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Relation between Voltage, Current, Power and Resistance.

Resistance (Ohm) = Voltage (V) / Current (A)

Resistance is, in a certain environment (temperature, humidity,…) constant. This means that current follows voltage variations and visa versa. It is impossible to force a power supply to deliver a certain current AND a certain voltage.

The resistance of an electrophoresis unit depends on its size, gel thickness, amount of buffer, buffer conductivity and temperature. This resistance will normally decrease in time due to a slowly increasing temperature. Electrophoresis units which have a resistance below the minimum load resistance of a power supply will trigger an alarm! Read the output voltage and current during a run to measure the resistance and use above formula (Ohm's law) to calculate the value.

Power (W) = Voltage (V) x Current (A)

This means that the power depends on both voltage and current. But since current depends on the resistance of the connected unit it's impossible to force to generate a certain power. The only thing that can be done is to limit the power supply to generate a certain power.

Behaviour of a power supply / Constant modes

When RUN is pressed, the internal generator will start building up the high voltage at the output terminals while voltage and current are constantly measured and power is calculated. When one of the pre-set parameters is exceeded, the generator will stop rising and starts to adjust the Voltage to keep that parameter constant.

When one of the other parameters is exceeded while the power supply is already in constant mode, it will switch to the newly exceeded parameter to be held constant.

Make sure that the limits of the parameters that are not ought to be held constant, are set to a sufficient high value:

Constant voltage

To keep the voltage constant, program the desired voltage and a higher current and power than the maximum expected values:

  • Current > Voltage / Resistance
  • Power > Voltage x Current

Constant current

To keep the current constant, program the desired current and a higher voltage and power than the maximum expected values:

  • Voltage > Current x Resistance
  • Power > Voltage x Current

Constant power

To keep the power constant, program the desired power and a higher voltage and current than the maximum expected values:

  • Voltage > Current x Resistance
  • Current > Voltage / Resistance

Why are my output values different from those of a similar experiment?

Either your programmed parameters are not equal to those described or the resistance of your electrophoresis unit is different (see above).

It cannot be due to e.g. an other model of power supply as the relations between Voltage, Current, Power and Resistance are monitored in the same way by any instrument (the electrical laws cannot be disregarded!).

What about connecting more than one unit to the same power supply?

The outlets being in parallel each electrophoresis unit will be supplied with exactly the same voltage. However, current and power may differ due to differences between them even when the same model, gel, buffers, etc… are used. Therefore, it is recommended to run several electrophoresis units only in the constant Voltage mode on the same power supply.

The total current and power of the power supply will rise for each tank connected to it. The sum of the currents of each tank may not exceed the maximum current of the power supply, the same for the sum of the required power.

ps_faq.txt · Last modified: 07/08/2018 14:43 by pieter