# a couple power supply questions

• posted

Hello,

I gotta couple questions...

1. When you put a volt-meter across something, and you immediately read a voltage, let's say 5V for example and then it slowly decays down to zero.... what does this mean? What's potentially goin on here?

1. Every once in a while I'll measure the resistance across the positive and negative ends of a DC power supply output (when it's off) and find a low resistance... sometimes it's higher... I've seen like

200ohms and I've seen like 2kohms.... what's typical here and what does it mean when I see a very low one and a very high one?

thanks!

• posted

1. You're on the AC volts range.

1. You have a series capacitor in there someplace.

2. The supply is turned off, and you're discharging the filter caps with the meter.

As above. Ohmmeters typically shove a current through the DUT, measure the voltage drop, and divide the two to get a resistance number. If what you're measuring isn't a resistor, you have to remember what it's actually doing.

Cheers

Phil Hobbs

```--
Dr Philip C D Hobbs
Principal```
• posted

1. Capacitor discharging through your volt meter.
2. Output filter capacitors "trying" to discharge through your ohm meter. Try shorting the circuit with a 1K resistors for a few minutes before reading with an ohm meter. Unfortunately the ohm meter is going to charge the caps to some degree (depending on polarity). Do not short power supply outputs powered or unpowered with screwdrivers, pliers, tweezers etc. It is a bad habit to get into. It is fun to see a bright blue spark now and again but the output of power supplies is not an appropriate venue.

Best, EdV

• posted

For #1 the impedance of the meter (10Meg ohm) is most likely discharging the circuit your measuring. Sometimes we will take a 10K ohm resistor and discharge the circuit first if it doesn?t already have a bleeder resister installed. Large electrolytic caps and store a charge for a while.

For #2 we usually see 2k-5k depending upon the circuit for a 5V rail. I use it as a pre-test before powering a circuit up. To eliminate the Woosh Bang Pop.

Reversed electrolytic caps are another story.

Cheers

• posted

```--
#1 has been adequately covered, so I won't go there.

As far as #2 goes, some power supply designs require a minimum output```
• posted

Remember the desktops of the olden days, where if you didn't have a hard drive yet, you had to put a dummy load on the PS, just so it could maintain regulation? :-)

Cheers! Rich

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.