DC blocking capacitors voltage rating

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.

--
Cats, coffee, chocolate...vices to live by

Search Google for "galvanic capacitor".

Also:

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?

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.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

Or buy three .047uf 100V and put them in parallel.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

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.

Not obvious what you are trying to block, but first off, don't use the boat as a power negative return. Run two wires from the Icom to the battery. Now, if you want to connect the ground connector on the back of the Icom to some kind of boat ground, you could put a capacitor in that line. The capacitor will be is series with several feet of wire; so, the characteristics of the capacitor are pretty much moot. I would look for a 200 - 300 V Mylar cap, or the like.

Tam

The article Marine Grounding Systems,

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.

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

Yes alright, I understand what you=B4re 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=B4ll 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.