Hello,

Please see my attachment, it is a DC supply setup using to provide 24VDC and 48VDC. I don't know why as soon as I applied the 120vac in from wall, the R1 (0.2 Ohm, 80W) always blew up and caused no 48vdc, I tried replace R1 two times and it is the same problem. Without R1, I can powered up DC1 by itself, it is 24 vdc, but when I connect R1 to the setup in series with DC2 to make 48vdc, the R1 dead right away. I need your advises.

Thanks

CN

 

Are they SMPS or transformer types ?

 

If you have only 120VAC connected and nothing connected to the 48V and 24V outputs, you must not have it wired as you have it drawn. To blow an 80 watt resistor, those must be some terrific power supplies.

 

when I connect R1 to the setup in series with DC2 to make 48vdc, the R1 dead right away.

R2 is in series with DC2, not R1. Do the supplies have their - terminal connected to mains ground? If they do then you're connecting R2 directly across DC2 (both - terminals would be connected together via mains ground).

24 / 0.2 = 120 amps drawn by R2 meaning 2.88KW power dissipation. (assuming your supplies can put out that much power for a breif moment)

 

This is not my setup. I got this fixture from somebody else, and I just looked on the setup and redrew it. DC1 and DC2 are AC/DC power supply and used to apply 24V or 48v to Unit Under Test depend on the model. The rating of 2 DC supplies is 24V, 25A each. If we test UUT for 24V, we use DC2 only, and if we test the UUT for 48V, we use both DC1 and DC2 in series. The only we concern is R1 had blown up 2 times. As BillB3857 mentioned the DC1 may be the cause, I also thought so. But DC1 is still turned on and produced 24V at its output terminal.
I ran out of R1, is there any part out here can be substituted for R1?

Thanks

 

What is the expected load at 48V? Does R2 get hot also? If the power supplies are truly in series, both resistors should be carrying the same amount of current.

 

R2 is still good (not getting hot) even R1 gone bad couple times. After I replaced the R1, I tried to turn on both DC supplies to measure the DC output 24V and 48V before connecting 48V to Unit Under Test. When I measured it, I got 24 vdc, but 0v at 48v, and then I know R1 is dead.

Is it possible output at DC1 producing inrush current or spike when turning on the power?

Thanks

 

Is there continuity between either GND and DC1's secondary, or either N or Line to the secondary? That could cause problems.

 

If they are SMPS, it might too ?
He did not answer that and how are we to give any remedy?

 

Post a picture of the power supplies and the 80W resistor.

What is the DUT's resistance?

 

I just came back from vacation, so sorry I did not anwer R!@@ 's question.

This is a Lambda power supply. Here is the link
http://www.alliedelec.com/images/products/datasheets/bm/LAMBDA_AMERICAS/70176986.pdf

I checked all continuities, every thing looks normal.

And here is the link of foil power resistor
http://parts.digikey.com/1/parts/1222542-res-1-ohm-80w-1-foil-227-fnr-2-t227-0-100-ohm-1-20ppm.html

DUT's resistance is 0.2 Ohm