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Author Topic: Using Bypass Capacators as Filters  (Read 611 times)
VE3XKD
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Posts: 51




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« on: December 16, 2006, 09:32:43 AM »

Hi All

Some of you are aware that I have been tackling HF noise issues in a Toyota Prius Hybrid for the last year or so. I am about to obtain a used but serviceable Prius High Voltage DC-DC converter (Toyota calls it a Synergy Drive) and investigate filtering out the tremendous HF noise levels that it radiates. This is the source of the most annoying  noise that radiates from the Prius. If I can filter out the noise using bypass capacators inside the box I might be able to reduce the noise at source, rather than trying to shield it after it exits and radiates throughout the wiring harness of the Prius. Believe me when I say it radiates like mad on ALL wiring harnesses. I have wrapped many of the  wiring harnesses in copper foil, bonded and grounded and still the noise on 40 and 80 is bad.  I have heard from an experienced RFI engineer that I need to bypass each  DC HV line in the converter with  0.1Ufd paralleled by a 0.01Ufd Capacator in order to filter/reduce HF noise spikes carried by the HV DC lines. The noise from the DC-DC converter starts at 2 Mhz, and has a general peak from 3.5 to 13 MHz then drops off. I was wondering if anyone has any experience with such solutions. Are HV (2 KV) disk ceramic capacators suitable? The Converter runs at about 6oo volts. Any and all comments appreciated!

73

John

VE3XKD
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N1QKH
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Posts: 27




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« Reply #1 on: December 17, 2006, 06:58:38 AM »

There are capacitors made for bypassing DC lines as they pass through a chassis or bulkhead. These have leads at both ends (connected to each other, internally; the case of the capactor is the other connection. Many times the case is threaded and has nut and star washer to contact the chassis. Military surplus vendors would be a good place to look for these.

73 Don
N1QKH
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N1QKH
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Posts: 27




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« Reply #2 on: December 17, 2006, 08:46:13 AM »

Take a look at this website. I have no financial interest in these guys. Your mileage may very.
http://www.surplussales.com/Feedthrus/FTflangeMt.html

Note: If you go with military surplus parts, stick with capacitors made in the last 20 years and test them BEFORE installation. Old paper insulated caps from WWII are not a good choice.

73 N1QKH
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N1QKH
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Posts: 27




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« Reply #3 on: December 17, 2006, 11:38:30 AM »


To address your original question: using a large capacitor (usually electrolytic which is good a low frequecies and poor at high frequencies) in parallel with a small capacitor (usually ceramic, silver mica or monlithic which are good at high frequencies but become physically too large when large values of capacitance are needed)is and old and successful strategy. It is the most ecconomical approach. However, the inductance of the capacitor leads becomes an issue when, you are dealing with fast-ristime transients (switching spikes). You can try the 2KV ceramic disk caps but, it may not produce as much attenuation as you need. This would be a cheap way to test the idea. The coaxial-caps, mentioned in my other two posts, are better in this kind of situation.

73 N1QKH
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VE3XKD
Member

Posts: 51




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« Reply #4 on: December 17, 2006, 04:44:52 PM »

Hi

I can't use coaxial caps as the Hybrid Synergy drive requires to pass large amounts of current and there is no room to install co-axial capacitors. I therefore can only use bypass capacators. I need to install 16 bypass capacitors. If I need to attenuate 3.6 MHz and 7 MHz noise, what value and type of capacitor is recommended?

73

John

VE3XKD
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N1QKH
Member

Posts: 27




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« Reply #5 on: December 17, 2006, 07:23:09 PM »

I am not sure that, I understand how a Toyota Prius works. This would be a very good time to have a schematic and see how the motor drive is referenced to vehicle chassis ground. Adding caps to DC lines is one thing; adding caps to lines with a chopped square wave is more complex. I don't think the Prius has a simple DC motor because I don't recall seeing anything that looked like a commutator in the Toyota marketing literature. Is this impression correct?

73 de N1QKH
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VE3XKD
Member

Posts: 51




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« Reply #6 on: December 18, 2006, 04:33:39 AM »

Hi

Right now I am trying to tame the charging circuit for the NiMh battery pack in the Prius. The battery lines comming from the DC-DC inverter radiate huge amounts of rfi. I've tried shielding the entire cable system, added ferrites, etc, but nothing cures the noise as well as I'd like. Though I've gone from 10 to 20  over Nine noise levels, the noise is still S6 on 40 and I'd really like to get it down to something like S2 thus I thought to experiment with better bypassing inside the DC-DC inverter. This means buying a spare (used) inverter box and opening it up, trying different bypass techniques and then swapping the box in and out of the vehicle.

73

John
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N1QKH
Member

Posts: 27




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« Reply #7 on: December 18, 2006, 06:12:33 AM »

Hi,

What year is your Prius? I need to surf around the Internet and gain a better understanding of how this works. I would like to understand if the added caps will introduce some new (and expensive) problem. Also, I need to get some idea of what the current through the caps would be. We don't want any caps to get hot and burn up out on the highway in the middle of February.

73 Don N1QKH
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VE3XKD
Member

Posts: 51




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« Reply #8 on: December 18, 2006, 07:48:46 AM »

Hi

The best description I have been able to find, along with partial schematic diagrams, of the DC-DC inverter/converter, is at the following URL:

http://www.osti.gov/bridge/servlets/purl/890029-WIfqPO/890029.PDF

From the pictures, it appears as if some form of bypassing or snap-on ferrites could be added inside the DC-DC converter box.

Cheers

John
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KC8VWM
Member

Posts: 3124




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« Reply #9 on: December 18, 2006, 10:17:54 AM »

OK, allow me to play the devil's advocate on this question for a moment.

Why mess with the electrical system in the first place? Those vehicles are connected to all sorts of electrical sensors to monitor the many checks and balances of the vehicles electrical system.

Perhaps installing a capacitor in the electrical system will throw off the equilibrium of the vehicles electrical monitoring system?

Is it possible to install another separate battery which is completely independent of the vehicles unique electrical & charging system to operate your radio equipment?

73
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VE3XKD
Member

Posts: 51




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« Reply #10 on: December 18, 2006, 12:55:06 PM »

Hi

The problem with the Prius is that the DC-DC converter generates tremendous RF noise that interferes with mobile HF radio.  I have extensively shielded the converter, all cables going to and from it, bonded it to chassis ground, etc. Rnfortunately the noise radiates along all of the wires, and eventually leaks out. I was hoping that opening up the DC-DC converter and adding in some correct values of bypass capacitors or ferrites would reduce the noise at source sufficiently.

Cheers

John
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KB1NCP
Member

Posts: 10




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« Reply #11 on: December 21, 2006, 10:30:42 PM »

Hi, I had a similar problem with my company work vehicle. We have computers and printers on board and one of the two problems I have solved. The computer runs on 24 volts and has a rather small inverter. Grounding by means of an extra wire run from a ground terminal on the board of the inverter only helped a little. What I found that cured my problem was to only run a hot wire to the battery, and run a short and seperate ground from the radios fused cord set to the chassis. My other problem is that the printer is really noisey. So I only turn it on when I need to print, otherwise I turn it off. When I ran both the hot and ground to the battery directly the RFI was intolerable. By putting only the hot to battery, and getting the ground from the chassis worked for me from S9 to gone. Try it.
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