I just want control the speed of dc motor.( without using pwm,can we control speed by varying voltage and current). any way to control??

Depends on what you mean by "control". If you mean keep the speed constant even as the load changes, then just controlling the voltage won't do it. You need some kind of feedback (which can be nothing more than YOU turning up the voltage as the load slows the motor down).

Yes, you can do this without PWM and just by varying the voltage, but you have to provide it with a low impedance voltage source. A rheostat, probably in the 10Ω to 20Ω range, or an adjustable regulator are options.

 

this is not a English grammar forum. its electronics forum, if u know answer then tell or else keep quite. u r not only the person knowing abt electronics, we r here to just discuss abt electronics not English language.

This will not get you anywhere fast. This is an English-language forum and not an SMS or text-speak forum. More to the point, YOU came here seeking assistance from people that owe you nothing and have no obligation to do anything for you. Thus, the burden is on YOU to make those same people want to choose to spend their time assisting you instead of one of the many other things they could be doing. Think of it as an opportunity to develop and practice your written interpersonal communication skills.

 

Well said Wbahn. If the OP can't be bothered to ask POLITELY in ENGLISH, then I can't be bothered to help him. I won't help some brat who thinks he's entitled to everything. And I'm sure I'm not the only one who feels this way.

 

If you want to avoid a situation where the motor becomes current instead of voltage driven the current through the divider must be considerably above the motor current (10 times for example). This means 1A for a 100mA motor. 5V/1A = 5Ω or two 2.5Ω resistors for the divider. This results in 5W of standby power, which is unnecessary wastage and heat.

Better is to use a voltage regulator IC or, if this is too complex, a 2.5V/1W Zener diode or eventually a couple of series diodes (as someone already mentioned). Four 1N4148 or 1N4001 diodes in series result in a voltage drop of about 4 x 0.6V = 2.4 V. Power loss is 2.4 x 0.1 = 0.24W, which is considerably less. A diode is a non-Ohmic device which means that the resistance varies with the voltage across its terminals. This means a voltage driven motor.

The reason why a voltage driven motor is preferential is that the torque of a DC motor is proportional to its current. If you restrict the current and therefore torque in any way, you run into the risk of a motor unable to start under load, since a motor draws a much higher starting current.

 

Four 1N4148 or 1N4001 diodes in series result in a voltage drop of about 4 x 0.6V = 2.4 V.

1N4148s would not be a good choice for this application if the current might be an amp or more under some conditions.

 

The problem with using the voltage drop diodes at all is that the OP has now said that we wants to be able to control the motor speed (it's so nice when we get the specs piecemeal, isn't it?). You might still use one or possibly two, but you need something else to permit controlling things.

Given the current we are talking about, it is probably reasonable to just use a transistor in an emitter-follower configuration and two diodes in the collector circuit to limit the max voltage to the motor and shift some of the heat away from the transistor. If you use a diode that are only going to drop about 0.5V or less, then you might use three.