I picked up a Knight Star Roamer. It does have a transformer DC supply. I was planning on installing a polarized cord. Do radios with transformer supplies need safety caps, like X or Y types?

Hi, here is some information for you,http://www.justradios.com/safetytips.html, and a good place to buy them also.

Does the radio have capacitors on the transformer primary to the chassis, or across the primary winding?

If so, those caps should be UL Rated safety caps.

Pete

I started reading the link on safety capacitors from JustRadios:



How does connecting a capacitor from the hot side of the power line to an un-grounded chassis "keep you safe?" Seems counter-intuitive. Especially with a transformer-operated radio, the only reason for a line to chassis capacitor would be as a line filter.


I think I read somewhere on this Forum that the name "Safety Capacitor" has nothing to do with preventing shocks. The term refers to the fact that when/if these capacitors fail, they won't connect the line to the chassis via a short?

See page 4: http://www.newark.com/pdfs/techarticles ... 008-01.pdf
Rich

The more I read, the more I feel my original comments are correct.
http://www.seered.co.uk/sunvic_capacito ... mation.pdf
Safety Capacitors have nothing to do with protecting the user from shock under normal circumstances. Their purpose is to act as RFI/EMI filters.

They are called "safety capacitor" due to their resistance to failure and when they do fail, they don't fail as a "short."

I don't use them. I do use (rarely) "brute force" line filters that do contain safety capacitors and inductors. Switch-mode power supplies use line filters, sometimes to prevent EMI generated by the power supply from getting back onto the power lines.

Rich

The safety aspect does protect the consumer, the capacitor fails in an open condition preventing the chassis from being hot. They are also constructed to prevent fires when they fail. Other than those two conditions they are no different than a regular capacitor.

Dave

Exactly. The capacitor protects the consumer after the capacitor fails, by not failing short.

So I don't understand this statement:



I don't see how a capacitor isolates the chassis. It does the opposite: it allows a low level AC current to flow from the hot side of the line and through the consumer, should he touch the chassis.

Rich

As written the line is incorrect, add to the end of the line " in case of capacitor failure"

Dave

My understanding is these were also supposed to be flame proof, and not catch fire.

I could be wrong, since that is based on anedoctal internet info.
Leakage currents should be limited to a safe value if line to ground caps are limited to
.015mFd or less.

Pete

Peter,

I agree. All the "safety" aspects are after the cap fails. Doesn't short and doesn't burn, but it doesn't give any additional protection to the consumer while it's still alive.

So I would use a "safety cap" if I thought I needed a line filter. I wouldn't add a safety cap because it's gong to make me safer somehow.

Rich

I am a strong proponent of using "safety caps" as can be found in many of my posts. This has been a good discussion that finally puts the correct perspective on their use. I just wish I had been the one to sum it all up so succinctly. I believe also that the fact that they are specifically designed and tested for more or less continuous AC exposure makes them less prone to fail in the first place.

There is no caps on the primary side of the radio as is, or in the schematic.

Also, I was under the impression that safety cap fail open and to not burn or flame. Since I have seen 2 radios in the last year with the bumbles bees across the line scorched and chard, I would think that they are a hell of a lot more safe than the old style caps.

Yes, modern line or mains rated caps *are* safer then 50 year old cap technology.
But adding a line cap to a radio that the radio manufacture chose to leave out does not add safety to the radio. It subtracts safety.
The line cap function in a radio is not to provide safety. Not all radios came with line caps. Not all radios need line caps.

Replacing an existing Bumblebee line cap with a safety rated cap is an excellent idea.
Placing a cap across the line (or line to ground) where one did not exist, for no reason other then to add "safety"---foolish.

Safety refers to the fail mode of the cap. It does not refer to the cap circuit function in the radio.

Amen!

Rich

[url]Placing a cap across the line (or line to ground) where one did not exist, for no reason other then to add "safety"---foolish.[/url]

Don't they double as inference suppression?

Also the manufacturers most likely left it out for cost, and because they were not required by the gov't. I am not going to install one in this particular radio. I did get some good info off this thread.

Vinnie,

Yes, a cap across the line can perform noise suppression. That would be it's mission. It's only mission. It is not doing double duty with safety being it's primary mission. Adding this cap where one does not exist, can not and will not add safety to the radio. It would subtract safety. Using a line rated cap will help mitigate some of the safety risk but not all.

I cringe when folks want to add a "safety cap" to a radio--when they think they are engineering a better radio by adding some missing safety. If they want to add a cap for purposes of noise suppression or hum modulation control, fine. I doubt they would notice a difference. But they do it with a reduction in safety not an increase.

"Safety cap" is a poor term. Unfortunately the geenie is out of the bottle and can't be put back in. The primary winding of the power transformer is the most hostile location in a radio for a capacitor. Also often not necessary. Thus why we (I) can be so wordy about the subject. Understand I wasn't addressing you in particular. Just the concept.

Dale,

That is very good info. I always figured it primary use was noise suppression, but I did misunderstand the safety aspect. Anyway, this was a very good thread for novices like myself.

I believe they are refering to the line isolation caps used in AC/DC radios which have "floating" or isolated circuit grounds. The chassis in them have to be bypassed to circuit ground to prevent the various stages of the radio from coupling unintentionally. The alternative is to use the chassis as circuit ground, in which case it is "hot."

In the context of a transformer-operated tube radio, it's more common to see two capacitors, from either side of the line, to chassis. Their purpose is to shunt the inter-winding capacitance to the HV secondary (through its grounded center tap), and protect the insulation on the secondary from the effects of line surges or transients. Since old paper or "bumblebee" capacitors should never be trusted on AC line circuits, Y2 (or XY2) line-to-chassis safety rated caps of the same values as the originals should be used for replacements.