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Old 30th Jul 2018, 8:17 am   #1
egerton
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Default Cap. dropper + full wave bridged set

Having got my Bestone TRF working quite nicely, I'd like to dispense with the lamp limiter and find an integrated solution to power. It's a midget set so no room for a transformer so I am turning my attention to a capacitor dropper and wondered if a full wave bridge to supply DC to the set is better than a cap. drop alone. I'm considering the 2 comments I received in another thread about this subject. I am concerned by the statement in Chris's reply ..the whole set will be up at half mains or so. Is this more risky than a cap. dropper alone? The set has a 'live' (connected to neutral) chassis anyway.

As always any comments welcomed. Paul

Provided that the heater load is >>HT load it will work, but if the heater rms volts are correct, the HT dc will be a bit off, which may or may not matter. You could feed the whole set with positive dc by using a bridge rectifier with a series capacitor on the ac input, but then the whole set will be up at half mains or so.
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Capacitive droppers really need a symmetrical load, otherwise you will get a badly distorted waveform.

Probably the best way to get a transformerless radio to work with a capacitive dropper is just to wire a bridge rectifier around the set; so its rectifier, as well as the valve heaters, will be passing current on both half-cycles.
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Old 30th Jul 2018, 10:39 am   #2
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Default Re: Cap. dropper + full wave bridged set

A capacitive dropper alone for a series chain with HW rectified HT is a disaster unless the HT current is massively less than the heater current.

I tried knocking this about in a circuit simulator and it looked to me that it was hard to get a viable solution, so I think the FW bridge to even out the cycles is absolutely necessary. Even then there is a problem of waveform distortion generated by the HT so a little care may be needed to get the right heater power rather than the right peak voltage.

I would start by measuring the HT current and then either using a circuit simulator or building a little testbed to try some experiments before you go for the real thing.

The bridge will make it a true live chassis set of course so there absolutely has to be no accessible metal (don't forget aerial and grub screws).
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Old 30th Jul 2018, 10:53 am   #3
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Default Re: Cap. dropper + full wave bridged set

Chris is correct, a bridge rectifier would result in an AC voltage with respect to earth on the chassis.

I would still recommend an external 240-120V isolating transformer and put a US style plug on the set.

I have rewired US sets by half wave rectifying the heater circuit and rewiring the field coil in series to drop the HT voltage:
1. Put a 100 ohm resistor between the mains and the 1D6 anode.
2. Rewire the field coil in place of the choke
3. Put a 1N4007 in series with the heater chain.

A single capacitor dropper is not going to be a good idea on this set as the HT current includes the field coil current and will be >100 mA.

Last edited by PJL; 30th Jul 2018 at 11:05 am.
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Old 30th Jul 2018, 11:10 am   #4
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Default Re: Cap. dropper + full wave bridged set

A diode in series with the heater chain does not give an RMS voltage of 120v, it is 170v, so the heaters will be overloaded.
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Old 30th Jul 2018, 11:56 am   #5
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Default Re: Cap. dropper + full wave bridged set

Yes, half power, Vout = Vin*0.707, if diode is used in heater chain then series R needs to be approx. 175 ohms, P diss. approx. 15 watts.

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Old 30th Jul 2018, 12:17 pm   #6
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Default Re: Cap. dropper + full wave bridged set

Quote:
Originally Posted by winston_1 View Post
A diode in series with the heater chain does not give an RMS voltage of 120v, it is 170v, so the heaters will be overloaded.
Ooops! I did this a long time ago and I am supposed to be working. It needs a series resistor...

Even with this, moving the field coil may still have reduced the overall heat dissipation in the box? but a capacitor dropper for the heater chain with the field coil modification might be better.

Last edited by PJL; 30th Jul 2018 at 12:29 pm.
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Old 30th Jul 2018, 1:00 pm   #7
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Default Re: Cap. dropper + full wave bridged set

Replace the rectifier with a semiconductor and series resistor and use 2 capacitor droppers, one for heaters and another for HT?

Lift all the chassis connections, fit a busbar for grounds and isolate the chassis?
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Old 30th Jul 2018, 4:34 pm   #8
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Default Re: Cap. dropper + full wave bridged set

You cannot connect the chassis to 'neutral' and to the output from a bridge rectifier. You can either have a conventional AC/DC arrangement, with 'neutral' chassis and half-wave rectification via a dropper resistor, or you can have a bridge rectifier and the circuit completely isolated from the chassis. Both have dangers.
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Old 30th Jul 2018, 7:27 pm   #9
egerton
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Default Re: Cap. dropper + full wave bridged set

I hadn't thought that through; but I see that now. Thank you all for the informative replies. I'd like to use this as a bedside radio as it's small enough and does work well on MW. The power of the set is only 43.2 W. I've found an 45W AC-AC converter with a 110V and USA outlet. I now intend to put a NEMA 1 15A USA plug on the set and use it with the converter, which I believe is an auto transformer. So the chassis would not be isolated but it would at least be at neutral potential and this seems a simple solution to allow use of the set day to day but only by me.

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Old 30th Jul 2018, 7:34 pm   #10
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Default Re: Cap. dropper + full wave bridged set

The AC to AC "converter" might not be an auto transformer it might be a switch mode job that generates a pile of hash across the spectrum.

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Old 30th Jul 2018, 9:15 pm   #11
egerton
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Default Re: Cap. dropper + full wave bridged set

I hope not but will find out soon as I've ordered it. It's cheap enough and has a auto transformer symbol on the case so hopefully it will be the real thing. I've also ordered a selection of 'motor run' capacitors to play with with a view to experimenting with a dummy load. These may come in useful for droppers. I'm not sure if they are true X types, but they are 250 V rated.

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Old 3rd Aug 2018, 9:43 am   #12
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Default Re: Cap. dropper + full wave bridged set

Hello.
Motor run capacitors are not rated as X types as such as they are designed for a different purpose in life. In a Radio they are under considerably less stress than in a motor so have a long life. One proviso though is to use ones rated at 400 VAC.
You can use two caps, one for heaters and one for HT. Yes and with half wave rectified HT. Ideally you need a valve HT rectifier that has its Anode connected to the supplying capacitor dropper. Then also from the valves anode a silicon diodes cathode connected to it and the anode grounded, a 1N4007 is ideal. This actually works like a doubler so the valve rectifier is essential as the HT could rise over 400v with silicon in both positions. The value of the cap dropper will only be about 1uf for HT using this method, dependent on HT current.

Doing it that way will ensure you have the chassis at neutral and not floating at near half mains.
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Last edited by D Cassidy; 3rd Aug 2018 at 10:06 am.
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