**Power Supply from a Magnetic Tape**

Comes from http://www.energynews.gr/electric.htm and http://www.energynews.gr/question.htm

Basil
Dimitropoulos

you may call me at +30-10-9590530.

**Common questions and answers here**

The required knowledge for the understanding of this information concerns
Electronic Applications in regard to **Magnetic Recording Of Signals Onto
Magnetic Tape.**

To come to the point, my project is the production of ** Portable Power
Supplies (PPS).** For the needs of this product, it is necessary a special
kind of Magnetic Heads that is not available in a Standardized Form. As a matter
of fact, a

I suggest the __along-the-tape fulltrack recording format__
(longitudinal recording), using a VHS videocassette. __The tape speed is 1
m/s (nominal)__ and the magnetic tape is moving into the VHS videocassette
meaning that the tape motion is linear, like the audio applications, with a
foamy plastic material behind the tape at the head / tape point (see Fig. 1).

In the traditional applications of magnetic recording, we have the positive
and negative half-period of the sine wave (see Fig. 2a) while in my project the
period of the sawtooth wave has only positive values (see Fig. 2b). This
condition allows us to use the whole linear section for the positive values of
the sawtooth wave meaning that the induction B is more than twice the RMS value
of the sine wave. In audio applications of course, we have larger linear section
– due to the ultrasonic bias current – but we have also very significant
self-demagnetization losses due to the field alternation of the magnetic
particles. In my project, if we use HF bias current, these losses will be even
larger due to: **1.** The higher frequency, **2.** The close adjacency of
the opposite magnetic particles. This is why we must use **DC (not HF) bias
current.**

The output voltage of a playback head depends on the tape permanent magnetic flux [Ör] (depending on track width), reproducing frequency [fp], number of turns [N] and efficiency factor [n].

In audio applications, we have Ör of order nWb, fp of order KHz, N of the order hundreds and n about 0.3. These result in EMF of order mV.

In video applications, we have fp of order MHz but we have also very small N due to winding self-capacitance and very low Ör due to very small track width (49 ìm in VHS).

My project combines mainly the higher Ör due to wider recording track (1/2
inch), fp of order MHz and the higher n due to non-short
wavelengths – ** there are not opposite magnetic particles as in sine wave**
– (see Fig. 3). This means that, after the recording mode, the tape magnetic
particles will have much higher retentivity and coercivity than specifications
of the magnetic tape manufacturers;

Given that there are not initial magnetization curves, based on the above conditions, of any magnetic tape, I do not have specific information about the recording signal level; both recording and bias current. As a matter of fact, these characteristics will be determined after the manufacturing of the Magnetic Heads.

However, you may have a test using a standardized ferrite head (e.g, data
tape head, either 1/4 or 1/2 inch tape). You will see that __the output voltage
of this head is much higher than RMS value of sinusoidal recording.__ The
recording process is as follows:

We consider that there is not distortion due to non-linearity of the magnetization curve concerned the ferrite core because, in magnetic recording applications, the recording signals into the core are very low; about 10 milliTesla or less.

The voltage waveform on the coil winding must be trapezoid so that the current waveform is sawtooth (see Fig. 4). We may use a Blocking Oscillator like those in TV yokes (deflection coils) but the IC which leads the oscillation must be different from such applications because in my project we have 1 MHz recording frequency. For example, the IC 555 is proper in frequencies of order KHz, not MHz.

__C O N C L U S I O N__

In the traditional applications of magnetic recording (analogue storage), the waveforms of the recording signal are sinusoidal. Therefore, the waveforms of the playback signal are identical with the recording waveforms – the derivative of the sin(x)dx is cos(x). Given that the internal impedance of the standardized playback heads is high due to the high frequency current, concluded that the current of such a playback head is little.

My project concerns recording and reproduction of non-sinusoidal and
non-exponential waveforms. Therefore, the waveform of the reproducing current is
different from the recording waveform. In this case, the reactance (XL)
of the playback head does not obey the relation XL=jù*L
because this relation is only for periodic functions whose values depend on time
__continuously.__ In other words, the "connection" between the
angular frequency (ù) and self-inductance coefficient (L) is different from the
usual applications. This results in very low impedance meaning that ** a
large current can pass through the playback head winding** – the
internal impedance of the playback head becomes very low. So, the coilwire
diameter of Playback Heads for the PPS must be large with respect to the
coilwire diameter of standardized magnetic heads. In the sequel, I specify the
technical characteristics of the required

Figure 5 is a schematic of the required Recording Head.

I recommend Heavy Insulation coilwire: 0.76 mm total diameter or 21 AWG (0.031 inches total diameter).

** About the number of turns (N).** Because of the waveform of
the reproducing current is nearly like the Direct Current, we may use a larger N
than N of the Recording Head. However, if N is too large then it will cause:

We should avoid both of them.

** Front gap material:** Glass Bonding as the Recording Head.

** About the front gap length:** Although the required gap
length is 1 micrometer, as the Recording Head, I suggest that the Prototypes
Playback Heads have distinct gap lengths so that we perform tests regarding the
accommodation of the output voltage and current. For example, when the gap
length is small, we have lower voltage than voltage of long gap because of a
very small gap length implies that the magnetic conductance of the gap becomes
comparable to the magnetic conductance of the head core (the head efficiency is
proportional to the front gap length). On the other hand, we have larger current
because the coercive force against the coercivity of the magnetic tape is lower
than coercive force of long gap (the longer the gap the more the wavelengths
which are included into the front gap area, for a given reproducing frequency).

When the gap length is large, we have higher voltage but we have also less current.

** Core Material:** Hot Isostatically Pressed MANGANESE-ZINC

** The rear gap** must have the lowest reluctance possible because
this reluctance reduces the magnetic conductance of the head core, it reduces
the head efficiency. We may, for example, use a

** The entire head** is protected against parasitic magnetic fields
like the Recording Head.

The Figure 6 is a sketch of the required Playback Head. The Recording Head is
the same except for:

**1.** Coilwire size, **2.** Number of turns and **3.** Reluctance of
the rear gap.

This sketch gives you the required core size with the critical dimensions and
the Figure 7 shows the detailed actual size of the required Recording &
Playback Head for the PPS. The Terminals are tinned wires or contacts (not shown
at the following Figures). Outriggers or other tape guides are not necessary.

Both Recording and Playback Heads do have polished gap surface – as the
Standardized Ferrite Tape Heads – so that the head wear, due to the tape
motion onto the head, is minimum.

** The output voltage of each Prototype Playback Head is some volts**
(about 10 or 12 V depending on the front gap length and reproducing frequency)
while

I have already taken measurements concerned permeability versus temperature and permeability versus frequency using standardized E-shape ferrite cores. The results are very different from the manufacturer measurements. My opinion is that these differentiations occur because of the ferrimagnetic resonance appears at higher frequencies (due to the non-opposite magnetic flux). This is a fundamental property of the Ferrimagnetism that concerns the magnetic anisotropy. For example, the manufacturer diagram shows that the initial permeability (ìr) is abruptly reduced after the 500 KHz. This behavior concerns only sinusoidal waveforms.

Basil Dimitropoulos

Electrical Engineer

1st E-Mail: pps@groovy.gr 2nd E-Mail: basild@in.gr

104 - 106 Kremou Street, Kallithea, Athens 176-76 GREECE

TEL: +30-10-9590530, FAX: +30-10-9615853

URL: http://users.groovy.gr/~pps