Some old electronics tricks - maybe for Fundamentals Friday ?

[G0FTD]

Greeting All and Dave.

I'm a radio amateur who loves building stuff and generally messing about.

I also love reading OLD radio and electronics mags and sucking up all the old ideas and information that has been lost over
the years.

I came across two old tricks that I've never seen used in modern times at all, and might be worth taking a look at.

Yeah they are easy, but despite this I never seem to find the time as I'm always playing with other stuff, grrr..

1) I saw this in a 1950's Practical Wireless magazine. How to make a variable capacitor using a fixed cap and a variable
resistor. Since radio was king in them olden days, and parts were expensive,keeping the cost of receivers down was pretty
high on the agenda. To make a fine tune control, a small value capacitor, say 10pf was put in series with a variable resistor
to make a variable capacitor.  I think the end of the variable was put to ground. One cheap variable capacitor it seems.

2)"Full wave" rectification using only two diodes. Erm, yeah but this one didn't use a centre tapped transformer.
It had a diode on each side of the secondary, and in parallel with the diode was a capacitor. Presumably some of the AC
was fed back at a different phase to smooth it out a bit. I've never seen this before. Not sure what the values for the caps
were needed though. Maybe this old trick could be reviewed ?

Kind regards,

Andy

[mikerj]

2)"Full wave" rectification using only two diodes. Erm, yeah but this one didn't use a centre tapped transformer.
It had a diode on each side of the secondary, and in parallel with the diode was a capacitor. Presumably some of the AC
was fed back at a different phase to smooth it out a bit. I've never seen this before. Not sure what the values for the caps
were needed though. Maybe this old trick could be reviewed ?

Are you sure it wasn't the classic full wave voltage doubler circuit?

[GreyWoolfe]

I've seen them done with center tapped transformers with the center tap going to ground.

http://www.industrial-electronics.com/Industrial_Power_Supplies_Inverters_and_Converter/4_Two-Diode-Full-Wave-Single-Phase-Rectifiers.html

[GNU_Ninja]

I've seen them done with center tapped transformers with the center tap going to ground.

http://www.industrial-electronics.com/Industrial_Power_Supplies_Inverters_and_Converter/4_Two-Diode-Full-Wave-Single-Phase-Rectifiers.html

The two diode full wave rectifier with centre tapped transformer was popular back in the day because valve rectifiers (eg GZ34) were quite expensive (and also quite inefficient). Nowadays, solid state bridge rectifiers are ten a penny so you hardly ever see the two diode rectifier topology (not that I've looked), also centre tapped transformers may be more expensive now 

With the two diode full wave rectifier, you only suffer one diode voltage drop I suppose. As opposed to a two diode drop with a bridge rectifier, so there is an advantage, of sorts.

[vk3yedotcom]

Greeting All and Dave.

I'm a radio amateur who loves building stuff and generally messing about.

I also love reading OLD radio and electronics mags and sucking up all the old ideas and information that has been lost over
the years.

I came across two old tricks that I've never seen used in modern times at all, and might be worth taking a look at.

Yeah they are easy, but despite this I never seem to find the time as I'm always playing with other stuff, grrr..

1) I saw this in a 1950's Practical Wireless magazine. How to make a variable capacitor using a fixed cap and a variable
resistor. Since radio was king in them olden days, and parts were expensive,keeping the cost of receivers down was pretty
high on the agenda. To make a fine tune control, a small value capacitor, say 10pf was put in series with a variable resistor
to make a variable capacitor.  I think the end of the variable was put to ground. One cheap variable capacitor it seems.

Possibly this approach has fallen out of favour because of the availability of cheap silicon diodes, eg 1N4004, 1N4007 etc.

These can be used as a cheap varactor and get quite good frequency swings, even in VXO type circuits using ceramic resonators.

They're not as good as a variable capacitor but tuning is easier than a potentiometer as you've got about 80% rotation versus 50% with a capacitor.  Bigger knobs are also easier to find, making tuning even better.  I've had success with switching banks of diodes - eg 1 or 3 to get a wider tuning range.  Here you want the maximum capacitance of the smaller group of diodes (or just one) to just exceed the minimum capacitance of the larger  bank.

[bitseeker]

1) I saw this in a 1950's Practical Wireless magazine. How to make a variable capacitor using a fixed cap and a variable
resistor. Since radio was king in them olden days, and parts were expensive,keeping the cost of receivers down was pretty
high on the agenda. To make a fine tune control, a small value capacitor, say 10pf was put in series with a variable resistor
to make a variable capacitor.  I think the end of the variable was put to ground. One cheap variable capacitor it seems.

So, is the general concept here that in order to get lower capacitance, you let more current dump to ground via the resistor rather than fill the capacitor?