# Hello, how can I replace a variable capacitor with a varicap

#### michael1978

hello, can somebody help me? i found this am receiver, but is using variable capacitor, how can i replace of do with varicap this circuits(i hava varicap bb112)

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#### Svein

The varicap is voltage driven, so you must isolate it with a 1nF capacitor at each end. Or - bring one end of the varicap to ground like this:

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#### michael1978

The varicap is voltage driven, so you must isolate it with a 1nF capacitor at each end. Or - bring one end of the varicap to ground like this:
View attachment 221826
The varicap is voltage driven, so you must isolate it with a 1nF capacitor at each end. Or - bring one end of the varicap to ground like this:
View attachment 221826
I search a lot in internet and i so your circuits, and i did not find help so i come here to help me, because i have no variable capacitor
So to remove variable cabacitor, and to add R1 C1 VARICAP, but what about the antenna, how to make antenna, how to where to conected in the circuit?

#### Averagesupernova

Gold Member
this varicap is 520pF
Which varicap? The link I posted offers a number of them and nothing comes close to 500 pF.

#### Merlin3189

Homework Helper
Gold Member
Datasheet
min = 440 pF, typ = 470 pF, max = 520 pF

I can't vouch for the datasheet as I've never had a BB112, but Siemens should know.

Edit: PS and BTW, I'm testing some diodes - varicaps, LEDs and simple power diodes - at the moment, and the record is over 1 nF for a Lucas DD710 low voltage power diode

#### Merlin3189

Homework Helper
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As for the antenna, use the same connection as in the original diagram. It is shown as inductively coupled to the tuning coil by the 4-5 t winding.

#### Baluncore

i found this am receiver, but is using variable capacitor, how can i replace of do with varicap this circuits(i hava varicap bb112)
A minor difficulty with that circuit is the floating voltage of the tuned circuit secondary.
If it is to use a varicap, controlled by a DC voltage, it will need to be referenced to ground.

But there is an envelope detector embedded in that circuit, which must not be broken by the change in front end bias.
That might explain why the input RF transformer has a floating tuned secondary.

The supply voltage is 9V, the varicap will need most of that. The control voltage pot needs a regulated voltage.
Without regulation, any change in battery voltage due to audio output load will de-tune the RF circuit.

The pot and regulator will use more current. So power consumption will rise with the change to a varicap.

#### michael1978

Which varicap? The link I posted offers a number of them and nothing comes close to 500 pF.
No this was orginal circuits so i replace varicap bb112, but i hear only noise

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#### michael1978

Datasheet
min = 440 pF, typ = 470 pF, max = 520 pF

I can't vouch for the datasheet as I've never had a BB112, but Siemens should know.

Edit: PS and BTW, I'm testing some diodes - varicaps, LEDs and simple power diodes - at the moment, and the record is over 1 nF for a Lucas DD710 low voltage power diode
yes me too, but i connect without antenna i hear noise

#### Baluncore

No this was orginal circuits so i replace varicap bb112, but i hear only noise
@michael1978. That looks like LTspice. Have you been successful running that simulation ?
There are some obvious problems, it is easier to edit your file than create another.
Can you please attach your file.asc to a post with the txt extension as file.asc.txt
Attach the plot file in the same way if the simulation worked. file.plt.txt

#### michael1978

@michael1978. That looks like LTspice. Have you been successful running that simulation ?
There are some obvious problems, it is easier to edit your file than create another.
Can you please attach your file.asc to a post with the txt extension as file.asc.txt
Attach the plot file in the same way if the simulation worked. file.plt.txt
yes i send you, if you can help me.... I RENAME am.asc into am.txt

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#### Baluncore

Here is a modified bias circuit, no model for BB112 or Pot, but circuit should work.

C1 and C2 must be low leakage caps. Avoid power supply bypass caps.

