Power Supply from a Magnetic Tape
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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 New Product Development is necessary. This Development concerns Custom Design Ferrite Tape Heads.
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). The recording frequency is 1 MHz (nominal).
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; the force of cohesion between the tape magnetic particles becomes very strong.
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 Prototypes Magnetic Heads.
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:
1. High self-inductance coefficient (L) in a sense that the internal impedance of the playback head will be somewhat increased due to the inductive reactance during the return time of the sawtooth wave (reset mode).
2. High back-magnetomotive force (I*N) in a sense that the coercive force at the front gap will be too high to the coercivity of the magnetic tape although this coercivity will have much higher value than specifications of magnetic tape manufacturers due to the non-opposite magnetic particles.
We should avoid both of them. The N must be one hundred (100).
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.
The front gap length must have three values: 0.5, 1 and 2 micrometers.
Core Material: Hot Isostatically Pressed MANGANESE-ZINC FERRITE MND 5100 (American Trade Name) or equivalent.
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 straight foreword manor meaning that the rear gap does not have silicon oxide, it is nearly closed gap. In this case, the rear gap is essentially zero or very small in relation to the front gap.
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 the voltage drop into the head is approx. 0.3 - 0.5 V. We may use the Playback Heads in various connections: series, parallel etc. The above mean that the output power of the PPS is practically unlimited. Merely, we must use a percentage of the output power for the motor; to start the motor, we may use an electric source. For example, if the motor consumes 6V / 0.3A then the total current into the Playback Head is the nominal 1A + 0.3A = 1.3A. To this point, I explain that the overloading tolerance allows us to overload the Playback Head up to 3A because of: 1. The maximum back-magnetomotive force possible is I*N = 3A*100turns = 300A. 2. The copper wire resists many times the nominal current (1A) for many hours meaning that an 1.3 or 1.5A total current is allowable almost continuously.
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