"Dennis Monticelli" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- My company makes chips for switching supplies. One important design choice is whether or not to allow the filter inductor to operate in "discontinuous mode." This simply means that the current through the inductor is not continous; it runs out of stored energy each cycle as the current drops to zero. When that happens its filtering action is impaired.
An old fashioned tube supply with a choke is no differrent even though the frequency is much lower. If the choke has insufficient inductance for the minimum load current, then it will "run dry" before the next cycle of the line voltage replenishes the energy. Wwhen that happens its ability to regulate the output voltage is seriously impaired and the output soars to 1.4 times the RMS. This is why Gerry recommends that the minimum load represented by the stock bleed resistor be heeded. To paraphrase Gerry, only use choke input supplies when the load current range is relatively narrow; 10:1 is too much. Denny AE6C On 12/12/06, Gerry <[EMAIL PROTECTED]> wrote:
"Gerry" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- What you describe is typical of choke input power supplies. Text books tell us that choke input supplies are better at regulation but that is simply not true if you have a wide ranging dynamic load. Choke input power supplies require a minimum load. There are formulas in the old handbooks to calculate the value of resistance to draw a proper load current. Without a minimum load it would be as if no choke existed (except for its DC resistance) and the filter capacitors will charge to 1.414 times the value of AC rms. In other words, if you have 750VAC secondary the capacitors will charge to over 1,060 volts with no load! With a properly chosen resistor this would be 0.9 times 750 volts or 675 volts. Please note the HV filter capacitors are only rated to 350 volts for a total of only 700 volts. So as you can see, the 10k, 20w load resistor on the output of the HV side of the supply is crucial. That resistor dissipates around 16 watts. Notice that the low end is referenced to +250V and not ground. It should be rated for twice the actual power dissipation or 32 watts. Put it another way, without the load resistor, key up voltage would be around 1kV while key down would drop to about 675 volts. I should mention that the choke has DC resistance which may be a significant contributor to voltage drop. If you draw 450 mils and your DC choke has 50 Ohms of DC resistance, you will drop an additional 22.5 volts across the choke. Also look at the +250 volt side and see that the filter caps are rated to only 300 volts. This is much too close for comfort and is really a very slim margin of safety. Typically the voltage rating of electrolytics is related to the maximum voltage it can withstand for a given numbers of hours at some temperature. Component manufacturers usually try to embellish the specifications and say their caps will take 1000 hours of use at room temperature. But in actual operation ambient temperature will be higher than 25 degrees C and they typically don't say too much about self heating due to ripple current which brings up temperature even more. This is why it is always best to over rate voltage in electrolytics when selecting components. I see that Drake realized this was not a good design and soon changed to a capacitive input supply which is better suited to SSB service, and less expensive. As far as I'm concerned I would avoid the original AC-3 in daily use. You wisely chose to use it on the test bench instead. -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Donnie Garrett Sent: Tuesday, December 12, 2006 5:27 AM To: Tom Taylor Cc: drakelist@www.zerobeat.net Subject: Re: [drakelist] AC-4R upgrades "Donnie Garrett" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- Tom: I have one of the early AC3 supplies that used a HV choke. Garey K4OAH and I had a lot of discussions about its design some time back after I discovered the ceramic 25W load resistor was open. You better check yours to see if its OK. It will still operate but the no load HV will float very high with an open load resistor. I was never able to find an exact replacement at the time I needed it so I installed a 25W wire-wound resistor in a vertical fashion with a small bolt running through it. Somewhat similar to the original setup. I choose a 25K 25W instead of a 15K to reduce the heat dissipation produced by the resistor. The 25K still loaded down the HV and was near the same as was prior with the original 15K 25W resistor which seems to regularly fail due to running it at or near it's maxium wattage dissipation. Also one last thing, If I recall the HV winding on this early choke version AC3 was higher than the later AC3 and AC4's that didn't use the HV choke. Seems I temporally jumped across this choke just to see what effect it had on the HV and if I recall correctly the HV jumped up in voltage considerably, and was then out of speck and was near the max voltage ratings of the 2 HV caps. (No Good) I would appreciate your findings on the HV secondary windings if you don't mind. Too put it quite frankly I don't care for the early model AC3 choke type supply due to these issues. The only nice thing about this model is that all the caps mount on the underside of its chassis. This AC3 supply now serves as my test bench supply and is not used in my station. Hope this helps, Don / WA9TGT On 12/11/06, Tom Taylor <[EMAIL PROTECTED]> wrote: > I have a number of Drake A, B, and C twin combos. I decided to upgrade all the power supplies with AC-4R boards. Since most of the caps are now 30 to 40 years old, I don't want to risk a bias supply going out and ruining the finals or some short ruining the transformers. I order a bunch of blank boards since I already had some of the parts. The rest of the parts I ordered from DigiKey. I assembled all the boards at the same time and then started upgrading the AC-4s, one by one. > > So far, all of the power supplies are different in one way or another and all of them have been modified by previous owners. Here are some examples: > > #1) A previous owner replace the mid-voltage supplies' capacitors with a bundle of three caps mounted to the upper end the supply. > > #2) A previous owner replaced the sand resistor below the chassis with two higher wattage resistors mounted to the upper end of the supply. > > #3) A previous owner replaced one of the diodes in the HV supply. The transformer in this supply has transformer bolts inserted the wrong direction to mount the AC-4R board. Unfortunately, it's not as simple as removing the bolts and inserting them in the other direction because the bolts are just long enough to reach, but not long enough to add the board spacers. > > #4) This is an AC-3, serial #101. I've never seen a Drake power supply like this. There's a large choke mounted adjacent to the transformer. All of the other parts, including the capacitors, are radial lead caps that are packed underneath the very low chassis. Has anyone else seen an early AC-3 like this? I'm planning to remove the choke and rebuilding the supply just like all of the others since all of the circuitry is now on the AC-4R board. > > Thanks, > Tom N7TM > -- WA9TGT / Don Garrett / Muncie, IN "Unique Radio Parts" www.wa9tgt.com ---------------------------------------------------------------------- Submissions: drakelist@www.zerobeat.net Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net ---------------------------------------------------------------------- ---------------------------------------------------------------------- Submissions: drakelist@www.zerobeat.net Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net ----------------------------------------------------------------------
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