Title: RF transitting capacitors - current ratings at different frequencies Post by: Opcom on April 10, 2020, 12:30:26 AM I made up a spreadsheet to help me figure out how much to derate a "RF transmitting" type doorknob capacitor that gives a rating of 15A @ 30Mhz, to use on lower bands.
Does this make any sense or are there factors I missed? I don't know so much at all, but believe they have to be derated for lower frequencies. The output of the sheet gave this chart. The chart has two caps. 470pF, 15A @ 30MHz 1000pF, 25A @ 5Mhz. The green column shows the derated current at a given frequency. The blue column is just the spec of the cap in question. Lines 1-4 show different currents allowable to lower frequencies for the first cap. Lines 5-9 show different currents allowable to lower frequencies for the second cap. (ignore the lines 10-12 - the current was calculated to be higher at frequencies above th4 cap's specified rating. I don't think that's appropriate. Title: Re: RF transitting capacitors - current ratings at different frequencies Post by: Steve - K4HX on April 10, 2020, 11:17:50 AM What was the derating formula or factor?
Good stuff. Title: Re: RF transitting capacitors - current ratings at different frequencies Post by: W1ITT on April 10, 2020, 11:46:45 AM I am guessing that you are talking about doorknob capacitors of the Centralab type. I see that Comet vacuum capacitors have a similar derating curve and it appears that heat generated by losses is the limiting factor. They go so far as to make water cooled vacuum capacitors!
This leads me to believe that if we, in common amateur fashion, wish to push the ratings a bit, some sort of cooling might be advantageous. While we probably wouldn't use a blower in an antenna matching unit, it might help to arrange that any doorknob capacitors in a transmitter be placed where the cooling air could get on them. If nothing else, this would at least provide a bit more safety factor. https://www.comet-pct.com/getmedia/47fcbd2f-1729-4ffe-800e-14b62c58561d/SB-44_Current_Limits_for_COMET_Vacuum_Capacitors.aspx 73 de Norm W1ITT Title: Re: RF transitting capacitors - current ratings at different frequencies Post by: K8DI on April 10, 2020, 12:11:18 PM Interesting stuff. I get that as frequency decreases, heating loss goes up due to the reactance. The Comet data show a knee at varying frequency where the current rating also drops as frequency increases, which I’m finding counterintuitive. Anyone know why it has a break over and what influences it?.
Ed Title: Re: RF transitting capacitors - current ratings at different frequencies Post by: WD5JKO on April 10, 2020, 01:28:56 PM Interesting stuff. I get that as frequency decreases, heating loss goes up due to the reactance. The Comet data show a knee at varying frequency where the current rating also drops as frequency increases, which I’m finding counter intuitive. Anyone know why it has a break over and what influences it?. Ed Take a .01uf ceramic disk, and wrap the leads around a Grid Dip Oscillator coil and tying the ends together. You will see a resonance around 7 Mhz. This is a resonance where Xc = Xl of equal amount and opposite sign. That leaves just the ESR, effective series resistance. The ESR is what gets hot from I^2*R loss. The charts in the Comet Word Doc seem to show the series resonance point at the slope change up top, or close to it depending on lead length. Above series resonance, a capacitor is inductive. Perhaps the Max current decline from there is due to skin effect losses increasing with frequency? Edit: Has more to do with the inductive reactance as frequency rises above resonance. Jim Wd5JKO Title: Re: RF transitting capacitors - current ratings at different frequencies Post by: Tom WA3KLR on April 11, 2020, 12:13:23 PM (This is what happens when I have time on my hands.) Attached is a photo of a ceramic transmitting cap I have. It has 3 current versus frequency ratings on the nameplate and I have wondered about this situation too. This is bigger than a ‘doorknob’ type, the round ceramic body about 3 ¼ “ diameter.
