New problem

It would appear that I have grossly underestimated the needs of my "little" motor. It draws 3.5 amps no load. and with a moderate load it goes up to 4-5 amps. The motor is attached to a gear box. It came from a "Power Wheels" Barbie Jeep my daughter had years ago.
My PWM circuit will not start it. I tried a smaller motor ( 1 amp ) and it starts that fine but only gets it up to about 1/2 speed .
with the computer fan attached it measures 10-11 V
with the 1 amp motor it measures 4-5 volts
What is limiting my current ?

 

Just finished reading the link . ( thanks ) Is not C1 in my schematic doing the same thing ? I believe the author called it a "kickstart". Does it need to be bigger ?

 

Re:C1

Yes and no. I thought C1 was to decouple the 555's . It would also be good to add a 0.1 uF ceramic cap between the power supply and ground pins of each (Pins1, 8) for the same purpose. 4QD recommends that the main cap be close to the output transistor(s). My advice is much along the lines of the pull-up. It costs nothing to add the extra pads. You can decide later whether to add the capacitor. I suspect with the low power you are at, the additional cap will not be necessary, particularly if C1 is near the output transistor. Don't forget the decoupling, however. The momentary drop in supply voltage as each 555 turns on acts like an additional switch in the circuit and can lead to strange behavior. As for size, I used a 3300uF/50V on the test circuit, because that is what I had handy.

However, the best advice is to get everything working well on the prototype board before making a PCB, which you are doing.

3.5A is really not much, but remember the transistor ratings are max values. Switching transistors can get pretty hot working near their max, so you will need a heat sink.

If it turns out that you really need more current than the TIP31 can provide, you might consider going to a Darlington power transistor (e.g., TIP142, 10A) or a mosfet. Both options have been discussed frequently on this forum. John

 

I don't have any caps electrolytic caps bigger than 220 but I will move it closer to the output tranistor.
I will also add some .1's to the chips.

I will report back

 

Please keep us posted.

One thing that occurred to me during dinner was the degradation of the current gain with collector current (see datasheet for the TIP31C). At 2A, it is 15; at 4A it is only 8. Your poor little 555 puts out 200mA. So, even with a gain of 15, you are limited to 3A. One, you could reduce the base resistor to 100 to 500 ohm (12V/100 = 120mA; 1.4W; be sure to use a larger resistor and expect it to get warm) to drive the transistor a bit harder. Two, in the alternative, I think you are looking at using a Darlington power transistor to keep the drain on the 555 within reason and yet provide enough current capacity. Radio Shack has some in stock, but I don't remember which ones. They are open until 9?

John

 

I have a TIP3055 NPN ( 10A collecter current )
and a TIP 120 Darlington ( 5 amp continuous 8 max )

 

Try the Darlington. John

 

Whoops I guessed wrong.
I just tried the 3055 ( no difference )
I then added another 220cap near the tranistor ( no difference )
I then added 4 .1uf caps pin 1 to - and pin 8 to + x2 ( this cut my output voltage from 5.7 to 2.9 at the motor.

 

Your circuit is full of voltage loss because the transistor is not used as a switch, it is a follower:
At 200mA, the output high voltage of the 555 has a typical voltage loss of 2.5V.
At 2A, the typical Vbe of a TIP31 transistor is 1.0V.
A TIP41 won't work for more current because the 555 cannot give it enough base current.
A darlington TIP142 follower has a typical Vbe voltage loss of 1.6V.
Max voltage losses are worse.
A Mosfet should be used as a switch for hardly any voltage loss.

 

Audioguru
I am thinking there may be more to it. I have a voltage loss of 6 + volts under load.
Any suggestions on which mosfet ?

 

Audioguru
I am thinking there may be more to it. I have a voltage loss of 6 + volts under load.
Any suggestions on which mosfet ?

There are thousands of Mosfets.
An IRFZ44 Mosfet is rated for 60V and 50A. When cool, its max on-resistance is only 0.028 ohms so 10A will produce a voltage loss of only 0.28V.
There are better Mosfets available.

 

I think I might try this one http://www.uoguelph.ca/~antoon/circ/pwm555.html

I am only missing a couple of the components Q1 & D3

 

Scott,

What are you using to power your timing circuit and eventually, the motor?

I'm assuming you want your circuit to handle at minimum 5 amperes (the loaded motor). What do you plan to use to power these two circuits?

 

A 15 amp 12V power supply. I am not sure what brand it is. It has no labels on it. I got it from my father, he used it as a bench supply for many years. It appears to be built pretty well. And quite heavy.