Replacing half wave with full wave rectifier.

['LIVEWIRE?']

Can someone refresh my memory as to the DC o/p with various types of filter circuits? DC = AC x 1.414 with acapacitor after the rectifier, whether said rectifier is half-wave, full wave or bridge, right?, but what is the effect on DC of including, e.g., a choke in series with the rectifier. ( I should know all this after all the years I've been repairing radios, etc., but wouldn't pass a theoretical exam now!!)

[ms660]

With no load connected, the DC voltage across the filter capacitor will equal the peak AC for half/full wave, full wave bridge and CRC,CLC and LC filters so far as I know.

Lawrence.

[TrevorG3VLF]

If sufficient current is drawn, a choke input filter will give an output of the average voltage which is Vrms times 0.9. At very low currents the output will rise to Vrms times root 2. A small load resistor is sometimes included to limit this.

The current is almost constant so it is much kinder on the rectifier and transformer.

[Radio Wrangler]

Ah, don't use 1N4007 family rectifiers if you can avoid it in a receiver. They are very slow and turn off with a snap which has harmonics right across the HF bands. Buy some 'ultrafast' rectifier diodes instead. It's another one of those counter-intuitive trip-ups that faster switching means less noise in this case. Slow diodes without a lot of careful filtering could make things fail EMC tests.

David

[turretslug]

BA159 are cheap as chips and widely available- I threw away 1N4007s a long time ago. (1NOISE4007!)

['LIVEWIRE?']

Aren't the UF400x series better than the 1N400x diodes, David?

[MALC SCOTT]

UF = ultra fast.

[AC/HL]

I think this came from an old RS catalogue.

[bikerhifinut]

Originally Posted by Radio Wrangler

Ah, don't use 1N4007 family rectifiers if you can avoid it in a receiver. They are very slow and turn off with a snap which has harmonics right across the HF bands.

It's not exactly a cutting edge radio David but I get you.

Something tells me what I think are 1n4007 in my box of goodies are UF5407 anyway. In any case theres no difference in price in any substantial way so it makes sense to keep a stock of the Ultrafast or even schottkies in. I'll bet the existing diode is even worse than a 1N4007 being of an early generation of these beasties.

A.

[bikerhifinut]

Originally Posted by AC/HL

I think this came from an old RS catalogue.

Brilliant!

That's a useful chart i will store that in my file of useful info!

It more or less tells me that I have no issues using a FW Bridge in this cheap and cheerful radio. It's destined for a workshop receiver so mostly as long as I can get BBC Cumbria once moved, and I'll be building up a pantry Tx for use with the MP3 player thats got an FM radio in it so I'll be sorted.

As noted in a previous thread that I found about this particular radio, the main smoothing and reservoir capacitors are only rated at 150V and although they havent gone pop yet, I am minded to replace the dual can with something a bit more robust. its a 40 + 60 uF unit but I reckon a 40+40 or 50+50 will do the same job.

OT a bit but it would sort out an issue I have with a mains transformer that is slightly over voltage for the SE EL84 amp it's destined for. I can use a LCRC filter instead of the CLCRC that is there now and 90% of the voltage is about right.

Thanks Bill.

A.

[Ed_Dinning]

Hi gents, attached a copy of the Mullard "rectifier circuits table" from on of their app notes.

This has the ACrms to peak/ average and Dc voltages for various rectifier configurations


Ed

[G8HQP Dave]

Originally Posted by bikerhifinut

90% of the voltage is about right.

Just checking that you are clear that choke input means 90% of Vrms, not 90% of the Vpk you get with capacitor input? Switching from half-wave to full-wave means a tiny increase in voltage; switching from cap input to choke input means a big decrease in voltage.

[ms660]

http://www.r-type.org/articles/art-144.htm

Lawrence.

[kalee20]

Originally Posted by G8HQP Dave

Switching from half-wave to full-wave means a tiny increase in voltage; switching from cap input to choke input means a big decrease in voltage.

Absolutely! Though the choke input filter does have a stable output voltage, once you get above the minimum current.

For 100V RMS input, the DC voltage starts at 141V and as load is increased, sags to 90V and then stays pretty steady.

[bikerhifinut]

I do know the difference.

A.

[Brigham]

Do you have enough room for an AZ31?

[bikerhifinut]

Originally Posted by Brigham

Do you have enough room for an AZ31?

It's not relevant here Brigham, it's a silicon rectifier and the transformer's not centre tapped. Nice idea mind, and I think there's room for a 6X4 in there and that could be used as part of a hybrid rectifier, but I dont need the voltage drop as it now happens and I fear the mains transformer is already probably working at its limits. Plus its not best practice to run a rectifier valve heater off the same winding as the rest of your valves.

I think we've covered most of the initial question now, and it basically boils down to, 1) it's probably not going to make any noticeable difference as paul first suggested, and 2) put a bridge in as it should take some strain off the mains transformer.

Thanks to everyone who contributed, some very useful hints and information's come out of this from my point of view.

A.

[julie_m]

You can use a full-wave rectifier valve and two Silicon diodes in a bridge configuration, no need for a centre tap. But semis are cheating, really

Edit: Also, EZ40 / 80 / 81 are specifically designed with excellent H-K insulation so as to be able to share a heater supply with other valves.

[bikerhifinut]

Sure Julie, what you say is absolutely right.

I think I did mention a hybrid rectifier in the previous post, there can be advantages in as much as you get a gently rising HT. And It may well be cheating I guess but its an easy way to get the advantage of a valve rectifier I guess?

I stand four square behind my comments re its not best practice to heat a rectifier with the same winding as the rest of your valves.
I know some gear did that but that will only have been to save on a winding.

regards

Andy.

[G8HQP Dave]

EZ80 etc. were specifically designed to run off the same heater chain as the other valves. An indirectly heated rectifier intended for its own winding could have much thinner heater-cathode insulation and so draw less heater current and warm up much more quickly.