A Day in the Lab

10 04 2010

Excellent day in the lab.

I got the elevator key copied, so I’ll never get locked out of the lab. I did a successful test of the mechanical relay.

I rearranged the equipment rack to make room for the coil power supply:

Using 80/20 I constructed a 4U rack mountable chassis for the coil power supply:

Next I mounted the electrical hardware on the sheet aluminum. I started by using a transfer punch to make guides for the drilling. Then I drilled and tapped the holes for M3 bolts:

The first circuit I wired up was the 2 KΩ bleed resistor. Here you can see the bleed resistor wired to the capacitor via a switch. Turning ON the switch bleeds the capacitor:

Next I wired the DC side to the capacitor while minding polarity. Put a voltage meter across the capacitor:

Using the variac I slowly brought up the capacitor to 300V:

Once the capacitor is charged… it holds its charge without much dissipation after the variac goes off.

I was planning to power the relay using this ATX power supply (24V across +12V and -12V):

But during my initial testing it started squeeling and soon blew its fuse!

There was no short circuit that I could see, so perhaps it just died of natural causes?

Anyway… it reminds me: I should fuse the  coil power supply before I go much further!



5 responses

11 04 2010
M. Simon

The computer supply is not designed to be used as a 24V supply the way you are using it. I think you killed the supply.

11 04 2010

Perhaps. All I did was briefly put a voltage meter across the -12V / +12V.

It blew its fuse long after I removed the voltage meter.

11 04 2010

i read somewhere that atx power supplys are designed to be under load or will fail, perhaps it died because you had it on without load

13 04 2010

I use old ATX supplies all the time for various projects. IMO they can take a bit of abuse. I’ve left them on with no load for extended periods without issue, but maybe I’ve just been lucky. Dust is their main enemy. I haven’t tried it but I’m pretty sure you cannot connect power between the different ‘rails’, e.g. don’t take the -12V from one connector and the +12V from the other. It needs to come off the same rail, which also limits the wattage per rail.

15 04 2010

The other posters are correct, that you can’t use a computer power supply in this way. If you are looking just to charge the capacitors, a simple 24V DC power supply isn’t hard to make. It may be worth it to buy a used bench power supply with a variable output so it can be used for different applications.

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