probably better off using a FET rather than a bipolar transistor

Dave

doubly agreed

 

[(*steve*)]

The reason I recommend steve because I'm not really an electronic engineer.

Thanks for the flattering comments, but please jump in. I'm not an EE either

I'd just be looking at voltage, current, power dissipation and drive requirements to get a short list and then (generally) suggesting the one that happened to be on the top of the list.

Looks like davenn has offered a suggestion, and I'd normally not second-guess him.

So I'd go to digikey, look up mosfets, select something capable of switching say 10A at 30V, and capable of at least a couple of watts of dissipation, and pick one. (oh, obviously N channel, and a logic level device sounds like a good idea.

I get suggestions like

NTD5867NL-1G, IPS105N03L G, FQU13N10LTU, and RFD3055LE.

Really, there are hundreds of devices that would suit this purpose.

I would also think very seriously about wiring the op-amp up as a Schmitt trigger in this application

 

[Rleo6965]

Realistically if its drawing ~ 4 amps or so, you are probably better off using a FET rather than a bipolar transistor

Dave

doubly agreed

I would also think very seriously about wiring the op-amp up as a Schmitt trigger in this application

This is the reason I'm sticking here in Electronics Point. I'm really learning new things to me from you guys.

 

[pizoman]

Sorry guys, I retested the current from my motor as per Dave's instructions and get about 8 @ the 200m DCA setting on my multimeter.