Servo / ESC tester with 555

[polashd]

I've made the circuit (attached) to produce signal to drive my BLDC ESC. In Ltspice it shows producing required signal, almost (50 Hz, duty 5% to 10%) with the variable R (R5=10k).

But when I made and tested it practically the ESC (& motor) not running giving err tone. I tested with DMM and found frequency and duty% far from what was demonstrated in Ltspice. I did some trial and err using different values for R1, R2 & C2, finally reached a point where it is working but the running of the motor is not smooth enough.

I want to use this diagram. I need equation to calculate frequency & duty% for this diagram so that I can determine values of resistors and capacitor(c2).
Can any one provide me the equation, pls.

 

[alec_t]

Your sim looks good.
The pulse width and period are each approximately 0.8*R*C2, where R is the total charge resistance (R1 + pot portion) or total discharge resistance (R2 + pot portion) respectively. For more accuracy you would have to take into account the 1N4148 characteristics and the fact that the 555 output is always a volt or so below the supply voltage (unless the 555 is a CMOS type).
Perhaps your ESC is loading the 555 output pin 3 too much and affecting the results. Try connecting a 1k pull-up resistor from the discharge pin (pin 7) to the +ve supply and driving the ESC from pin 7.

 

[Tony Stewart]

The 555 uses a Vcc to R current source to charge a cap and your design with a narrow pulse uses only RC so the formula is incorrect and non linear.

I suggest if you want a more precise frequency use the internal method and generate a one shot pulse for your 5% duty cycle in the next stage using a dual 555 chip or an RC delay to a Schmitt inverter.... or create a dead band pulse using an RC delay on one side of an XOR gate with the other side direct and delay the other clocks to fall in the middle of the deadband transition pulse.

 

[MikeMl]

This circuit would do much better being operated from Vcc >=12V than from 5V. A standard 555 is less stable at near its minimum operating voltage. Your sim makes no allowance for any load current being drawn from pin 3 (Out) so I agree with Alec; that load current changes the pin3 voltage that goes into the timing network. You should put a buffer between pin 3 and the load...

Finally, the 555 model in the LTSpice misc library is an idealized "behavioral" model which does not account for Voh and Vol under varying source and sink currents in-to/out-of pin 3.

Get a "real" model of a specific 555 IC chip (TI bipolar or CMOS), link it into the LTSpice library, connect a load which mimics your load in parallel with the timing network, then the sim will match your reality. What you modeled, and what you built are two different things (as you found out)...

 

[MikeMl]

The 555 uses a constant current source to charge a cap...

??? Huh?

 

[Tony Stewart]

??? Huh?

I must be getting tired.disregard

 

[Tony Stewart]

The Op should give a timing spec as a test criteria before further design changes.

 

[spec]

You do not say what is connected to the output of the 555, but in general it is ill advised to use the same signal that is used for timing to drive a load, especial reactive or low resistance, as Alec said in post #2.

As Mike says in post #4 the 555 is not at its best running off 5V, but the following mods will help.
(1) Decoupling on supply line: eliminates current glitch as both output transistors turn on during transitions.
(2) Discharged via DIS: eliminates the forward drop of one diode.
(3) Charge diode replaced by a low forward drop schottky diode.
(4) Q output pulled up to V1. If running off 5V change pull up resistor to 470R.

The two decoupling capacitors should be disk ceramic X7R dialectic, and the timing capacitor should be metal film, ideally polypropylene dielectric.

spec

ISSUE 2 of 2016_09_03

​

 

[alec_t]

Reverse D2?

 

[spec]

There was more errors so I updated the sketch: does it look OK now?

spec

 

[spec]

Here is a version where the timing is separate from the Q output:

ERRATA
(1) LMC555 should read LM555

 

[Tony Stewart]

If you want to design a precision pulse width and variable frequency or variable pulse independent of frequency, both can be done, if you specify it.

 

[kal.a]

I would humbly suggest variable pulse width and fixed frequency for his application.

 

[spec]

I take it that:
BLDC = Brushless DC Motor
ESC = Electronic Stability Control (for a vehicle)

spec

 

[alec_t]

Spec, you're right about BLDC, but to modellers ESC means 'Electronic Speed Controller'.

 

[spec]

Spec, you're right about BLDC, but to modellers ESC means 'Electronic Speed Controller'.

Thanks Alec: I wish people would not use abbreviations without first explaining them, bearing in mind that ETO is for general readership, including NNES, not just the cognoscenti.

A particular military customer we worked with was very fond of abbreviations and I just used to go along with it not having a clue what they were talking about. But at one meeting I asked them what one of their abbreviation meant: this turned into a long discussion, because none of them could agree what the abbreviation meant.

One of the most awkward and misused abbreviations is AC (Alternating Current). So you get phrases like 'AC voltage' (Alternating Current voltage), 'AC current' (Alternating Current current). DC (Direct Current) is similar.

When I was in the RAF (Royal Air Force) we talked mostly in numbers: 1250 was your ID (IDentity) Card, 252 was a charge form, 3562 was a leave form, etc, etc. So you may have, 'He lost his 1250 and was put on a 252 and his 3562 was cancelled.' Civvies just could not comprehend what we were talking about.

Rant over.

spec

 

[alec_t]

I wish people would not use abbreviations without first explaining them,bearing in mind that ETO is for general readership, including NNES,

Me too. NNES stands for Norfolk Needle Exchange Scheme .

 

[spec]

You got my little joke (Non Native English Speaker).

spec

 

[polashd]

You do not say what is connected to the output of the 555, but in general it is ill advised to use the same signal that is used for timing to drive a load, especial reactive or low resistance, as Alec said in post #2.

In my post #1 I wrote the following

I've made the circuit (attached) to produce signal to drive my BLDC ESC. In Ltspice it shows producing required signal, almost (50 Hz, duty 5% to 10%) with the variable R (R5=10k).

Now I assume you were confused about BLDC ESC
BLDC= Brush Less DC Motor, ESC=Electronic Speed Controller
My circuit is to connect with ESC (which is powered by a 2S (ie. 7.4V) Lipo battery. The 555 circuit doesn't drive any load. It gets its power from the the ESC (which is 5V as I measured) and just send a signal(50Hz, Duty(min)=5%(1ms), & max=10%(2ms)) to the ESC, The ESC drive the motor.

If the signal is incorrect the ESC won't run properly.

 

[Colin]

try this