I think this is the only cap I have like this and it has been lying under the workbench for many years. Apparently at one time probably more than 10 years ago I did some measurements, and some are calculated, I don’t recall how I came up with the series resistance. I think I was starting to investigate the seemingly counter-intuitive ratings. Most of the name plate may not be legible in the photo unfortunately. Here is what it says: Aerovox Type 1960-304 mica capacitor .003 Mfd. 7000 Volts 60 C ambient 20 Amps at 3000 kcs 16 Amps at 1000 kcs 10 Amps at 300 kcs. Also showing is the paper tag I made stating I measured 2950 pf at 1 kHz. Somehow I measured the self-resonance at 22 MHz. This calculates to 18 nH series inductance at 22 MHz. Also stated is the series resistance of less than 1.2 milliOhms at 3. 9 MHz. Again, I don’t recall how I did this measurement. I guess we would need a ceramic capacitor engineer to explain everything but here is my take. Basically the part is made of thin metal layers and ceramic dielectric. No rating can be exceeded. There is a voltage breakdown rating, maximum current determined by the cross-section of the metal layers & number of layers, maximum temperature reached based on the acceptable maximum of each of the materials used. Temperature rise from internal dissipation plus ambient also gives a limit. Just an intuitive guess, I think this big flanged part could handle 50 – 100 Watts at 60 C. I’m just throwing together some numbers that may be needed to figure what is going on. Margins placed on ratings. I would model the part as, between the two end flanges, the series resistance which I presume in this case is not frequency dependent, next in series - the series inductance - not frequency dependent, then the capacitance – perhaps slightly frequency dependent, and the dielectric loss which is frequency dependent as a resistor in parallel with the capacitor. Reactance in itself is lossless. Reactance goes up as the frequency goes down. Dielectric losses increase as the frequency increases. 20 Amps with 1.2 milliOhms series resistance gives 0.48 Watts. The dielectric loss I don’t know, I don’t think it would be too many times this? AC voltage developed across 3000 pf at 3000 kc and 20 Amps = 354 V rms, 500 V peak. AC voltage developed across 3000 pf at 300 kc and 10 Amps = 1768 V rms, 2500 V peak. My only guess is that the limiting factor is actually the voltage across the capacitor and they are adding dc voltage and margin? My conclusion, current-limit at 3000 kcs, voltage limit at 300 kcs. Just my guess. Perhaps someone like broadcast engineers Tim WA1HLR or Jamie N2VJ has an idea on this. Title: Re: RF transitting capacitors - current ratings at different frequencies Post by: Tim WA1HnyLR on April 12, 2020, 12:15:59 PM Hi all I got an E mail from Tom,KLR Re: RF handling of capacitors in RF service. In a way comparing mica to ceramic capacitors is like comparing apples to oranges. In the case of a mica condensher. , the ratings plate indicates an increase of current at progressively higher frequencies. as you can see the curve flattens out about the 20 A rating on the .003 Mfd cap. Micas are not used in many HF operations above 7 Mhz except for the much smaller types such as the molded postage stamp type and dipped silver mica's which have a low ESR compared to some ceramic designs. The physically large construction of the ceramic cased transmitting capacitors. will exhibit resonances that my wind up in the area of interest. It would be best to do the grid dip osc with a metal strap across the cap to see if there is a potential problem. It will be noticed that the smaller mica transmitting capacitors are rated to higher frequencies. It is also dependent on what purpose you are looking for. The large ceramic cased mica's will work well in 160 & 80/ 75 meter applications in the tank circuit such as padding the PA tuning,and addition capacitance needed for loading. A much larger 300Pf cap in a bakelite type case perhaps @5Kv makes a good permanent replacement in a Viking Valiant in place of that troublesome cluster"F" of small mica caps used for the 160/80 meter tank padder. Ceramics on the other hand behave differently. A typical 500Pf 20 Kv TV door knob capacitor does exhibit a high ESR and will not tolerate much RF current as I found out. Case in point: early experience with using 2 500 pf 20 Kv door knobs is series
as the replacement in a Valiant. It appeared to work at first but I noticed when making an old buzzard transmission the PA plate current kept going up as the the capacitance value kept going down. I had to re-dip the plate frequently. I felt the caps after obviously going into standby. The were quite hot. . These caps are ok for medium powered transmitters like a single 813. I have had large ceramic caps that were like TV door knobs on steroids , 2000 Pf @ 30Kv blow up and catch fire in my 4-1000 transmitter years ago. Only a very large RF rated 500Pf 15 Kv was used did the problem end. I am not sure how well a large ceramic cased mica would have handled 10 meters. There are now a number of good RF rated ceramic caps that are out the such as the Coment brand that Norm W1ITT mentioned. The construction is like that of a shopping cart wheel. Once again it depends on your application. when in doubt,always overkill. Tim WA1HnyLR Title: Re: RF transitting capacitors - current ratings at different frequencies Post by: Opcom on June 14, 2020, 12:33:51 AM What was the derating formula or factor? Good stuff. I missed getting back to answer the question. the chart was based on the VAR calculated from the capacitor's current rating @ MHz as stated on the capacitor. It turned out to be the same as the very simple concept of derating the current by % decrease in frequency. The VAR is what I believe heats the capacitor due to current, and the caveat is that as the frequency goes down, the voltage across the cap will go up, so be careful of the voltage rating of the capacitor. However, I can see from other replies that the chart is not necessarily to be trusted. It is not for "DC filter" doorknobs as from old TV sets and laser power supplies, but was intended to be for 'solid' RF-rated caps like the micas and the RF doorknobs. I can see from the good information in the other posts that the chart is not reliable and shouldn't be used. That is a good answer. Title: Re: RF transitting capacitors - current ratings at different frequencies Post by: VE7RF on June 14, 2020, 01:22:48 PM You can obtain the RF current ratings for any TX doorknob cap, from HEC (high energy corp) website...for any frequency.
