90V DC Power Supply Sanity Check

[dksoba]

Hi All,

I need a roughly 90vdc power source for a brake on an AC motor. At this stage, I'd just like to simply release the brake. The resistance of the brake at room temperature is 1.2kohm. At 90v, this corresponds to a current draw of 75mOhms, correct? I'm not sure the best way to make a 90V power supply, so what I'm wondering is if I can make a simple full-bridge rectifier to rectify AC to DC, and have some filter capacitors and of course a fuse. I've not made such a power supply before and I've messed around w/AC *very* little. However, it seems like right now it's a good choice and I need to learn about AC electronics more and more.


Right now, my plan is to use a 2kv variac (which I have on hand) to slowly raise the voltage in a full-bridge rectifier -> capacitor -> load circuit. This way I can test if my rectifier is working as I hoped at *really* low voltages, and slowly raise it up to the voltage I need. However, AC scares the crap out of me so as a sanity check I thought I'd run it by ya'll.

Please advise,
Thanks,
Matt.

 

[mister_rf]

Yes, It’s OK now to use a variac for testing purposes . Need to use some 400V rectifier bridge (1 Amp minimum current handling capability) and for the capacitor use some polarized electrolytic type capacitor value starting from 47uF/350Vdc up to 100uF/350Vdc. A high voltage capacitor it’s recommended now as during the tests accidentally you may reach the upper limit of the power supply line, but later on, using some fix transformer (75-90Vac output) you may use standard 160Vdc -250Vdc capacitors.
Try to respect correct polarity(between rectifier bridge to capacitor), if the polarizes capacitors is connected in opposite polarity, it may explode.
Small correction for current drawn, at 90V and 1.2k load,
I=U/R= Volts/ohms=Amps
I= 90/1200= 0.075A= 75mA

 

[dksoba]

Thanks Mister_rf.

Yes, my units were wrong on the current rating! 11pm... How do you determine the upper capacity limit for the capacitors? I Have some 20000uF 400V capacitors here, can I use them? Since I will slowly raise the voltage, they shouldn't draw too much current. I can probably put a current limiting thermistor or resistor in addition to a fuse there as well.

The capacitors are for a large unregulated DC power supply that I will need to make in the future to supply motor drivers.

Thanks,
Matt

 

[mister_rf]

Yes you can, but that’s way too much for this application and may create additional problems to the rectifier bridge or just in case of short-circuit. A NTC thermistor plus a more powerful bridge rectifier is also required.
Some data about how to calculate the capacitor:
**broken link removed**

 

[dksoba]

Thanks,

I bought some 100uF 450V capacitors from a local surplus electronics supply store. Can't wait to see how it goes!

Matt