I have a 72 volts to 12 volts isolated power supply.

When I gave it 72 volts input I used a resistor in series with the +72 volts so I don't send in too much current on startup, I wanted to charge the caps first. I usually do this with other things so I don't get a huge spark.

I tried both a 60 ohm and 600 ohm resistor. Both resistors started to heat up instantly and burn. Nothing was connected to the output of the power supply.

Can I assume the power supply is bad? I assume if I gave it full 72 voltage without the resistor in series it would just burn my connectors.

I assume there is an internal short?

Thanks

 

Post a schematic of the circuit you used.

 

I have a 72 volts to 12 volts isolated power supply.

What type of '72 volt to 12 volt supply' is this? Is this a switching supply?

A switching supply can/will draw more current at a lower input voltage than at a higher voltage. By using the resistor on the input, you are reducing the input voltage and therefore increasing the current the supply wants to draw from the source.

I'd go ahead and just hit it with the full 72 volts and see it if doesn't just start up normally.

 

there is something seriously wrong. Without more info you are going to be on your own.

 

OK, so I think I may know what happened.

The sticker was somehow reversed and I believe I connected the 72 volts to the output 12 volts.

I could have even reversed the polarity on the output voltage connectors.

I don't know how the sticker was reversed, but I should have known when the pins didn't match up on the sides from a normal power supply.

Here is a similar power supply to what I used, except that the sticker is reversed that shows all the polarity. So where it says "t" I actually don't have a terminal on that side but the other. The output voltage terminals are also thicker.

So if I put 72 volts into the output, and reversed the polarity, do you think I killed the power supply?

http://www.ebay.com/itm/Vicor-Conve...258668?hash=item2385c6e06c:g:TlcAAOxyjxlTNHeJ

 

If you look at that ebay link above that is a normal power supply.

Look at mine, the sticker is reversed but the sticker is not removable. Look at the pins on the left and right, you will see the middle pin missing. The output posts also should be thicker on the output.

So what I did was give 72 volts input connected to the 12 volt output, and I also reversed the polarity.

Do you think this killed the power supply?

 

So if I put 72 volts into the output, and reversed the polarity, do you think I killed the power supply?

Quite possibly. The output stage will almost certainly include an electrolytic capacitor, which won't have enjoyed that treatment.

 

I hooked power back up to it in its proper location and the power supply actually works. I then hooked up something to the 12 volts out and pulled up to 8 amps without any issues.

The question now remains, why did reverse polarity, and hooking the 72 volts into the 12 volts output, cause the large current draw? The resistor I used burned bright red, but the power supply did not get hot during this incorrect hookup.

Can I assume the power supply is OK if it seems to be working normally?

 

Hello,

Perhaps there is a big diode inside as reverse voltage protection that burned your resistor.

Bertus

 

If the polarity was reversed, you will probably be forward biasing the power supply output rectifier diode. These are usually have a half wave rectifier as the output. When they are connected correctly, the rectifier diode is blocking but reverse it and you will have the diode conducting in series with the transformer secondary winding. HIGH CURRENT!!

 

That is a first-gen Vicor brick. Variable frequency, zero-voltage switching, strange version of a forward converter.

https://en.wikipedia.org/wiki/Forward_converter

As shown in the concept schematic, the output is a transformer secondary and a diode. If you connect a DC source to the output with the right polarity, it will see one power diode forward voltage in series with two inductors. The diode prevented a large reverse voltage across the internal capacitors. You really lucked out.

Are you stuck with using that series of parts? Their 2nd gen bricks (Maxi, Mini, Micro) are much better behaved.

ak

 

I just bought this off EBAY to use on my electric bike. What I do like about this unit is that it has very low standby current draw.

The Mini or Micro will draw 8 watts on standby (no load power dissipation) or when on and not being used, the vicor I have only draws like 1 watts. Maybe this has to do with the transformer? I know my cheap 72 volt to 12 volt china power supply which has a transformer inside doesn't draw many watts when in standby.

SInce I'm using this with a battery to power the 72 volts I don't want to draw 8 watts in standby. That would be a whopping 192 watts a day if I forgot to turn off the power supply, it would kill the battery really fast.

BTW, I hooked up one of those fluke amp meters to check current draw when I had the voltage hooked up incorrectly. For some reason my fluke didn't read any current when the resistor was glowing red hot. But it worked fine when I hooked it up normally and drew 8 amps from a RC fan.
Any idea why when I was shorting the power supply it didn't read at all on my fluke?

 

The bike has a 72 V battery?

ak

 

Yes it is a 72 volt battery.