a couple power supply questions

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

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

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

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

As far as #2 goes, some power supply designs require a minimum output load to maintain regulation, and that load is included in the supply itself and is connected across the output terminals, internally.

Then, when you measure the resistance across the terminals, with the supply turned off, what you're measuring is the resistance of that load.

Other power supplies, notably high voltage, have what's called a "bleeder" resistor tied across their outputs so that when the supply is powered down, any energy stored in its output is dissipated safely, and quickly, so that any inadvertant human/machine contact won't result in harm to the human.

--- JF

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

in

I used to use a 12 volt automotive tail light as a load on the 5 volt output to get the thing to turn on.

-Bill