TTL "Merging"?

[James Lehman]

Don't you loose about 0.3 to 0.7 volts off the top that way?

James.

[buffo]

Yes, you will loose whatever voltage is dropped across the diode. (.7 volts is typical.) That's no problem for TTL lasers though, as most TTL lasers will turn on at between 1.5 and 2 volts anyway. He'll still have plenty of voltage available even after the loss across the diode.

If he was using an analog-modulated laser, then yeah, he'd end up loosing ~20 percent of the max power.

Adam

[James Lehman]

Yes, you will loose whatever voltage is dropped across the diode. (.7 volts is typical.) That's no problem for TTL lasers though, as most TTL lasers will turn on at between 1.5 and 2 volts anyway. He'll still have plenty of voltage available even after the loss across the diode.

If he was using an analog-modulated laser, then yeah, he'd end up loosing ~20 percent of the max power.

Adam

And He's also loose the low end, below the diode junction voltage.

James.

[sonaluma]

Hi James, These instructions are for TTL signals for a TTL laser, so the forwards voltage drop does not present any issue. Regards, sonaluma

[buffo]

And He's also loose the low end, below the diode junction voltage.

As sonoluma pointed out, that's not relevant for a TTL laser. It's either on or off.

For a laser that supported analog modulation, you don't "loose" anything on the low end, because the laser won't turn on until you get above 1.1 to 1.5 volts anyway. The net effect of having the diode in the modulation line is that it shifts the whole power curve down. So when you're putting out, say, 2.3 volts on the modulation line, the laser is only seeing 1.5 volts (because of the voltage drop across the diode).

Thus, if we assume that the laser will turn on at 1.5 volts, the laser would just be turning on when you had the modulation up to 2.3 volts already. (Normally a voltage of 2.3V would give you significant output.) You can still hit all the low power settings though.

Your loss comes at the high end, when you've got the full 5 volts on the modulation line but the laser is only seeing 4.3 volts, so it's not maxed out yet.

Adam

[mixedgas]

You can make a "zero loss diode" for analog voltages with low forward drop (well, .05 V ) or so in some cases using a N channel mosfet by tying the gate lead to the drain lead. And then conduction is from drain to source. I dont know off which small signal mosfet off hand, but I will try tonight with a mpf102 (available from radio shack or maplin/radio spares in europe), Or use a schottky diode and only loose about .29V.
Steve Roberts

[z-tek]

ive just tryed this but it didnt work!

ive connected all 3 of the ground, to the ground on my ttl input on the laser
and on each signal wire ive put a 1N4148 diode (black band pointing towards the laser driver) then from that all 3 of the dioes are connected (black band end) and then it goes to the laser signal input!

any ideas people?

Paul

[mixedgas]

ive just tryed this but it didnt work!

ive connected all 3 of the ground, to the ground on my ttl input on the laser
and on each signal wire ive put a 1N4148 diode (black band pointing towards the laser driver) then from that all 3 of the dioes are connected (black band end) and then it goes to the laser signal input!

any ideas people?

Paul

Laser has too high a input impedance, or is not true ttl Try either of the two buffers in attached BMP The transistor one has the most swing, especially if the input to the laser is actually a optocoupler, which it often is.

Steve

[z-tek]

how can i find out?
or what else can i do, im in the same boat as the guy asking the origional question!

Paul

[mixedgas]

how can i find out?
or what else can i do, im in the same boat as the guy asking the origional question!

Paul

I just edited the post. The usual way is to connect a 10K potentiometer to 5V regulated and
the wiper to to the laser input and the other leg to ground, and see if the laser snaps on at 1.6V or so, in which case its TTL. See the attachment I just added to my last post. It has buffer circuits.

Steve