Page 2 of 2 FirstFirst 12
Results 11 to 17 of 17

Thread: TTL and PWM

  1. #11
    Join Date
    Feb 2008
    Location
    East Sussex, England
    Posts
    5,174

    Default

    Chris, yes one of the attractions of the units is a single cable. Only single colour, very simple, easy to rig in strange places to get the distributed beams effect we're looking for.
    Running 40+ sets of DMX and power isn't really very desirable!
    I also did some calculations and found similar to you on the voltage drop.
    Currently we don't know what the current draw is on them, which makes things tricky, but the highest power unit is a 500mW 445. Red 120mW, green 80mW. We're going to get a blue sample to do some testing on. Is 500mA a reasonable [over] estimation per head?
    Worst case scenario is that we build more controllers and have shorter runs to the heads.
    4*12ch controllers with max 30m cable run for example.
    30m 20AWG @500mA = 11V
    If they're a bit tolerant to slightly over voltage we could dial up the SMPS and hope the close to home ones take a little over voltage.
    12.5V @ 10m = 12.4V
    12.5V @ 30m = 11.75V
    The PWM driver doesn't care about voltage, it can take wide input range.
    |
    |
    |
    VDX, your stuff sounds excellent, but is just waaay beyond the electronics knowledge we currently possess, hence the desire to use as much off the shelf equipment as possible. I also want to get this from concept to operational with as little fuss and time as possible, and paying a few extra ££ to shorten the time involvement is money well spent!
    http://www.facebook.com/SubsonicSystems
    http://www.frikkinlazors.co.uk

    You are using Bonetti's defense against me, ah?

    I thought it fitting, considering the rocky terrain.

  2. #12
    Join Date
    Oct 2012
    Location
    Germany
    Posts
    962

    Default

    Hi Norty,

    attached is the schematics for the LM-drivers with optocoupler - it's designed for currents of up to 8.34 Amperes when all LM's are active.

    For lower constant currents you can drop the other 4 LM's - a single LM338 is capable of up to 5 Amps, the cheaper LM317 with same pin-out are meant for only 1,5 Amps, if this is enough.

    To "set" the current [I] to another value this formula for the shunt resistor [R] is used: R = 1,25V / I

    Viktor
    Attached Thumbnails Attached Thumbnails LM-driver4-schematics_MOD2.jpg  

    Aufruf zum Projekt "Müll-freie Meere" - https://reprap.org/forum/list.php?426
    Call for the project "garbage-free seas" - https://reprap.org/forum/list.php?425

  3. #13
    Join Date
    Feb 2008
    Location
    Northern Indiana
    Posts
    903

    Default

    445nm at 500mw should be around 500mA. I bet you will be fine. With a 12V input driver you should have plenty of overhead in the driver for fluctuations. If you do have TTL problems a optocoupler at the laser driver as victor suggests could solve that problem.

  4. #14
    Join Date
    Feb 2008
    Location
    East Sussex, England
    Posts
    5,174

    Default

    Ok, I think I'm understanding how the optocoupler works (remember, I'm not an electronicist! Be gentle)
    It can still work under lower voltage levels than the equipment requiring the signal, so could be used to switch power from a supply closer to the head, even if the line voltage had dropped low?
    And you could build a repeater box with multiple optocouplers and it's own PSU to drive further down the line?
    E.g. 12V drops to 7.5V over a 70m line which might be too low for the laser driver, but ok for the optocoupler.
    The optocoupler modulates the ongoing 12V from the repeater box another 30m down the line?
    http://www.facebook.com/SubsonicSystems
    http://www.frikkinlazors.co.uk

    You are using Bonetti's defense against me, ah?

    I thought it fitting, considering the rocky terrain.

  5. #15
    Join Date
    Oct 2012
    Location
    Germany
    Posts
    962

    Default

    ... yes - you can use the nearest line plug to your LED's to source the power ... and send the (optical) PWM-signal throug a plastic (or glass) light guide over some hundred meters (or even kilometers).

    The PWM-frequency can be as fast as some hundred kHz to MHz - you only need a LED on the sender side and a photo-transistor as reciever ... did some DIY-optocoupling with cutting common optocouplers in half and placing plastic light feeders with some meters length inbetween ... used this in a university project to "isolate" some hundred Kilovolts

    Viktor
    Aufruf zum Projekt "Müll-freie Meere" - https://reprap.org/forum/list.php?426
    Call for the project "garbage-free seas" - https://reprap.org/forum/list.php?425

  6. #16
    Join Date
    Feb 2008
    Location
    Northern Indiana
    Posts
    903

    Default

    Quote Originally Posted by norty303 View Post
    Ok, I think I'm understanding how the optocoupler works (remember, I'm not an electronicist! Be gentle)
    It can still work under lower voltage levels than the equipment requiring the signal, so could be used to switch power from a supply closer to the head, even if the line voltage had dropped low?
    And you could build a repeater box with multiple optocouplers and it's own PSU to drive further down the line?
    E.g. 12V drops to 7.5V over a 70m line which might be too low for the laser driver, but ok for the optocoupler.
    The optocoupler modulates the ongoing 12V from the repeater box another 30m down the line?
    Think of the optocoupler as an isolator. It is just a LED shining on a phototrasistor. No electrical connection inside. It is commonly used for level shifting and isolation.

    You have 2 possible problems in the 3 wire send cable. First, the voltage drop not supplying the driver with enough voltage. Second, the voltage drop changing the voltage level of the TTL ground. Lets say at the end of the 70m line the voltage is 8V from a starting 12V. At the starting point the ground is 0V and hot wire is 12V, but at the end the ground is now +2V and the hot wire is 10V referenced to the starting point ground. Because the TTL line pulls little current it will still be around 5V referenced to the starting ground. So, at the head end the ground is fluctuating between 0 and +2V referenced to the TTL line which is 0-5V. It could freak the TTL input out because the fluctuating ground. If you put the opto before the TTL input at the head it will turn on/off as long as the input exceeds the turn on voltage of its LED. The opto output is like a switch that is isolated from the fluctuations of the ground wire. When the opto output is hooked to the fluctuating ground it "rides" the fluctuations providing a better 0-5V signal to the TTL input. Basically it provides a new ground reference at the head end. Also, it is very important to maintain isolated grounds on the 12V system. The 12V ground should not be earthed especially at the head end.

    Make any sense? Kinda hard to explain without pictures.

    Ask the laser module manufacture what voltage the "TTL" line takes. It might be 0-12V not the standard TTL 0-5V.

    I overlooked something when thinking about the LED driver used to create TTL PWM signal. The output will be inverted. This could cause some grief.

  7. #17
    Join Date
    Feb 2008
    Location
    East Sussex, England
    Posts
    5,174

    Default

    Thanks Chris - a bit of that made sense!
    And it had also occurred to me that the TTL input could be TTL+ or TTL- too.
    I think the test unit will reveal this info, and I would trust it more than the responses I might get from the manufacturer.
    http://www.facebook.com/SubsonicSystems
    http://www.frikkinlazors.co.uk

    You are using Bonetti's defense against me, ah?

    I thought it fitting, considering the rocky terrain.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •