Known current stimulation apparatus have a current waveform generator with a constant-current output stage, which operates with a relatively high operating voltage (up to several 100 Volts), in order that the necessary currents can be applied even in the case of extremely high patient resistances. In the application of large-surface electrodes, the load resistances are generally comparatively low. The operating voltage required for the necessary currents for such normal instances is here substantially lower than the high operating voltage actually available, which leads to unpleasant irritation phenomena due to an increase in the current density when the resistance is suddenly altered due to the separation of the electrodes or the like. In accordance with the illustrated embodiment, the constant-current stage and the operating voltage supply are connected to an operating voltage control circuit which, in dependence upon the load resistance and/or the current intensity, regulates the operating voltage (U.sub.B) for the constant current stage from an initial voltage value (U.sub.V) to higher values with an increasing load resistance, whereby, however, the control time constant is such that, in the case of a differential resistance change which exceeds a specifiable threshold, the change in the operating voltage proceeds less rapidly than the resistance change. In the case of rapid change, for example, due to the separation and falling-off of electrodes, the operating voltage is thus only slowly adjusted upwardly such that undesired irritation pheomena are prevented.

BACKGROUND OF THE INVENTION 
The invention relates to an electromedical apparatus, particularly a 
stimulation current apparatus comprising a current waveform generator as 
well as a constant current stage for controlling the output current 
through a load resistance, preferably the human body of a patient, which 
functions with a specifiable operating voltage. 
Known apparatus of this type, particularly electromedical stimulation 
apparatus, have a constant current output stage which operates with a 
relatively high operating voltage (up to several 100 V). This is intended 
to guarantee that, even in the case of extremely high patient resistances, 
the currents necessary for therapy or diagnosis, respectively, can be 
applied with the correspondingly high operating voltage requirement. In 
the case of a conventional method of treatment, the patient resistances 
are indeed relatively low (e.g. in the case of application of 
large-surface electrodes); thus, during application in these normal cases, 
an operating voltage is needed for the required currents which lies 
substantially lower than the high operating voltage available. This 
discrepancy leads to very unpleasant occurrences in those instances when 
e.g. the patient resistance is suddenly varied; this particularly applies 
to the instance in which the current supply electrodes have become 
disengaged at the application location and have finally completely dropped 
off. At the moment of detachment, namely, the area of current passage 
becomes rapidly reduced such that, in conjunction with the characteristic 
of a constant current source, the current density on the skin takes on 
very high values at the application location. This leads to a very 
unpleasant, painful irritation of the patient treated. In many instances, 
also, direct burns of the skin are brought about as a consequence. If the 
supply electrodes have finally completely dropped off, the full, 
dangerously high operating voltage is connected between the electrodes as 
long as the apparatus is still in operation and continues to function with 
the previously set intensity. 
SUMMARY OF THE INVENTION 
In an electromedical apparatus of the type initially cited, it is the 
object of the present invention to ensure that unpleasant current - or 
voltage - effects are avoided with certainty in the case of a sudden 
deviation from the normal operating conditions. In particular, in the case 
of stimulation current apparatus, the occurrence of unpleasant or even 
dangerous irritations is to be prevented when the current supply 
electrodes become detached and drop off. In addition, the dropped-off 
electrodes are to be prevented from carrying the high operating voltage. 
In accordance with the invention, the object is achieved by virtue of the 
fact that the constant current stage and the operating voltage supply are 
connected to an operating voltage control circuit which regulates the 
operating voltage, in dependence upon the load resistance and/or current 
intensity, from an initial voltage value at an initial intensity, 
preferably zero, to higher values with an increasing load resistance, 
whereby, however, the control time constant of the operating voltage 
control circuit is matched relative to the differential resistance change 
of the load resistance such that, in the case of a differential resistance 
change which exceeds a specifiable threshold, the change in the operating 
voltage proceeds less rapidly than the resistance change, as a consequence 
of which the voltage control circuit is switched to a lower operating 
voltage value, preferably to the initial voltage value of the operating 
voltage. 
In the apparatus in accordance with the invention, in the normal case, 
there is always a relatively low operating voltage connected to the load 
resistance; for example, the patient during stimulation current treatment. 
Only when the patient resistance increases or e.g. the intensity is 
adjusted upwardly, does the operating voltage automatically adapt itself 
correspondingly to the higher required value. As long as a change of the 
cited magnitudes proceeds relatively slowly, the operating voltage follows 
these changes in direct proportionality. However, if the change proceeds 
very rapidly--which is e.g. the case when the electrodes become detached 
and drop off or if there are very rapid intensity changes--the operating 
voltage is upwardly adjusted only very slowly. Due to the changeover 
switching to the lower operating voltage value which then takes place, 
unpleasantly high current densities are prevented and, in the case of 
dropping-off of the electrodes, only the low operating voltage value is 
then still connected to these electrodes. As already indicated, unpleasant 
irritations can also be brought about through a too rapid upward 
adjustment of the current intensity by means of an intensity adjustment 
member. Thus, in an advantageous further development of the invention, 
even in such a case wherein the current intensity change takes place with 
a rate exceeding a specifiable threshold, the change of the operating 
voltage by means of the operating voltage control circuit should proceed 
with a greater time constant than the differential voltage change via the 
load resistance. In this specific instance, precisely the same effect 
occurs as, for example, during the detachment and subsequent dropping-off 
of electrodes; i.e., the operating voltage value, just as in the above 
instance, is switched to a lower value. 
Further advantages and details of the invention are apparent from the 
following description of a sample embodiment on the basis of the drawing 
in conjunction with the subclaims; and other objects, features and 
advantages will be apparent from this detailed disclosure and from the 
appended claims.

DETAILED DESCRIPTION 
In the basic circuit diagram, 1 designates a current waveform generator for 
stimulation currents to be selected. Current waveform generator 1 is 
connected via a current intensity adjustment member 2 (potentiometer), 
with a constant current stage 3 which consists, in a conventional manner, 
of a voltage amplifier 4, transistor 5, and transistor-emitter-resistance 
6. As indicated by broken lines, the variable patient resistance 9 is 
connected between the current supply electrodes 7 and 8. The operating 
voltage for the patient circuit is referenced with U.sub.B. In addition, 
there is disposed in the patient circuit a measuring resistance 10 for the 
actual value of the patient current which is connected via an amplifier 
stage 11 to a display unit 12 for indicating the actual value of the 
patient current, on the one hand, and which is connected to a comparator 
input of a comparator 13, on the other hand. The respective nominal value 
of the current intensity, which is determined by the output voltage of the 
intensity adjustment member 2, is connected to the other comparator input 
of comparator 13. In the case of deviations between the actual and nominal 
values, comparator 13 generates an output signal which switches transistor 
14 into the conductive state, as a consequence of which a relatively low 
voltage U.sub.V (U.sub.V may be approximately 10 to 20 V) is connected, in 
the illustrated manner, into the functional circuit of an operating 
voltage control unit 15. The operating voltage control unit 15 comprises a 
voltage amplifier 16 for a voltage tapped via the collector-emitter-path 
of the transistor 5 in the constant current stage 3. It comprises, in 
addition, the resistance 17, connected to the output side, as well as a 
voltage adjustment member 18 (operational amplifier) to which a comparison 
voltage U.sub.V is connected (which in the present sample embodiment is 
equal to the voltage U.sub.V of transistor 14). Voltage amplifier 18 is 
followed, via a diode 19 at the coupling point of transistor 14, by a 
storage capacitance 20 with a succeeding first control transistor 21 which 
is wired with resistances 22 and 23. Transistor 21 is followed by a second 
control transistor 24 with the wiring resistance 25. Control transistor 24 
is connected at its collector-end to the DC voltage supply U.sub.O. In the 
operating voltage control circuit 15, in addition, there is arranged 
before voltage adjustment member 18 an additional switching transistor 26 
which connects, in dependence upon the current waveform of the current 
generator 1, to the voltage adjustment member 18 voltage signals from the 
constant current stage 3 only when current maxima occur in the load 
circuit. 
