Boosting PWM voltage, suggestions

[DaveDoesIT]

Hi,

A newbie to this forum with limited electronics knowledge. I have built many electronic kits over my time but not so good on the design side.

I have a 5KHz PWM 0 - 95% output of about 450mV. I need to up that output to 5v.

Is there something I can build or buy cheaply that will take the 450mV and output 5v PWM at the same frequency?

Thanks

Dave

[wackyvorlon]

You can have it drive a transistor that switches a five volt supply. Not very difficult to do. How much current does it need to supply?

[DaveDoesIT]

Originally Posted by wackyvorlon You can have it drive a transistor that switches a five volt supply. Not very difficult to do. How much current does it need to supply?

Thanks for the reply, sorry forgot to add the current, this from the manual...

"Output Current, max -- 100mA through a 50 Ohm CMOS driver"

Thanks for the help so far.

Dave

[wackyvorlon]

Okay, 100mA isn't much at all. I'm thinking something like the 2N7000 or a 2N3904. Now, I just have to see if I can remember the biasing



M1 is a 2N7000 mosfet, R1 is selected to give 100mA at 5v. 5/.1 is 50 ohms. Vcc is your +5v supply. Now, this circuit may be wrong. I'm a little fuzzy on the exact configuration you need. You might need a P-channel mosfet instead of an N-channel like the 2n7000. I'll look it up and make sure, but I wanted to give you the basic idea.

[DaveDoesIT]

Originally Posted by wackyvorlon You might need a P-channel mosfet instead of an N-channel like the 2n7000. I'll look it up and make sure, but I wanted to give you the basic idea.

Hi WV,

Thanks for that. Please do not rush, I live out in the boonies of AZ and I have to order stuff which takes about a week to get here. So I am happy for you to get comfortable with the correct parts before I hit the Mouser catalog.

I very much appreciate your help with this.

Dave

[DaveDoesIT]

Originally Posted by wackyvorlon You might need a P-channel mosfet instead of an N-channel like the 2n7000. I'll look it up and make sure, but I wanted to give you the basic idea.

Hi, Any progress on looking up which MOSFET I should use?

Thanks

Dave

[Lexx0001]

Hi,

I don't mean to confuse things here, but I've got a couple of questions. The 450 mV mentioned earlier, Where did you get that? From the manual or measured at the output? If it's measured, that voltage could be across an open output. I'm reading that the output current will source (or sink) 100mA to the load. Which could be a small motor. Or a transistor that would in turn power a larger motor.

Is this something you are building or modifying? Do you have a schematic? I'm pretty sure I can help you get this running, but I need more info.

Bill K.

[DaveDoesIT]

Originally Posted by Lexx0001 If it's measured, that voltage could be across an open output. I'm reading that the output current will source (or sink) 100mA to the load.

Thanks for the reply Bill.

I measured the 450mV PWM while attached to the CO2 Laser intensity I am trying to power and that needs 5v PWM at 5KHz and 100mA max. I tried with it open and got the same results.

I suspect it may be a programming problem as there are 4 EEPROMs on the board and I have no idea what they do. My guess is there is an input that requires raising to maybe 5v as an interlock for doors, shutter etc.

I have tried all of the options I can think of and the only one that did anything, I think was for an E-stop as the PWM stopped instantly when I applied 5v and would not restart until I applied 5v to an adjacent line. Getting a bit cavalier waving 5v around but there was little to lose.

I am flying blind here as the company that made this no longer has schematics or parts for it. They can't even find a user manual.

I am thinking of tearing all the electronics out and getting a couple of Gecko drives and run it under EMC2. I found a circuit for a manual PWM control what I can try. I am not sure it is worth any more effort. I was trying to keep the stock electronics as it is configured as a printer and I just send the "picture" from Corel and the on-board software figures it all out. I have everything working except the intensity.

Thank again for the reply and I would like to hear any comments you may have.

Dave

[Lexx0001]

Dave,

I hear what you are saying. Sometimes the trick is to know when to move on. I haven't learned that trick yet at least not in a timely manner. I don't know about the 5 kHz PWM. While that speed would work well for a laser, it might be a little fast for anything but a small motor. But then, with only 5 volts to work with, the motor has to be small.

What you could try is hooking up a LED. I like the ones that have an internal resister in them, hook the anode to the +5v and the cathode to the output, and try changing the voltage. If nothing happens, then hook the cathode to ground and the anode to the output. What we are trying to do here is see which way the current is flowing, if it's sourcing or sinking. If still nothing happens, verify the LED by hooking it from +5 to ground. If that still don't work, try reversing the leads. I like the 5 volt LED's because you don't have to worry about them being real bright for an instant, then dark forever. Once the magic smoke is let out of them, they are worthless. Girls don't seem to be too impressed when you give them earrings made from them. At least my daughter acted like she wasn't.

Anyway. once you get the LED to work, the next step is to find something to drive the motor with. But, I guess lets see what happens.

This isn't something you would want to reverse is it?

[DaveDoesIT]

Originally Posted by Lexx0001 which way the current is flowing, if it's sourcing or sinking.

Sorry for the delay but been a bit swamped here.

I hooked it to a scope and the pulses are positive going. Does that help?

Dave

[Lexx0001]

Dave,

Ahh, no, not directly by itself. Do you have control over the pulse width? This is hooked up to a laser, right? If you can make the laser brighter when the width of this pulse decreases, then it's pretty safe to say that the output is sinking the current. In other words the current goes from the +V supply, through the load, and then the output to ground. The term output is a little confusing because it doesn't supply voltage, but rather a current path to ground in this case. Remember, when a transistor is on, it's conducting and should only have a very low voltage across it. When it's off, it will have the full voltage.

As a disclaimer, I should mention that I've made some assumptions, that the scope is hooked up to circuit ground and the voltage has got a fairly nice square wave. Square as in nice sharp edges, not square as in 50% duty cycle.

Bill K.