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Author Topic: cnc screwcutting???  (Read 19887 times)

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Offline rcaffin

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Re: cnc screwcutting???
« Reply #40 on: January 25, 2011, 04:59:10 PM »
I've connected things as you describe and am getting a clean signal from the emitter when it is not connected to the input on the BOB. as soon as I connect to the BOB it drop's to 1.5v. Just to clear up I'm not doing anything stupid on my part, I taking 5v from a pin on my BOB (next to pin 10 on BOB) to give 5v to the sensor and to the collector on the transistor.

Hi Dale
No worries. We'll get there.

It may be that the 10k pull-up resistor in the OPB916B cannot send enough base current into the 2N3904 to pull the emitter any higher than 1.5 V, but I find that rather hard to believe if everything is per the book. You see, with 1.5 V on the emitter that's 2.1 V on the base, or 2.9 V across the 10 k pull-up. That puts ~0.29 mA into the base. Allow a typical beta of 100 and you get 29 mA out the emitter - although most 2N3904 transistors should do better than that. Anyhow, 29 mA through 4.7 k should give well over 100 V drive! Odd, very odd, and not believable.

One possibility is that the front end of the buffer chip has been damaged, such that it can effectively short out any input current. (They all run on smoke: let the smoke out and they don't work any more.) You can check this by connecting the input pin to +5 V on the BOB and measuring the voltage at the pin. It should be of the order of 4.1 V. If it is much lower than this then I would suspect damage to the input buffer chip.

Fortunately, all the chips on the C11 appear to be in sockets, and they can be easily replaced. Spares can be bought through CNC4PC or through many electronics stores (Tandy?). But first, do the check with a 1 k resistor and let us know the results.

Cheers
Roger Caffin


Re: cnc screwcutting???
« Reply #41 on: January 25, 2011, 05:41:00 PM »
Thanks Rcaffin, I'll check the volts across my input first to see if it's ok, I have checked on mach diagnose page to see the input is working by manually adding the 5v to the input and it is flashing up ok on screen.

Do I add the resistor to the base pin?

Thanks for the heads up on the spares, I checked out cnc4pc a few day ago when I thought I smoked the BOB.

Kind regards

Dale gribble

Offline rcaffin

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Re: cnc screwcutting???
« Reply #42 on: January 25, 2011, 05:49:33 PM »
I have checked on mach diagnose page to see the input is working by manually adding the 5v to the input and it is flashing up ok on screen.
Good check for the opto isolator etc, but does not check the buffer chip which may still be damaged. Howeevr, obviously not totally cremated. odd...

Quote
Do I add the resistor to the base pin?
Not for the test. Disconnect all the sensor wires from the BOB and just put the 1 k resistor between the input pin and +5 V and measure the voltage on the pin. This will tell us what the BOB input looks like electrically.

Cheers
Roger
Re: cnc screwcutting???
« Reply #43 on: January 25, 2011, 06:25:22 PM »
Hi roger,

Volt without resistor 4.29
Re: cnc screwcutting???
« Reply #44 on: January 25, 2011, 06:26:39 PM »
Hi roger,

Volts without resistor = 4.29v
volts with 1k resistor = 2.96v

Regards

Dale

Offline rcaffin

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Re: cnc screwcutting???
« Reply #45 on: January 25, 2011, 06:31:22 PM »
Volt without resistor=  4.29 V
volts with 1k resistor = 2.96 v

Hi Dale
Sorry - I don't quite understand this.
Do you have the BOB internal resistor tied to the +5 V or Gnd?
Did you add the 1 k resistor between the input pin and +5 V, or between the input pin and Gnd?

Cheers
Re: cnc screwcutting???
« Reply #46 on: January 25, 2011, 06:51:08 PM »
Hi roger,

Resistor between 5v and the input pin.

Dale

Offline rcaffin

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Re: cnc screwcutting???
« Reply #47 on: January 25, 2011, 07:15:35 PM »
Resistor between 5v and the input pin.

In that case I am mystified ...
Aha - are you measuring the voltages between gnd and input pin, or between pin and +5 V?

Cheers
Roger

Offline rcaffin

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Re: cnc screwcutting???
« Reply #48 on: January 25, 2011, 07:25:10 PM »
Volts without resistor = 4.29v
volts with 1k resistor = 2.96v

Hum ... I will assume that you are measuring the voltage between pin and +5 V. I will translate that to between pin and gnd, and assume BOB input resistor is connected to gnd. Please confirm.
No pull-up resistor: 0.7 V   wrt gnd
1k pull-up resistor: 2.0 V   wrt gnd

That means that there is 3 mA going through the 1 k resistor to a 4k7 resistor to gnd and the input pin of the buffer. Now at 2 V above gnd, the 4k7 resistor will account for ~0.4 mA to gnd, leaving 2.6 mA going into the buffer pin. That is a shade high! It is possible that the buffer chip has been cooked and needs replacing.

Can you read a part number off the top of the buffer chip? I can look up its specs if I know what it is.

Cheers
Roger


Re: cnc screwcutting???
« Reply #49 on: January 26, 2011, 12:51:49 PM »
Hi roger,

re checked input pin with 1k resistor with reference to grd
volts output from BOB 5.12v (
volts @ input pin with I k resistor 2.16v

Don't know if these figures make more sense, so I've add an image to help explain what I've checked

Regards

Dale