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#### michael1978

Here is a modified bias circuit, no model for BB112 or Pot, but circuit should work.
View attachment 221877
C1 and C2 must be low leakage caps. Avoid power supply bypass caps.
Here is a modified bias circuit, no model for BB112 or Pot, but circuit should work.
View attachment 221877
C1 and C2 must be low leakage caps. Avoid power supply bypass caps.
thank you very much, but i have just 2 more question, why you pick 47K to potentiometer, normally they say a few megaohms, and last where to connect antenna and i need simple inductor 200u ? and that was only for test trnsformer thanks

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#### Baluncore

but i have just 2 more question, why you pick 47K to potentiometer, normally they say a few megaohms, and last where to connect antenna and i need simple inductor 200u ?
1. The resistor needs to be big to keep the Q of the LC circuit high. Any coupling capacitor or varicap reverse leakage current will flow through the resistor, so I use 47k as a starting value. Any resistor over 100k is likely to leak due to environmental contamination.
2. For MW reception I would wind 75 turns on a ferrite rod and use that for the tuning inductor and the antenna. No external antenna is needed.
You could experiment with 50 turns of wire on a 2" or 50mm plastic pipe or cardboard roll. Wind a temporary primary of 12 turns over the coil to couple an external antenna.

#### michael1978

1. The resistor needs to be big to keep the Q of the LC circuit high. Any coupling capacitor or varicap reverse leakage current will flow through the resistor, so I use 47k as a starting value. Any resistor over 100k is likely to leak due to environmental contamination.
2. For MW reception I would wind 75 turns on a ferrite rod and use that for the tuning inductor and the antenna. No external antenna is needed.
You could experiment with 50 turns of wire on a 2" or 50mm plastic pipe or cardboard roll. Wind a temporary primary of 12 turns over the coil to couple an external antenna.
hello
but can i use inductor in form of resistor? in place of ferrit rod they are the same of not?

#### michael1978

hello
but can i use inductor in form of resistor? in place of ferrit rod they are the same of not?

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#### davenn

Gold Member
hello
but can i use inductor in form of resistor? in place of ferrit rod they are the same of not?

an inductor ISNT a resistor

follow @Baluncore 's instructions, he is wise in such things

#### berkeman

Mentor
an inductor ISNT a resistor
He might be asking if he can use a high-resistance wire-wound resistor as an inductor, but I'm not able to decode his questions very well.

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#### Baluncore

The 200uH inductor is the secondary of an RF transformer. It should have low loss and low wire resistance. It is normal to use an air cored coil wound on a cardboard tube, or a smaller coil wound on a ferrite rod.

The primary of the air cored RF transformer is maybe 6 turns of wire wound over the secondary and used to lightly couple the antenna current into the tuned secondary.

The alternative is a ferrite rod that acts as an antenna wound with wire to make the tuning inductor. That may eliminate the need for the antenna and primary winding. https://en.wikipedia.org/wiki/Ferrite_core#Ferrite_rod_aerial

You might find a 200uH RF “choke” inductor with low loss characteristics, but I doubt it. If you use a small inductor, wound on a small ferrite slug, you will find it very difficult to couple the antenna signal into the small tuning inductor.

The reactance of 190uH at 530kHz is about 630 ohms. The BB112 varicap has a series resistance of 1.5 ohms so the Q will be about 300 which is OK on the AM broadcast band. Keep your tuning inductor wire resistance below 1.5 ohms to have good Q = selectivity.

There are many calculators, such as here; http://zpostbox.ru/how_to_calculate_inductors.html
Try 85 turns, spread over 40mm along a 40mm diameter former.
Work out the length of wire. Then from wire tables, the resistance. Will 85 turns fit along 40mm? 40 / 85 = 0.47mm diam wire.

#### michael1978

He might be asking if he can use a high-resistance wire-wound resistor as an inductor, but I'm not able to decode his questions very well.

an inductor ISNT a resistor

follow @Baluncore 's instructions, he is wise in such things
sorry you misundestand me, i have a inductor(i dont know how they call? they look like resistor, but they are inductor)

#### michael1978

an inductor ISNT a resistor

follow @Baluncore 's instructions, he is wise in such things
sorry you misundestand me, i have a inductor(i dont know how they call? they look like resistor, but they are inductor)

"Hello, how can I replace a variable capacitor with a varicap"

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