ac or dc

[cueshark]

i wonder if anyone can tell me how to tell me how to tell if 80 volts at my ps output is ac or dc? i was told by a gecko tech that my motor hum sounded like ac 60 hertz. so i need to know how to tell. thanks for any help anyone can give. greg

[Al_The_Man]

What kind of PS is it commercial? Home made? You should definately have DC.
if you have a bridge and large Electrolytic capacitors on the output of the suply then it is DC.
If the caps had AC on them, they would have overheated and probabally blown up by now.
Al.

[cueshark]

it is homemade according to cambells tutorial. it is a mackie 1400 watt toridal
transformer [aprox 57 volt ac out] with a 100 volt 35 amp bridge rectifer &a 100volt 21000 uf cap.
so is there no way to check [cheap & easy] to tell if it is ac or dc.
the tech at gecko had me put the phone up against the motor so he could hear it. he said it was a 60 hertz hum. thanks greg

[Al_The_Man]

It has to be putting out DC otherwise you would see drastic symptoms if AC was present.
Make sure it is off load and use a Multimeter on the output using both AC & DC ranges.
The ripple output of a Bridge should be 120hz. But with no load you should expect virtually pure DC with no ripple.
It is possible you have a capacitor failed or dried up.
Without any capacitive effect, the DC level output of the bridge would not be the expected ACx1.414 either.
You really could use a 'scope to tell accurately what quality of DC you have there.
Al.

[picbits]

How much current are you drawing from it ? At 35amps the 21000uf capacitor is going to struggle to smooth the power supply at all.

You can add more 21000uf capacitors in parallel with the existing one (or any other high value as long as the voltage rating is correct) until your hum goes away.

[mackeym]

use a multimeter that can read both AC and DC and check both on the meter. try to same thing on a battery (DC) and then the same thing in a wall socket (AC) (be careful). You may be able to tell from the difference in the reading, wether you have AC or DC. Some calculations might prove what relationship you should get.

[mackeym]

This is one method that will show wether you have AC or DC...buy a 1uF film capacitor (radioshack has this) that's rated to at least 120V DC or 80VAC. Connect the capacitor to the PS output. Then measure the AC and DC current through the capacitor with a multimeter (my meter has a setting for AC and a setting for DC current). Measure both the AC and DC current through the capacitor. If you have a DC PS then you should measure almost no current in both the AC and DC settings on your multimeter. If your power supply is AC, then the AC current that you should measure is (2*Pi)(80V)(60Hz)(1E-6F)=0.03A. The DC current will probably be some reading that's unstable.

[Al_The_Man]

This test is OK if it is actually AC if it is a 120hz 100% ripple the cap will charge an indicate no ripple if no load.
If it is rectified and electrolytic smoothed and the DC output off load is practically the same as the RMS, this indicates bad Caps.
Al.

[vger]

Yes a scope would be the best way. There is a way to determine if you have ripple on the PS without a meter or a scope.... The load (machine or dummy load) must be connected with a multi turn coil of wire in series with the load. Just take some insulated wire that is the same guage or larger than the PS output leads and make a small coil about 1 to 2 inches in diameter with a bunch of turns. Connect coil so that it is in series with the load, so that the current goes through the coil then to the load and back to the supply. Power everything up and then bring a small magnet near the coil. If you feel the magnet vibrating in your fingers, you have either AC or a fair amount of ripple.

Steve

[GrahamIT]

A quick test for AC on a supposedly DC supply can be, connect another transformer primary across the DC output and measure the AC voltage on the second secondary winding while under load.

A scope is better bet though.

[unterhaus]

as makeym suggested, a digital volt meter that has both ac volts and dc volts should tell you the answer. If there is no ac, it will read very close to zero on VAC readings. If there is significant DC, most dvm's will not give a steady reading on vdc readings.