Post by thread:[EE:] EMI Filtering for Linear Power Supply

Hello all, I am designing a 12V linear power supply and now in the final stage I see I have some problems with the EMI (or so I guess). I have a circuit powered from this supply and when the refrigerator in my room starts, it resets the circuit. In your oppinion, is the EMI the cause of this problem ? What could I do to overcome it ? I hope I have explained it clearly. Thank you in advance for your answers. Lucian -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

At 11:25 PM 5/11/2004 +0300, Lucian wrote: >Hello all, > >I am designing a 12V linear power supply and now in the final stage I >see I have some problems with the EMI (or so I guess). I have a circuit >powered from this supply and when the refrigerator in my room starts, it >resets the circuit. >In your oppinion, is the EMI the cause of this problem ? What could I do >to overcome it ? >I hope I have explained it clearly. Thank you in advance for your >answers. You might not have enough transient rejection on the supply side, but this isn't an EMI problem. The question is, what happens on your 12V output, when the fridge switches, and what happens on the high side of the regulator at the same time.? You may need a larger input cap, if the fridge causes the input voltage to sag. I've always tried to design for an input range of 70 - 140VAC for a nominal 120V input. Any tighter than that, and you start seeing problems in the field. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

At 12:37 AM 5/12/2004 +0300, Lucian wrote: >The input is of 220V, Just scale the numbers then. >and I don't know for sure what happens when the >fridge switches, but I do know that there is also a problem with my >computer speakers - a short interruption in the sound playback - when >the fridge switches. >I will try with a larger input capacitor. It's probably a sag as the motor inductance is placed across the line. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Thank you for your reply. I will try with a larger capacitor. Lucian -----Original Message----- From: pic microcontroller discussion list [spam_OUTPICLISTTakeThisOuTMITVMA.MIT.EDU] On Behalf Of David VanHorn Sent: 12 mai 2004 01:03 To: .....PICLISTKILLspam@spam@MITVMA.MIT.EDU Subject: Re: [EE:] EMI Filtering for Linear Power Supply At 12:37 AM 5/12/2004 +0300, Lucian wrote: >The input is of 220V, Just scale the numbers then. >and I don't know for sure what happens when the >fridge switches, but I do know that there is also a problem with my >computer speakers - a short interruption in the sound playback - when >the fridge switches. >I will try with a larger input capacitor. It's probably a sag as the motor inductance is placed across the line. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Is it when the fridge switches on or off? If when switching on, then it could be voltage sag as the motor starts, or a voltage spike when the start windings are disconnected a few hundred mS later. If it happens when the fridge stops as well, then it is almost certainly a spike as the run windings are disconnected. But don't discount a spike at startup as the start windings will draw more current and the switch is near the motor, compared to the thermostat mounted inside the fridge itself (for the turn off). A larger cap will help fix a voltage sag and may change the spike behaviour. If it is spikes on the mains, then it is better fixed at the source - if you are prepared to attack your fridge , or try adding a spike supressor between the wall socket and the fridge plug. But regardless, you don't want your regulator upset by external events so I guess if you can sort it out 100% at that end it may be desirable also! I guess I'm suggesting you use the fridge to sort out the regulator problem & then use the speakers to sort out the fridge problem! Richard P At 12:37 AM 5/12/2004 +0300, Lucian wrote: >The input is of 220V, Just scale the numbers then. >and I don't know for sure what happens when the >fridge switches, but I do know that there is also a problem with my >computer speakers - a short interruption in the sound playback - when >the fridge switches. >I will try with a larger input capacitor. It's probably a sag as the motor inductance is placed across the line. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Do the lights in the room dim when the fridge kicks on? if you have a decent multimeter, check and see if the voltage sags when the fride kicks on. I'm not sure how to fix that, but finding the problem is the first part of the soluton. Lucian <lucifferKILLspamHOME.RO> wrote:Hello all, I am designing a 12V linear power supply and now in the final stage I see I have some problems with the EMI (or so I guess). I have a circuit powered from this supply and when the refrigerator in my room starts, it resets the circuit. In your oppinion, is the EMI the cause of this problem ? What could I do to overcome it ? I hope I have explained it clearly. Thank you in advance for your answers. Lucian -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads --------------------------------- Do you Yahoo!? Win a $20,000 Career Makeover at Yahoo! HotJobs -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

What about running the circuit off a battery supply that is charged with your typical supply. Then you don't have to worry about spikes per se, and you have a backup system. Lucian wrote: >Yes, you are right, but I woudn't like to risk that in other >environments, this source resets again and I want to know the cause of >this behaviour. It is a source for an alarm system and it is not allowed >to reset. I will try first with a larger capacitor, even if it is now of >3300uF and I consider it big enough... > >Lucian >{Original Message removed}

> The input is of 220V, and I don't know for sure what happens when the > fridge switches, but I do know that there is also a problem with my > computer speakers - a short interruption in the sound playback - when > the fridge switches. I will try with a larger input capacitor. You need to filter the fridge. It takes >15A surge on start and puts a ~1kV spike on the mains when it switches off. There is nothing wrong with your psu. Just the fridge imho ;-) Peter -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

When the frigde STARTS the line voltage DROPS. When it STOPS it produce spike and EMI. As other say, desing so it accept lower input voltage /Morgan Lucian 22:25 2004-05-11: >Hello all, > >I am designing a 12V linear power supply and now in the final stage I >see I have some problems with the EMI (or so I guess). I have a circuit >powered from this supply and when the refrigerator in my room starts, it >resets the circuit. -- Morgan Olsson, Kivik, Sweden -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Ok, I admit, my fridge has a problem :) So this is not an EMI issue, but a spike one. Ok, the solution I found is to insert an 100nF capacitor between VCC and GND, this will solve the reset problem. I have 2 boards, one with an 100nF cap and one without. The one without the capacitor was resetting, so I will take this as a general rule with my microcontroller boards, inserting the capacitor. As with the fridge, I'm too busy to fix his problem :) Thank you for your kind support ! Lucian {Original Message removed}