I typ will use the HT-50/58 series for some applications....and the larger HT-57's for plate block caps. The...'go to' cap in the HT-57 series is the 200 pf unit.. rated at 15A. Typ 4-8 are used in parallel for plate block caps on the metal tubes. Sometimes the even larger HT-59 caps are used for plate block service. The ..'go to' cap in the HT-59 series is the 250 pf unit, rated for 35.5A. The current flowing through any plate block cap is substantial at higher freqs, like 18-60 mhz. I just finished designing a YC-179 6M amp for a fellow, and with a modest 4.5 kv under load, on 50.125 mhz, a whopping 44A flows through the plate block cap assy. 4 x HT-57's in parallel are used, all are 200 pf units. Download the HEC catalog here. http://www.highenergycorp.com/High%20Energy%20Corp.%20Ceramic%20Capacitors.pdf It will list max current vs freq (1 khz up to 100 mhz) for every cap in their catalog..in graph form. Starting on page 36, the theory is in great detail. Knowing the cap value, the XC for a given freq is easily calculated. Knowing the RF current flowing through the cap, the AC V drop across the cap is easily calculated..( typ barely minimal for a plate block cap at upper HF). V drop = XC X current. Peak Ac V will be 41% higher of course. What HEC calls....'DC bias', hams call.. "B+". B+ and the peak AC Vdrop across the cap, can not exceed the caps V rating. Calculating VAR power at any freq is also a simple matter. ( RMS V drop across cap X current through the cap) All of this is already done for you on most, but not all caps they list in their catalog. The graphs for each cap value will graph current, voltage, and VAR. Be careful, selecting a cap is going to be for a specific application. IE: if say padding a cap for a tuned input..or padding a load cap on the lower bands, typ current ratings of the HEC series caps will be lower. Padding caps are critical, since you don't want them to drift, since it's part of a tuned circuit. Plate block caps are coupling caps, drift is a non issue....ditto with bypass caps. Typ plate block cap assy won't handle as much current on 160 + 80m..vs 10m.... but this is a non issue. There is very little current flowing through a plate block cap on the lower freqs. Centralab caps have virtually identical specs to HEC caps. HEC HT-57 series caps are the same as centralab 57 series etc. Jim VE7RF Title: Re: RF transitting capacitors - current ratings at different frequencies Post by: VE7RF on June 14, 2020, 02:20:22 PM As a supplement to my previous post, calculating the RF current through a plate block cap at a given freq is as follows. AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
On upper HF freqs, like 15-12-10-6m, the tube C on a GG triode makes up most of the required C1 tune cap value for a PI, or PI-L output tank. The tube C is the C between the anode and the grid (grid bonded to chassis). The tube C is directly in parallel with the C1 tune cap. Problem is.... the plate block cap resides between the anode..and the hot side of the C1 tune cap. On 10m band, think of it as the tube C... plus a tiny variable padder(C1).. in parallel. All the RF current that flows from anode to grid..now also has to flow through the plate block cap. On the lower bands, like 160-40m, the tube C is only a very small portion of the total tune C required. Xc of the tube C is higher on lower bands. Xc of the tube is lower, on upper HF bands. To calculate the RF current flowing through the tube C, (and also the plate block cap) is just RF voltage / XC. (rms) RF voltage is .6 X loaded B+ voltage. As you can see, on upper hf bands, tube XC is much lower..and more current flows. Beware, on metal tubes like my 3CX-3000A7 etc, the tube C (24 pf for the 3CX-3000A7) will increase to 33 pf, when tube is inserted into the grid ring /socket. The increase in C is because of the proximity of the lower anode fins.... to the chassis below the fins. This is easily measured with any digital LCR meter (I use both a B+K 875A..and also a 875B). Tubes like the YC-179 and YC-156 have 35 pf of tube C (anode to grid)..which increases to 50-52 pf..when integral grid flange of tube, is bolted to chassis. Increased tube C with other metal GG triodes... like 3x6, 3x10 /15/20. Just something to be aware of when doing calcs. The 4500 vdc of loaded B+... and the 52 pf of tube C of the YC-179...and a freq of 50.125 mhz... results in a whopping 44A flowing through the tube C...and also the plate block cap assy. And it all gets worse, if loaded B+ is further increased. The same tube, same loaded B+ is a non issue on lower freqs. Plate block cap value is also a non issue for the most part. W8JI replaced the 1000 pf plate block cap in an Ameritron AL-80..with a 170 pf cap..... and on 160M, zero difference in power output..and only a tiny tweak on the tune + load caps. Sri for the diatribe. Jim VE7RF |