Help with PWM Circuit

Discussion in 'General Electronics Chat' started by blinky, Sep 24, 2010.

  1. blinky

    Thread Starter Member

    May 25, 2009
    15
    0
    helllo folks,

    I currently use a PWM circuit to control some home made heater bands that go on telescope optics to stop the formation of dew. Now i have noticed something strange happening when I use the bands on full power - I appear to get a voltage drop of a few volts from my 20 amp 12v PSU. I know I am drawing nowhere near 20amps but it is the PWM circuit that causes this. The circuit is bought from here
    http://www.edutek.ltd.uk/Circuit_Pages/LampDimmer.html
    And I am wondering if changing the value of C1 to something higher - say 100uF would resolve the issue?
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Is your 20A power supply regulated?

    Regardless, changing C1 isn't going to have any effect unless the power supply itself isn't filtered - even then 100 uF is a drop in the bucket trying to filter a 20A supply.
     
  3. blinky

    Thread Starter Member

    May 25, 2009
    15
    0
    Yes, it's a regulated psu and I have tried with another regulated unit as well. I just cannot understand it as I am sure even on max power that I am only drawing 6 amps max.
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    It sounds like your powersupply isnt as well regulated as advertised.

    Either that, or your output caps are failing.
     
  5. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    First you would need to do the following test to isolate the problem area.

    Connects the heater bands directly to the output of the regulated power supply along with a voltmeter.

    If the supply voltage does not drop, then the PWM unit is definitely causing interference to the power supply circuitry to the point of affecting its voltage regulating function.

    If that's the case, then a filter must be added between the power supply output and the PWM unit. From the PWM unit webpage, it looks as if the PWM frequency is about 2.7KHz.

    If the output voltage drops, then you maybe exceeding the output current capability of the power supply and only a new supply will fix the problem.
     
  6. blinky

    Thread Starter Member

    May 25, 2009
    15
    0
    OK, I will try and connect the bands straight to the PSU. I am positive that I am not overloading the PSU though, it's 20amps - no way am I producing 240watts of heat!
    What sort of filter would I require?
     
  7. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    How sure are you of your powersupplys rating?
     
  8. blinky

    Thread Starter Member

    May 25, 2009
    15
    0
    Well the first one was from Ebay and is a 10amp regulates unit - the type used to power CCTV Camera's. The new one I bought, because I thought the first one might be suffering from bad regulation is a 20amp unit and is intended as a supply for a battery charger or to power amateur radio's, it also claims to be a regulated unit. The new PSU has a current display and when I have the bands on full power is shows only 6.5 amps being drawn (That 6.5 amps also contains the current used to drive the telescope electronics and drive motors, as well as a TEC Cooler for a CCD camera). I know that the current display might not be that accurate but I assume since it's showing 6.5amps being drawn that it should be nowhere near the capacity of 20amps!
     
  9. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Do you have a make and model for the supply?
     
  10. blinky

    Thread Starter Member

    May 25, 2009
    15
    0
    This is the first 10 amp PSU
    http://cgi.ebay.co.uk/10-Amp-CCTV-P...?pt=UK_CCTV&hash=item2a08c44b07#ht_3717wt_911
    This is the new 20 amp PSU
    http://cgi.ebay.co.uk/ws/eBayISAPI....6142468&ssPageName=STRK:MEWNX:IT#ht_500wt_928

    Another thing......
    I first noticed the low voltage because when my telescope mount was slewing full speed an led started flashing to indicate low voltage. Now this was with the 10amp supply and I measured the voltage - sure enough when slewing full speed with the dew bands on full heat the voltage dropped from 13volts to 11.5volts. I thought this indicated that I was drawing more than 10 amps and the voltage was dropping off. Now I have the same issue with my 20amp supply, although I have not measured the voltage drop yet (today's job!) I cannot believe that if I was trying to draw more that 20amps from a 10 amp supply all that would happen is it would drop the output voltage by a couple of volts. I am wondering if there is some strange interaction between the PWM circuit of the dew controller and the drive electronics for the mounts stepper motors?
     
  11. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    You only need to test that for a few seconds until the current is stable.

    Probably an LC filter but the component values depends on the current of the heater.

    This is primary used to isolate the current pulses of the PWM unit from affecting the voltage sensing circuit of the power supply.

    I have not used any such filter before so maybe others can offer their valuable experience in this area.
     
  12. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
  13. blinky

    Thread Starter Member

    May 25, 2009
    15
    0
    Are you sure? If so then that's fine - I can use one output solely for the dew controller and the other for the telescope mount! I just assumed it had 2 sets of plugs on the front but internally they were connected to a single output?
     
  14. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Its not fine.

    More like dual output "socket" power supply.

    Power off the power supply from mains and measure resistance/continuity between the two red sockets. There will be 0Ω between them.

    Best still, open the cover and show us an image of the inside. You can also look to see if there is a short wire link between these two red sockets yourself.
     
  15. blinky

    Thread Starter Member

    May 25, 2009
    15
    0
    Just checked with a meter and it's a straight short between the 2 outputs - 0 ohms
     
  16. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    It was just a guess/suggestion.

    What is the voltage the heater and the scope motors are supposed to operate at?
     
  17. blinky

    Thread Starter Member

    May 25, 2009
    15
    0
    Everything is supposed to work off 12volts. I am about to go and connect the bands straight to the 12volt supply - I will then be able to see if there is any voltage drop. Also as there will be no PWM circuit in the loop the current reading on the ammeter display will be a true reading for the bands.
     
  18. blinky

    Thread Starter Member

    May 25, 2009
    15
    0
    Hmm.... The plot thickens!
    Just tested things - each band is 10ohms which equals 1.2 amps each band or 2.4 in total
    On the PSU display this is fine - I see 2.4amps drawn and 13.8volts
    I power on the telescope mount and slew the motors to get some current being drawn and guess what - all seems fine! The voltage stays at 13.8volts and does not drop and the low voltage light does not illuminate either! I also connected the bands back to the dew controller PWM circuit as well. Now the only other things I had connected to the 12 volt supply last night was the peltier cooler for the CCD Camera - thats rated at 60 watts or 5 amps so I reckon I should be drawing the following:

    Dew bands on full power 2.4amps
    Mount on full slew (both motors) 2 amps
    CCD Camera cooler 5 amps
    So on total thats just under 10 amps - I am now at a loss to explain why I had the low voltage light on last night!
    Just going to power up the camera and see what the current draw increases to......
     
  19. blinky

    Thread Starter Member

    May 25, 2009
    15
    0
    With everything connected up it draws more current - though not 10 amps. The voltage stays on 13.8 volts but the low voltage light on the mount illuminates, wierd! I guess I just ignore it?
     
  20. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Umm... you could.

    If the light comes back on, be sure to check the voltage at the supply with the meter.

    If the low voltage light comes on, but the supply still says 13.8v, then it may be a faulty voltage detection circuit or loose wire.
     
Loading...