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 Post subject: Full wave bridge rectifier
PostPosted: Nov Wed 22, 2017 4:05 am 
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Joined: May Sun 21, 2017 10:55 pm
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Location: Elk Grove, CA
Transformer primary: 115v, secondary: 182.9-0-182.9,
Using 182.9v into the full wave bridge, the output is 164.2v pulsating DC. My question is where did the 18.7 volts go? I thought 3 to 4 volts.


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 Post subject: Re: Full wave bridge rectifier
PostPosted: Nov Wed 22, 2017 4:12 am 
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Location: Maryland 20709, USA
What are you using to make the measurement?

Most meters won't read pulsating DC correctly.

- Leigh

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 Post subject: Re: Full wave bridge rectifier
PostPosted: Nov Wed 22, 2017 4:30 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Livermore, CA
Connect an electrolytic cap across the DC output. Voltage will go to 1.414 times 182. This will be a true reading on your DC meter scale.

Like Leigh mentioned with pulsing DC your meter is reading zero and the peak.

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 Post subject: Re: Full wave bridge rectifier
PostPosted: Nov Wed 22, 2017 4:49 am 
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Location: Albuquerque, NM 87123
Filtered DC will be close to 260 volts; you better use at least 350 volt caps.


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 Post subject: Re: Full wave bridge rectifier
PostPosted: Nov Wed 22, 2017 10:17 pm 
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Joined: May Sun 21, 2017 10:55 pm
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Location: Elk Grove, CA
Thank you very much for the input. This is my first venture into transformers and rectifiers. I did as suggested. I placed a 10 mf 450v electrolytic cap. Across the output and ended with 251.5 dc volts out digital and 260.0v analog.

Would i be able to use this transformer and resistors to lower the voltage to 155v needed for plate voltage on a Grunow 410 farm radio? Or should I stop for a transformer with lower output? If so what should I look for?

Thanks again for your input

Jim


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 Post subject: Re: Full wave bridge rectifier
PostPosted: Nov Wed 22, 2017 10:31 pm 
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Location: Livermore, CA
Jim

Loading by the radio and filtering will get you proper voltage. That's a good transformer to work with to get 155 volts. This wasn't a popular battery voltage.

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 Post subject: Re: Full wave bridge rectifier
PostPosted: Nov Wed 22, 2017 11:18 pm 
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Joined: May Sun 21, 2017 10:55 pm
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Location: Elk Grove, CA
My first venture into this new found hobby is a Grunow 410 farm radio. Used a 6 volt car battery to supply vibrator and filament voltage. Two tubes require 6 volts and two others through a choke (?)use 2 volts. Both transformer and vibrator are trash. Decided to try to come up with a new new power supply.


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 Post subject: Re: Full wave bridge rectifier
PostPosted: Nov Thu 23, 2017 2:12 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5158
Location: Rochester NY USA
You'll likely need at least two filter capacitors, wired: Bridge, then cap, then resistor, than another cap. The resistor will drop the voltage to 155V, and to calculate the value needed, you'll need to know the current drawn by the radio. Say it needs 15 mA, then you need to drop about a hundred volts (251-155V) So resistor needs to be 100V/.015A or 6667 Ohms. 6.8K should be close enough. Power is V^2 / R or 1.4W, so a 3 or 5 watt resistor would be best to keep the temperature down.

All of the above goes out the window if it uses a class B output stage like a 19 or 1J6, since this has a great variation in current with audio level, and a simple resistor will not be enough. But using a large choke (15-20 Henries) before the first capacitor would give about 160V, depending on its DC resistance.


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 Post subject: Re: Full wave bridge rectifier
PostPosted: Nov Thu 23, 2017 3:37 pm 
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Joined: Jan Mon 28, 2013 9:35 pm
Posts: 1311
Location: Santa Rosa, CA
Consider using an 0D3 (or mil VR-150) tube to give you about 150vdc
out. Battery sets are not all that sensitive to their B+ voltage.
You'll need to calculate a series resistor to supply the correct
current to the tube. Any tube manual will tell you how.

