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I have hooked up a power supply circuit consisting of a tranny, a bridge rectifier, 2 chokes, two strings of caps, resistors for a load. They are set up in an LCLCR format. The tranny is a 1450vct that I have hooked up to the bridge without using the ct. The chokes are 4 ohm with 40 watts dcr and the caps are 3 330uf/400v 'lytics in series with 330/1w resistors between them to spread the load. Resistance at the tail end is 3 17k/30 watt resistors in parallel.
Here's the question... when I start applying power with my variac the chokes start to hum at about 250 volts and the humming just gets loader as I increase the voltage. What have I done wrong?
Second question... is the minus leg of the dc output of the bridge supposed to hook up to mains ground? I don't have it that way right now.
Thanks
Vince
Follow Ups:
and you only have three 400 vdc caps in series , that adds up to 1200 vdc , you need at least 2500 to 2800 vdc rated cap string. The dc voltage will be about 1.4 times the secondary voltage unloaded.
That is about 2250 vdc.
The three 17k ohm resistors in parallel add up to about 6k or so. That will be about 300 milliamps at 1450 vdc , or about 500 watts dissipated in the resistors. That means you will need resistors in the 1000 watt range to dissipate thr heat .
These numbers are very approximate values but you get the idea.
THESE VOLTAGES AND CURRENTS ARE LETHAL , THEY WILL KILL YOU ! ! !
Hope this helps
Wrong numbers Ed. With that tranny it was about 1100 vdc after it came through the ss bridge. Even the surge didn't go that high thanks goodness. You are right though, it ain't nothin' to take lightly!
Mains Ground is for SAFETY in the event of a fault. Connect this to the chassis @ one separate point only. DO NOT connect Mains Ground direct to circut. Connect the circut to chassis @ one separate point as well to complete the SAFETY arrangement. Typicallly this circut to chassis point will be at the input today, but older amps with metal can pi-filters made the circut-to-chassis connection via the can shell.
Ron
I will remember that when I get to a point where I put the ps circuit back in the chasis. Right now I am just trying to get the ps circuit to work properly.
"Second question... is the minus leg of the dc output of the bridge supposed to hook up to mains ground? I don't have it that way right now."To the power supply ground, yes.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Please heed my Caution reply above. The bridge rectifier negative should be connected to power supply circut & thus audio circut Common return in order for current to flow, BUT should NOT be connected direct to the 120VAC (or 240VC) Mains supply Ground lest SAFETY be compromised.
Vince,The 1st inductor in a choke I/P filter has to be massive, as takes a BEATING. The "singing" you heard occurs when inductors are not suitable for choke I/P service.
Even when appropriate inductors are used, "singing" can be an issue that requires some viscoelastic damping.
A tweak you can try to stop "singing" with the parts you already own is putting a 100 nF. cap. in front of the 1st choke, making the filter cLCLC. Behavior remains essentially choke I/P and the B+ will rise only a little. As that small cap. increases in value, regulation deteriorates and behavior eventually becomes cap. I/P.
Eli D.
Eli
I would like to try this as I don't want to buy a choke just for this purpose. The two chokes I have are monster potted collins chokes that weigh 10 lbs each. I did notice that the buzzing decreased quite a bit when I set them on a cushion of foam and even more when I hooked the minus leg of the bridge to ground.
How do I know if I can use the choke this way without damaging it?
Vince
Vince,See Todduk's post. Your Collins chokes are not especially big. The Hammond 193Q is a 0.5 A./10 H. "gorilla" that's suitable for choke I/P service. A 193Q weighs (sic) 21 lbs.
Try the little cap. tweak, as it takes strain off the 1st choke. Since regulation remains decent until you push the DC voltage above the AC RMS value, try incrementing the "fudge factor" cap. until you reach that point. The larger the tweak cap. value, the more strain you take off the choke. Pseudo-choke I/P filters have their uses.
Eli D.
I think for now I am going to drop back to using the "big mutha" tranny with cap input as I can get very close to 1000vdc using parts I have I have. Does having all those amps available help with performance?
Choke input causes a massive AC component to flow in the first choke. You need to source one specially designed for this function. Sowter sells a few but no so cheap I guess. The old Admiralty pattern oil filled chokes seem to work well though if you can find one
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