Increasing PWM output voltage

Do you need to pull both up and down? What are you driving with the PWM signal?

At 450mV, not many mosfets will work directly in any case. The easiest thing would be to use a comparator, and use the output to control a driver chip like an SN754410:

Regards, Bob Monsen

Hi Robert, thanks for the reply.

I do not know what that means. I am trying to drive a 25W CO2 Laser marking machine's Laser intensity.

I was given the dead Laser and installed one I bought surplus. I now have everything working except the intensity. It is supposed to put out 5v PWM at 5KHz but it is only putting out 450mV. There are no schematics available as it is such an old model and there are several EEPROMs on board and I have no idea what they are coded to do.

The PWM is working just not at sufficient voltage to trigger the Laser. I want to use the existing 450mV PWM output to drive something that will output the 5v I need.

I have been told that this Laser requires "Output Current, max 100mA through a 50 Ohm CMOS driver" and that's all I have to go on.

If I can't do that then I will tear out the electronics and install a

2-axis CNC stepper driver board and run it under EMC2. Obviously getting it to work would be better since it is configured as a printer for Corel and a simple matter to "print" the part. If I go the CNC route I will have to add in two more programming stages for every project.

Dave

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What rise time, minimum pulse width, and fall time do you need?

JF

Maybe what they were referring to is that it must be current regulated to not exceed the 100 ma or set point since a Laser current is critical.

"

Hi again,

I don't know about rise times etc as the PWM is already being generated, but at 450mV and I need to boost that to 5v. I can see the pulses on a scope and the frequency is around 4.980KHz and they are nice clean square waves that I can vary by software.

I just need something that can be driven by the existing PWM output and it in turn outputs the same signal but at 5v.

Thanks

Dave

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I have a solution for you, but since it seems that you want to make some
money off of it, I do too.

Email me with your proposal.

JF

Have you tried adding a pullup resistor (to +5V) to the output pin giving you the 450mV? It may just be an open collector type output.

Regards,

Sarah.

Ok, the output is probably too low resistance for the driver you are using. If you disconnect the thing you are driving, then it probably will go up to 5V, right? If so, all you need is a follower, which is an NPN transistor, connected to the PWM output and the input, like this:

5V ---------------. | | collector base |/ PWM --------| |>

| emitter | '----------Laser

(created by AACircuit v1.28.6 beta 04/19/05

This will decrease the required current at the PWM to 1/50th of what was required before.

At 100mA, if your PWM drives it the base up to 5V, then the transistor will only dissipate about 40mW, so any little NPN transistor will work. Radio Shack will have them. 2N4401, 2N2222, 2N3904, etc.

This is probably worth a try.

Regards, Bob Monsen

Hi Bob,

Thanks for the reply. No, this is 450mV with or without the Laser connected. The output remains the same into load no load.

Dave

Hi John,

Yeah, you are right. Here I am retired and living on SS of $1200 a month. I am trying to make a few extra bucks to brighten my lifestyle a touch. I bought the replacement Laser about 7 years back when I was working and it has been sitting waiting to be used for some project or other. When I was given the dead marking machine, I thought I was on the way to a little more fiscal independence.

That seems a little ways off still, but I am hopeful and will keep plugging away at it.

I am sorry, but there seems little point in emailing you as I have little enough to spend as it is.

Thank you for the offer.

Dave

Ok, then you'll need to go the comparator route. Here is a simple circuit that might work, using a TL431 as a comparator:

5V--------------o----------o---------. | | | | | | | .-. | 10k | 1k | | | ___ | | | | Input -|___|------' '-' |e A | b |< | o-------| PNP | | |\\ | .-. |c | 1k | | | | | | | | '-' o-------Output | | | | k | | - TL431 .-. '----------------r- ^ | | 10k (probably not needed) a | | | '-' | | | | GND------------------------o---------'

(created by AACircuit v1.28.6 beta 04/19/05

The TL431 is a shunt regulator. If the voltage on the 'r' pin goes above 2.5V, it'll suddenly become very low resistance between the k and a leads. So, if you set the pot so that the r lead goes a bit above 2.5V when the input is 400mV, and a bit below 2.5V when the input is 0, it'll cause the PNP transistor to turn on and off.

Look at the datasheet link above for more information. You can get these at electronics surplus places online, or of course mouser or digikey.

I'm sure there are simpler solutions, but this one will give you a nice crisp square wave with a minimum of parts.

Regards, Bob Monsen

Hi Bob,

Thanks for that. I'll keep you posted

Dave