Snubbing mech relay EMI from inductive load- Need advice on cap to use on the AC side

Comments

• PJAllen Banned Posts: 5,065

  2012-01-18 17:19 edited 2012-01-18 17:19

  Are you using a flyback diode across the relay coil?
• bill190 Posts: 769

  2012-01-18 20:17 edited 2012-01-18 20:17

  Google the words...
  relay flyback diode
• bsnut Posts: 521

  2012-01-18 23:51 edited 2012-01-18 23:51

  I am going to answer your questions.

  My question is: Is this digikey part basically the same spec as the .56uF 400VDC / 200VAC Polyester Film Capacitor? I would assume so. Other than a slightly higher AC rating, it appears to fit the bill.

  That cap will work for what you are trying to do. Remember that the voltage of the cap needs be above the operating voltage for what you are trying to do.

  Are you using a flyback diode across the relay coil?

  If you don't have flyback diode across the relay coil, you will need one. This diode across the load (relay coil or solenoid) protects the transistor from spikes when the load is turn off.
• damage31 Posts: 21

  2012-01-19 06:02 edited 2012-01-19 06:02

  thanks all- yes I am using a diode across the relay coil. I forget the exact part number, but it's a 1N400x series... something i had in my parts bin.
  I am also using a 2n2222 to actually fire the relay coil. I can actuate the relay all day long with no AC load and not have any stamp issues.
  
  
  My problem is specifically when the relay turns off- AND the AC load is connected...
  my stamp will occasionally reset. (similar to what is shown in the video from my first post)
  
  I figure that there must be a EMI spike from opening the relay contacts that is causing my stamp to reset. (but i dont have access to a oscilloscope to confirm the back emi, I just assume it is there)
  
  
  Thus far in my attempt to fix this, I have sprinkled: .1uf caps around my stamp and the 4 other IC's power pins, as well as added a couple of 10uf, as well as a 4700uf just before my 7805 vin ...This has helped reduce the frequency of the stamps resets quite a bit, but not 100%.
  
  
  I am running 3 LTC1257's, 1 DS1302, and the 1 5v relay + the stamp from a 7805 with a heat sink. The 7805 is cool to the touch. My circuits power consumption is very low, i forget the exact spec i measured a few months ago, but it was near or just over 90-100ma with the relay on if i remember right.
• bill190 Posts: 769

  2012-01-19 06:20 edited 2012-01-19 06:20

  Where is the relay physically located?
  
  Near or on the same pcb as the Stamp? If yes, might try moving the relay away from the Stamp and see what happens. Then the AC wires will be away from the stamp.
• damage31 Posts: 21

  2012-01-19 11:49 edited 2012-01-19 11:49

  I'm limited at the moment- currently they share space in a project box, but theres ~1.5 inches between the relay and the stamp.
  
  I think ill try the cap, and if that doesnt work, ill have to break the relay out into a different project box.
  
  But I suspect that even a diff project box wont help till i can control the relay emi...
• Mickster Posts: 2,757

  2012-01-19 11:58 edited 2012-01-19 11:58

  Is it out of the question to use a "zero-volt-switching" Solid State Relay?
  
  Maybe a much smaller version of this:
  
  http://www.omega.com/pptst/SSRL240_660.html
  
  Mickster
• damage31 Posts: 21

  2012-01-19 12:28 edited 2012-01-19 12:28

  I did some reading and the best I can tell is that highly inductive loads are not recommended for 0 cross SSR because the current lags the voltage, so the reverse breakdown is still an issue, and could destroy the SSR.
  
  0 cross is a good choice for resistive loads
• Beau Schwabe Posts: 6,569

  2012-01-19 13:03 edited 2012-01-19 13:03

  Years ago I had a similar issue controlling a 240VAC industrial vacuum system.... BS2 controlled a DPDT 30Amp relay.
  
  I found that the reset line to the BS2 was sensitive to the equipment noise, especially since I had about a 30 foot umbilical link acting as an antenna between the BS2 and the PC.
  
  While a capacitor works well for inductive loads that are AC, and a diode works well for inductive loads that are DC, my solution was to 'harden" the reset-ability of the BS2 by tying a 1k resistor from the ATN (pin 3) of the BS2 to Vss (pin 4).
• Mickster Posts: 2,757

  2012-01-19 13:04 edited 2012-01-19 13:04

  damage31 wrote: »

  I did some reading and the best I can tell is that highly inductive loads are not recommended for 0 cross SSR because the current lags the voltage, so the reverse breakdown is still an issue, and could destroy the SSR.
  
  0 cross is a good choice for resistive loads

  
  I don't know where you got that from but.....
  
  http://www.opto22.com/site/pr_details.aspx?cid=4&item=OAC5
  
  
  Mickster
• damage31 Posts: 21

  2012-01-19 13:24 edited 2012-01-19 13:24

  that opto22 SSR is different than the couple I saw on digikey- i forget off hand who's SSR i was looking at, but i think it might have been a crydom
  
  that opto22 one looks interesting: "Turn-off Time 1/2 cycle maximum-zero current"
  
  I'll have a peek into it so see if i can get more info.
• Mickster Posts: 2,757

  2012-01-19 13:32 edited 2012-01-19 13:32

  They've been around for decades, actually. We switched to the Potter Brumfield version because they were significantly lower in cost but they didn't have the OPTO-22's retaining screw. We drove solenoid powered hydraulic valves and never had an issue. We have since switched to DC.
  
  Mickster