# Dual Offest Sub-audio Test Signal

Started by May 23, 2014
```On Sat, 24 May 2014 07:25:39 GMT, Jan Panteltje <panteltje@yahoo.com>
wrote:

>>>                      +12
>>>      10 Hz            |
>>> +12  signal        inductor2 (reverse connected)
>>>  |     |              |
>>> [ ]   ===          inductor1 (normal connected)
>>>  |     |    \         |
>>> [ ] <------|+ \       c
>>>  | bias    |   >--- b NPN
>>> ///      --|- /       e
>>>         |   /         |
>>>          -------------|
>>>                       |
>>>                     Rsense (1 Ohm?)
>>>                       |
>>>                      ///
>>>?
>>>
>>
>>Looks to me like two coils driven oppositely, or antiphase.
>>
>>Sorry, but I don't see how this would circuit provide the signal that
>>swings from -1V to -5V, as shown on my diagram.
>
>Well it can.
>

Maybe I am desne, or still learning, but I don't see any negative rail
on the circuit above. How can the output swing below 0V?

>
>Thats is not a diagram, but a wave form.
>I though tyou wanted to drive 2 coils with opposite phase?
>If only one use one coil ;-) (reverse connected).
>

The two coils are driven in pahse but with the offset shown in my
waveform artwork.

John Gallimore
```
```On Sat, 24 May 2014 08:46:15 +0100, piglet <erichpwagner@hotmail.com>
wrote:

>I am having trouble understanding what you want. The coils are unaware
>of ground or of their voltage with respect to ground or of each other.
>All they see is a current flowing through them. You want a steady
>current flow through them with 10Hz superimposed? Just wire them in
>series ??
>

The best way I can explain is this:

If you connect a CRO across "Coil A" you will see a 10Hz sinewave
swinging symmetrically between 1 and 5V.

If you connect a CRO acorss "Coil B", you will see the same but
swinging -1 to -5V.

That's it.

Here is the circuit again for reference:

In your other post in this thread, you seemed to think the above
circuit would work.

You pointed out the phases were opposed. So I can rewire the
connections of the coils to make them in phase as I originally
intended.

Is it worth building this up, or does it still need more work, or a
better approach?

John Gallimore
```
```On 24/05/2014 11:05, jgillmore@netscan.com wrote:
> If you connect a CRO across "Coil A" you will see a 10Hz sinewave
> swinging symmetrically between 1 and 5V.

OK so if I start to think in terms of the current in the 50R coil, at
the minima the current is 1V/50R or 20ma and at the maxima the current
is 5V/50R or 100ma. Then a CRO across the coil will see those voltages.

> If you connect a CRO acorss "Coil B", you will see the same but
> swinging -1 to -5V.

Same current will do. The voltage sign will depend on which way round
you attach the scope probe and ground clip.

And you want the current maxima to occur at the same time in both coils
or in other words be in-phase. So it looks very like just wiring the two
coils in series will do what you want?

piglet

```
```On Sat, 24 May 2014 08:46:15 +0100, piglet <erichpwagner@hotmail.com>
wrote:

Based on your suggestions, I have redrawn the circuit to put the coils
in phase, and also corrected the placement of the two pairs of diodes.

Here it is.

How does it look now?

John Galiimore
```
```On 24/05/2014 11:25, jgillmore@netscan.com wrote:
> How does it look now?

Worse! The coils are still in anti-phase, when the current in one is
increasing it is decreasing in the other. Also D5 and D6 are now reverse
biased so the lower coil will see nothing.

What do you not like about Jan's circuit?

But if what you want actually is anti-phase 10Hz (i.e. that when one
coil gets more the other gets less) then try with your earlier circuit.

piglet

```
```On 24/05/2014 11:53, piglet wrote:
> But if what you want actually is anti-phase 10Hz (i.e. that when one
> coil gets more the other gets less) then try with your earlier circuit.

Looking at the waveforms you posted at the start I do see that when one
coil has 1V across it you want the other to have 4V across it. In other
words the currents are in anti-phase.

Phil's scheme is fine. And the circuit you posted based on it is a valid
approach. I would suggest rearranging the amplifier so that the function
generator does not float. Most have an dc offset adjustment which you
might handy to set the midpoint.

piglet

```
```"piglet"
>> piglet wrote:
>
>> But if what you want actually is anti-phase 10Hz (i.e. that when one
>> coil gets more the other gets less) then try with your earlier circuit.
>
> Looking at the waveforms you posted at the start I do see that when one
> coil has 1V across it you want the other to have 4V across it. In other
> words the currents are in anti-phase.

** Yep.

> Phil's scheme is fine.

** No it isn't.

The OP wants a constant DC offset current  - cap coupling will not allow
that.

> And the circuit you posted based on it is a valid approach.

** But is full of horrible errors  -  too many to fix by mere NG posts.

> I would suggest rearranging the amplifier so that the function generator
> does not float. Most have an dc offset adjustment which you might handy to
> set the midpoint.

** Confucius say:

" Piglet cannot teach fool to sing "

....   Phil

```
```"Phil Allison" <

> ** You need to supply one end of each solenoid with a *solid* +/- 3V, then
> use a pair of electro caps of about 1000 uF to couple the 10Hz to the
> other ends -  orient the electros accordingly.

** Forget this if you need DC current offset.

....  Phil

```
```On 24/05/2014 13:25, Phil Allison wrote:
> ** Confucius say:
> " Piglet cannot teach fool to sing"

Amen. Piglet have too many chores.

```
```On Sat, 24 May 2014 22:25:30 +1000, Phil Allison <phil_a@tpg.com.au> wrote:

>
> "piglet"
>>> piglet wrote:
>>
>>> But if what you want actually is anti-phase 10Hz (i.e. that when one
>>> coil gets more the other gets less) then try with your earlier circuit.
>>
>> Looking at the waveforms you posted at the start I do see that when one
>> coil has 1V across it you want the other to have 4V across it. In other
>> words the currents are in anti-phase.
>
> ** Yep.
>
>> Phil's scheme is fine.
>
> ** No it isn't.
>
> The OP wants a constant DC offset current  - cap coupling will not allow
> that.
>
>
>> And the circuit you posted based on it is a valid approach.
>
> ** But is full of horrible errors  -  too many to fix by mere NG posts.
>
>
>> I would suggest rearranging the amplifier so that the function generator
>> does not float. Most have an dc offset adjustment which you might handy
>> to
>> set the midpoint.
>
> ** Confucius say:
>
> " Piglet cannot teach fool to sing "
>
>
>
>
> ....   Phil
>
>
>
>
>