555 - Maximum Frequency?

[Omar.M]

Hello! I'm working to get a PWM signal to my motor driver to get a lower voltage to my motors. I've got a basic astable multivibrator circuit here, and I've got the values to get about 50% duty cycles.

The thing is, the frequency is REALLY high. I'm using these values:
R1- 1hm:
R2- 470hm:
C1- 0.01:mu:

The frequency seems to be roughly: 153347.7
The fact that this is so huge, will that affect the LM555 or the circuit? I don't want anything exploding or breaking since the frequency is so high.

Thank you,
Omar

 

[j.p.bill]

That's way too high a frequency for a 555. The timing nomograms give up around 100 KHZ.

Why are you trying such a high frequency? Not only is 15 MHz much higher than what PWM requires, but you are going to have many problems finding devices to run that fast. Not to mention problems with RF noise floating out of your circuit.

The best way to put a lower voltage out to you motors is to reduce the supply voltage.

 

[Omar.M]

Well, the motors in the Tamiya gear box are quite small. The motor driver IC I'm going to use runs at about 4.5V whereas the motors are supposed to run at about 1.5-3.5V. So it was suggested I use PWM and bring down the voltage.

I'll use different values-- thanks .

 

[stevez]

Omar - others may offer you better help that I am able to provide but here's a few thoughts:

1. I may be mistaken but if the load you are driving with PWM is inductive (a motor is inductive) then you'll probably want to keep the frequency relatively low -possibly limiting to a few hundred Hz.

2. Take a look at the datasheet for a 555 as there are some very good indications of how to determine the frequency, duty cycle - and that indicate some limits to the values of R1, R2, the capacitor, etc.

 

[Omar.M]

Thank you very much, both of you.
I'll keep it to a few hundred Hz then. I think I've got the hang of how to guess the value and calculate it. I found a bunch of .01 uF capacitors, and I was trying to be cheap and using them. No worries, I've got many of them lying around and I'll play with the values a bit.

Thanks again!