[walter]

Hi guys, can I ask for your help?

I'm looking for a simple way of getting 12VDC and 24VDC from my toroid. The label says:

I/P:
1. 115V
2. 115V

O/P:
1. 58V
2. 58V
3. 12V
4. 12V

I measured 12VAC on the two unused pairs and my requirement would be 24VDC for motor brakes and separate 12VDC for the fans. Can this be done with rectifier and perhaps capacitor?

Your help will be appreciated.

[ger21]

Wire two fans in series and you'll only need 24V.

[Al_The_Man]

Wire the 24vdc brake from the two 12v winding in series, use a bridge rect. but do not use a capacitor, this will create 12x1.4v, besides it is not needed for a brake, it may be wise to use a BEMF diode across the brake .
You can try the fans on 24vdc with the ripple, they should be a matched pair for series operation.
Otherwise use a separate bridge on one of the 12v windings with both fans in parallel, if smoothing cap. is required. again the resultant DC will be ~16vdc.
Al.

[walter]

Great idea- I haven't thought of that. Al, could you elaborate on BEMF diode requirement, what will happen if I don't use it? (the motors are 1kW P5 model).

Thanks!

[cjmerlincnc]

Back EMF diode. As the brake is a coil when the transistor that operates it switches off a Back Emf voltage is induced in the coil. If there is no diode across the coil the BEMF can destroy the transistor it can also send a voltage spike on to your DC power line.

As an aside, If you need to drop a few volts off a DC supply, connect a few diodes in series. Each one will drop off about 1.2 volts.

Ugly but practical.


John

[Al_The_Man]

As John outlined the function is to provide a path for the BEMF to dissipate, the slight down side with the Diode is it causes a delay in drop out, not usually a problem with a brake.
The diode is connected so it is normally reverse biased, i.e. cathode to +v.
You do not need to drop any voltage if you do not use a cap, they are not normally used anyway for brakes, solenoids etc.
Al.

[walter]

Great info, thank you very much!