Why do I have to provide a current programming signal to the power supply?

Spellman’s power supplies have two regulating loops, voltage mode and current mode. Most people use our power supplies as a voltage source, controlling and regulating the output voltage in voltage mode.

The current loop of the power supply will limit the current drawn during a short circuit condition to whatever level the current loop (current programming) is set to.

To use the power supply as a voltage source most users set the current limit to maximum and control the voltage programming signal to obtain the desired output voltage. Operated in this manner the unit will function as a voltage source being able to provide programmable and regulated voltage (from 0 to 100% of rated output voltage) up to the unit’s maximum current compliance capability. If a short circuit occurs the unit will cross over into current mode and limit the output current at the unit’s maximum rated current.

If the current loop is mistakenly programmed to zero by leaving the current programming signal disconnected or left at zero, you are telling the power supply to provide “zero” current. The power supply will be happy to provide zero current, by providing zero output voltage. There is nothing actually wrong here with the power supply, the unit is just doing what it is told.

So if you have a power supply that “doesn’t provide any output voltage, even though you have the unit enabled and are dialing up the voltage programming…stop and see where the current programming is set. If the current programming is set to zero, you have found your problem.

Spellman’s rack mount units like the SL, SA, SR and ST have a handy “programming preset feature”. With the unit turned on and in standby, press in and hold the green front panel HV OFF button. With this done (no high voltage is being generated) the front panel digital voltage and current meters will display the user programmed kV and mA levels that the voltage loop and current loop are being provided in actual kV and mA. This is a simple way to check and confirm the programmed voltage and current levels provided to the power supply.