The method of operation of the basic circuit is as follows: 
Given zero intensity at intensity adjustment member 2, the control of the 
operating voltage U.sub.B proceeds via operating voltage control circuit 
15 such that the operating voltage value U.sub.B is adjusted to the 
relatively low operating value U.sub.B (U.sub.V) corresponding to the 
value U.sub.V of the comparison voltage of the voltage adjustment member 
18. If, in the case of applied electrodes 7 and 8, the intensity of the 
stimulation current in the patient circuit is slowly adjusted upwardly for 
the case of a specified patient resistance 9, the voltage across the 
patient resistance increases, whereas the voltage drop across transistor 5 
is slowly reduced. The slow reduction in the voltage across transistor 5 
effects in the operating voltage control circuit 15 a slow control of the 
operating voltage value U.sub.B to a higher value (U.sub.B greater than 
the value corresponding to U.sub.V). The time constant of the operating 
voltage control amounts to .tau.=C.multidot.R.parallel.R.sub.ET, wherein C 
is the capacitance of the capacitor 20 and R.parallel.R.sub.ET is the 
resistance value of the parallel circuit consisting of the resistance 22 
with the resistance value R and the input resistance of transistor 21 with 
resistance value R.sub.ET. As long as changes in the intensity of the 
current in the patient circuit proceed very slowly or also as long as the 
patient resistance itself changes only very slowly, the adaptation of the 
operating voltage U.sub.B proceeds with approximately the time constant of 
this change. However, if the change in the current intensity or also the 
change in the patient resistance exceeds a predetermined rate of change 
threshold--which is always the case when electrodes 7 or 8, on the 
patient's body become loosened and drop off, or if the current intensity 
at the current adjustment member 2 is inadvertently too rapidly 
increased--the operating voltage U.sub.B is adjusted less rapidly to 
higher values, by means of the operating voltage control unit 15, than 
would be required to maintain the corresponding stimulation current. 
However, there thus also results a disproportion between the actual value 
of the current in the patient circuit and the nominal value adjusted by 
means of intensity adjustment unit 2. The actual value drops, since, due 
to the very rapid increase of the patient resistance, for example, 
transistor 5 of the constant current stage reaches saturation, as a 
consequence of which the voltage across transistor 5 drops very rapidly. 
Since the drop in the voltage across transistor 5 proceeds with a very 
much smaller time constant than the control time constant of the control 
circuit 15, the actual value of the patient current decreases on account 
of the operating voltage U.sub.B following only with a very great delay. 
The decrease of the actual value; i.e., the deviation from the nominal 
value, is, however, detected by comparator 13 and responded to with the 
emission of a switching signal for transistor 14. Transistor 14 now 
connects voltage value U.sub.V to the capacitor 20 of the operating 
voltage control stage 15, which voltage value V.sub.V corresponds to the 
initial control voltage value at the zero intensity setting. In this 
manner, the relatively low operating voltage value U.sub.B (U.sub.V) is 
then automatically adjusted at electrodes 7 and 8. Thus, with the 
illustrated circuit, it has been made possible that, in the case of 
detachment and dropping off of the electrodes 7 or 8 from the patient's 
body 9, the actual value of the stimulation current in the patient circuit 
decreases very rapidly on account of the very delayed voltage 
readjustment. In the case of repeated contact of electrodes 7 or 8 with 
the patient's body prior to dropping off, high current densities during 
passage onto the skin will with certainty not occur; unpleasant or painful 
irritations or even burns on the skin are thus avoided. If the electrodes 
have finally fallen off, the very low and thus also non-dangerous 
operating voltage value U.sub.B (U.sub.V) is then connected to these 
electrodes, which operating voltage value corresponds to that present in 
the case of the zero intensity setting. The patient is thus protected from 
unpleasant effects in the case of electrode loosening or falling off. The 
same also applies to the instance of an excessively rapid current 
intensity increase. If, namely, the adjustment member 2 is carelessly 
adjusted upwardly too rapidly, the same effect will occur as in the case 
of excessively rapid resistance change. Transistor 5 is rapidly brought to 
saturation, while the actual value of the current in the patient circuit 
rapidly drops relative to the setting value at adjustment member 2. 
Comparator 13 again becomes activated, so that voltage value U.sub.V is 
connected via transistor 14 into the operating voltage control circuit 15. 
Thus, also in the case of excessively rapid current increase, the 
operating voltage value is adjusted to a relatively low value. Thus, also 
regarded from this point of view, the patient can no longer be burdened or 
even endangered by excessively high current densities. 
It will be apparent that many modifications and variations may be effected 
without departing from the scope of the novel concepts and teachings of 
the present invention.