HP Power Supply 6632A: Fast Mode vs Normal Mode?

[cook]

FAST MODE vs NORMAL MODE

I have recently acquired a HP 6632A DC power supply. I am just beginning to use it and I have been reading the manuals for it. It is the first power supply I have ever owned or used. I am a very beginner hobbyist and do not have an engineering background, as may be obvious by my question...

I can't seem to understand what the actual use is for the 'Fast Mode' or the 'Normal Mode' on the power supply. Although the manual states what the two modes do from a engineering/technical standpoint, I don't know enough to understand what it all means, other than the voltage output stabilizes faster in the FAST mode when there is a change in the load current.

Could someone who is familiar with this power supply provide a basic explanation about what the two modes are for and when you would typically use them?

My primary use for the power supply will be to power different parts of circuits, and to also help find shorted components by heating them up.

Thank you.

Below is the manual's explanation of two modes that HP provides. (Note that this is a HP-IB equipped power supply that is also programmable.)

"The power supply can operate either in NORMAL mode or FAST mode. These modes are selected by a switch located on the rear panel. If the FAST mode is selected, the supply will demonstrate enhanced characteristics in certain specifications and degraded characteristics in others. The following paragraphs discuss these specifications and the manner in which they are affected.

1. In FAST mode, the programming time for voltage programming is shorter than it is for NORMAL mode operation, but the output noise is greater. {The manual then references a table that lists various response times for NORMAL and FAST modes.}

2. When FAST mode is selected, there is no internal output capacitor, and any substantial addition of an external capacitor will reduce the stability of the supply in constant voltage operation. in NORMAL mode, a large internal capacitor is always present across the output, and in addition, the supply can maintain stability for large external load capacitors. {It then references a paragraph that covers capacitive loading.} In NORMAL mode, the internal output capacitor helps to control peak voltage excursions away from the nominal value for sudden changes in load current. However, if the output current is programmed to less than 40% of its full scale value, this capacitor may affect programming times, since the output voltage slew rate will be limited to the programmed current divided by the output capacitance.

3. When the supply is in constant current and FAST mode is selected, the absence of the output capacitor gives rise to increased output impedance and therefore enhanced overall performance. Under these conditions, the supply can drive an inductive load and exhibit greater stability. {It then references a paragraph on inductive loading.}

4. In FAST mode, the output noise can be substantially reduced if you disable the front panel metering using the DSP 0 command, and refrain from sending the VOUT?, IOUT? commands. This occurs because the largest contribution to output noise is from the internal A/D converter, which operates whenever the front panel meters are updated, or voltage and current queries are made. In NORMAL mode, the A/D converter does not contribute to the output noise. If your application is such that you have no strong reason to select either NORMAL or FAST mode, it is recommended that you use NORMAL mode."

[eccerr0r]

Re: HP Power Supply 6632A: Fast Mode vs Normal Mode?

Not familiar with your specific power supply, but this response is after reading the description by an engineer.

Sounds like normal mode is best for powering DC devices "normally". Keeps output voltage stable, best for constant voltage mode.

Fast mode is probably better for testing for shorts and charging batteries. Best for constant current mode. There is a warning that if you put your own capacitor at the end of your wire, the output voltage could end up oscillating (go unstable).

The selection of the switch is there because the power supply has no idea what it's connected.

For my use I'd probably leave it as Normal as more often than not, my power supplies are used to actually power stuff, and benefit from stable voltages.

[redwire]

Re: HP Power Supply 6632A: Fast Mode vs Normal Mode?

Almost all cheapola bench power supplies have a large output capacitor to keep them stable, which makes CC mode lousy.

You need FAST mode when doing CC stuff. It's important.
Example is set to 20V at 20mA, and testing unknown LED's. So the supply sits there at 20V out, and connecting an LED it should drop to say 3.4V at 20mA.
In SLOW mode, the output cap is charged to 20V which is a huge dump into the LED and kills it. Poof.
In FAST mode, there is no output cap so any overshoot is small due to the control loop, and the LED is OK.

[cook]

Re: HP Power Supply 6632A: Fast Mode vs Normal Mode?

eccerr0r and redwire,

Thanks for those responses. There is a lot of information in both responses. I will be going over them more in the upcoming days as time permits because I want to be sure I understand them so I can use the PSU appropriately.

Thank you.