Speaking of full wave bridges, there is a mostly forgotten way to
get two voltages out of a center tapped transfromer. Connect a
full wave bridge in the usual way (4 rectifiers) but instead of grounding
the center tap, ground the *negative* output of the bridge. The postive
output of the bridge will give full output (B+) and the center tap
will give you (B+)/2. Kinda neat and perfect for those of us that
like to homebrew regens. The lower voltage feeds the detector
tube (via an 0D3 regulator) and the higher voltage can be used
for an audio output stage. Look in any old ARRL handbook and
you'll see this technique discussed.
Steve

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 Post subject: Re: Full wave bridge rectifier
PostPosted: Nov Sat 25, 2017 9:00 pm 
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Joined: May Sun 21, 2017 10:55 pm
Posts: 21
Location: Elk Grove, CA
What size capacitors should I use for the filter capacitors? I have none on hand but would like to order a broad range in kit form. I have picked up another radio,a Silvertone 7036. and will need to change the capacitors out. On the current radio, Grunow410, I ordered them individually.

Jim


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 Post subject: Re: Full wave bridge rectifier
PostPosted: Dec Mon 18, 2017 12:07 am 
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Joined: May Sun 21, 2017 10:55 pm
Posts: 21
Location: Elk Grove, CA
Still working away on the power supply. Transformer output (secondary) 182.9 volts, into full bridge rectifier, output: 251.5-260.0 volts DC. Added smoothing capacitors before and after 6.8 k resistor. My question is this. How do I check the new dc voltage before I fire up the radio? I don't want to damage the tubes or radio. After the resistor and last capacitor, how do I take the measurement?


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 Post subject: Re: Full wave bridge rectifier
PostPosted: Dec Mon 18, 2017 12:29 am 
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Location: Maryland 20709, USA
Hi Jim,

The voltage across the firstf ilter cap should be the peak of the AC waveform - the voltage drop through the full-wave bridge, i.e. 182.9 * sqrt(2) - 1.5 volts.

That works out to 182.9 * 1.414 - 1.5 = 258.7 - 1.5 = 257.2 volts.
Sounds like you're getting the proper voltage.

If you have no leakage in the second filter cap, its voltage should be the same.

To determine the voltage delivered to the radio, you must simulate its current drain.
I don't know what that value would be.

- Leigh

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http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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 Post subject: Re: Full wave bridge rectifier
PostPosted: Jan Tue 02, 2018 4:58 am 
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Joined: May Sun 21, 2017 10:55 pm
Posts: 21
Location: Elk Grove, CA
Happy New Years Everyone! This radio was completely dead as I explained earlier. I Installed a 6.8 resistor to drop the plate voltage. I needed 155 v dc. The resistor dropped it to 140.7 V. using Tom Bavis ' input. The radio load was not known so 15ma was used as suggested. At this point using the same formula I need approx. 15 volts more, so does using a 5.6k resistor in place of the 6.8k resistor sound about right? Btw, the radio at least now has a hum and turning up the volume increases the hum. Will work on troubleshooting that problem later. But it was nice to hear something.

Thanks all for your input.

Jim


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 Post subject: Re: Full wave bridge rectifier
PostPosted: Jan Tue 02, 2018 4:13 pm 
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Posts: 13600
That 15v isn't going to be the difference as to whether the radio plays or not... No doubt has other issues...

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 Post subject: Re: Full wave bridge rectifier
PostPosted: Jan Tue 02, 2018 5:12 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 8258
Location: Long Island
Could you post a link to a schematic, or at least tell us what tubes are in the set? From what little can be found about it online, it appears to be a typical mid-1930s farm set with a mix of AC (cathode) tubes and filament type battery tubes. If that's the case you cannot operate the filament/heater circuit on 6-volts AC. The filaments in the battery tubes are their cathodes, so if you apply AC, 60-Hz hum will be impressed on the signals passing through them. Depending on the design of the set it may be easier to replace the filament tubes with 6-volt equivalents having cathodes, rewiring the filament circuit as needed, or it may be easier to construct a 6-volt DC power supply and use the original tube line-up.

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 Post subject: Re: Full wave bridge rectifier
PostPosted: Jan Tue 02, 2018 7:14 pm 
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Joined: May Sun 21, 2017 10:55 pm
Posts: 21
Location: Elk Grove, CA
Tubes are 1A4-IF, 1A6-1st Det. & osc., 75-2nd Det.-AVC, 41-Output (4 Tubes).

6.13 DC v. to the 41 & 75, 1.99 DC v to the 1A4, 2.19 DC v to the 1A6. The filament voltage is being provided by a 6 v wall wart (3 amps)

Wanted to get the power supply finalized before proceeding.

Corresponding with a tablet. Regular computer down. Will repair it soon and try again to upload a schematic.
Thanks,
Jim


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