DC blocking capacitors voltage rating

[joost]

Hi all,

I´m having problems on my boat with DC current going through the
ground connection of my HF transceiver. These currents cause all kinds
of (expensive) electrolysis effects on the steel hull and propellor.
Therefore I´m planning to build a DC block for this SSB ground strap.
The article
Marine Grounding Systems, http://www.sailmail.com/grounds.htm,
discusses how to make this and suggests the type of capacitor to use.
It´s the Type X7R Monolithic Ceramic capacitor, 0.15uF, 100 volt.
Unfortunately I can only obtain the 50 volt version in the place where
I am now (Whangarei, New Zealand). Are there any opinions on whether
this will also suffice? I´m transmitting using a 100W Icom 718 SSB
transceiver.

Joost Jager.

 

[Eeyore]

Hi all,

I´m having problems on my boat with DC current going through the
ground connection of my HF transceiver. These currents cause all kinds
of (expensive) electrolysis effects on the steel hull and propellor.
Therefore I´m planning to build a DC block for this SSB ground strap.
The article
Marine Grounding Systems, http://www.sailmail.com/grounds.htm,
discusses how to make this and suggests the type of capacitor to use.
It´s the Type X7R Monolithic Ceramic capacitor, 0.15uF, 100 volt.
Unfortunately I can only obtain the 50 volt version in the place where
I am now (Whangarei, New Zealand). Are there any opinions on whether
this will also suffice? I´m transmitting using a 100W Icom 718 SSB
transceiver.

It's not the voltage rating but the ESR and ESL ( equivalent series
resistance and inductance) that matter.

Do you have those figures for both parts ?

Graham

 

[Ecnerwal]

It´s the Type X7R Monolithic Ceramic capacitor, 0.15uF, 100 volt.
Unfortunately I can only obtain the 50 volt version in the place where

Buy four, and put them in series-parallel. Or buy two 0.3 uF and put
them in series (which is what four 0.15 uF in series-parallel are doing).

Two capacitors of the same value in series have half the capacitance and
twice the voltage rating of the individual capacitors.

Two capacitors of the same value in parallel have twice the capacitance
and the same voltage rating.

Four in series-parallel gets you the same capacitance, and twice the
voltage rating.

Good luck.

 

[joost]

Eeyore schreef:

It's not the voltage rating but the ESR and ESL ( equivalent series
resistance and inductance) that matter.

Do you have those figures for both parts ?

Graham

No, I checked on the digikey product page that is mentioned in the
article, but ESR and ESL are not mentioned

 

[joost]

Do you have an antenna tuner on the IC-718 radio? What kind of RF
grounding plate or system are you using? Sail or power? Steel,
aluminum, or fiberglass hull?

I have an AH-4 antenna tuner. The hull is steel, so I use no grounding
plate. Sailing boat.

All of the RF current goes through these capacitors. It's not so much
the voltage that will kill them, but the RF current. If you have a
fiberglass hull, with the recommended copper tape, you'll need lots of
these caps to distribute the current. If the capacitor ESR is low (it
will be for X7R ceramic), then the dissipation per capacitor is
minimal. Check for heating when you test it.

The required voltage rating is a bit difficult to calculate. If you
have a decent ground plate, the capacitors shouldn't see more than a
few RF volts across them. The rest is to isolate whatever power
source you're using for cathodic protection if it's a metal hull.
Those run at far less than 50VDC. I think you're safe using 50VDC
caps. However, if you're paranoid, just arrange two in series to get
the proper 100V rating. Don't worry about the resultant halving of
the capacitor value. The value is NOT particularly critical and
you'll have lots of them in parallel. Just cram as many extra
capacitors across the copper tape as possible to compensate.

Note that the 0.15uf 100v X7R need not be packaged with leads
attached. If you can get leadless chip versions, they can be sweat
soldered to the copper sheets. Also, the exact value is not critical.
My guess(tm) is that anything from 0.068 to 0.33uF will work. A
mixture of values is also acceptable. Just use lots of them as they
will carry considerable RF current.

Yes alright, I understand what you´re saying. Because the capacitors
let through the high freq, the voltage over them is low. Just as the
voltage over a piece of copper wire in a DC system is low. This makes
it all a lot clearer.

I´ll just go for the 50 volt versions of the suggested capacitors. In
the article I saw four capacitors being used and I assume that worked
fine with the amount of current that they have to carry. One thing
that perhaps may go wrong is that that ESR value of the 50 volt is
different from the 100 volt, but I couldnt find the values in the
information that I saw about these caps.

Joost.

 

[Eeyore]

Eeyore schreef:


No, I checked on the digikey product page that is mentioned in the
article, but ESR and ESL are not mentioned

You're not likely to find these values stated on Digikey's site, you'd need to
download the manufacturer's data sheet.

Graham

 

[Ross Herbert]

:
:
:Eeyore schreef:
:> joost wrote:
:>
:> > Hi all,
:> >
:> > I?m having problems on my boat with DC current going through the
:> > ground connection of my HF transceiver. These currents cause all kinds
:> > of (expensive) electrolysis effects on the steel hull and propellor.
:> > Therefore I?m planning to build a DC block for this SSB ground strap.
:> > The article
:> > Marine Grounding Systems, http://www.sailmail.com/grounds.htm,
:> > discusses how to make this and suggests the type of capacitor to use.
:> > It?s the Type X7R Monolithic Ceramic capacitor, 0.15uF, 100 volt.
:> > Unfortunately I can only obtain the 50 volt version in the place where
:> > I am now (Whangarei, New Zealand). Are there any opinions on whether
:> > this will also suffice? I?m transmitting using a 100W Icom 718 SSB
:> > transceiver.
:>
:> It's not the voltage rating but the ESR and ESL ( equivalent series
:> resistance and inductance) that matter.
:>
:> Do you have those figures for both parts ?
:>
:> Graham
:
:No, I checked on the digikey product page that is mentioned in the
:article, but ESR and ESL are not mentioned


Should be plenty of other sources for a 15nF X7R. eg. Epcos B37981M or F style.

Epcos data sheet for X7R caps
http://www.epcos.com/inf/20/10/db/cc_07/X7R_Leaded.pdf