[GUIDOSAN]

First this is a scrap magnet on an excavator so AC is unavailable.

I have a single 24volt charging source of 70 amps linked to four batteries (8D's) as buffers. I need to feed both 12 and 24 VDC into a control panel to run a magnet at either 12 or 24 volt.

It draws 100 amps on 24v and 60 amps on 12v. The 24v operation is at 75% duty cycle but the force is much higher and the 12 v is 100 % duty cycle. I need to be able to use both and I can't switch off the 24V since it is the control voltage.

I tried isolating with diodes but - no go. It would be simple to switch from parallel to series with SCRS or relays but I lose the 24V feed and the charging voltage when I do.

I considered using an SCR and pulsing it (PWM) to get a 12v output while still able to use the 24v for the control circuit and charging. The quality is not critical nor is the exact voltage as this is just feeding an electromagnet.

What other options are there ? I know i'm probably overlooking something.

Guy

[Jim Dawson]

Back in the day (early 70's) I used to work on heavy trucks. They used a series/parallel solenoid switch for the 24V starter. The rest of the truck was 12V, as was the charging system. I don't remember exactly how it was wired up but I'll bet Google does.

[GUIDOSAN]

It would be easier if I had a 12vdc charging system - but I don't.
The machine is 24vdc.
Thanks for the idea though - I'm hoping there's something like that I missed.
If I didn't need both voltages simultaneously it would be easy.

Guy

[Jim Dawson]

Assuming you only want to switch the magnet voltage, the series/parallel switch idea would still work, you just need to do the switching downstream of the control system. I would pull the control voltage from a separate feed, then that would allow you to select 24 or 12 V for the magnet. A battery isolator might do the trick also.

[skrubol]

Seeing as you can't run series and parallel at the same time, your best bet is probably some sort of PWM controller. A 24v DC motor controller that can handle the amps might do the trick. That would let you vary the equivalent voltage from 0-24V.

[GUIDOSAN]

Assuming you only want to switch the magnet voltage, the series/parallel switch idea would still work, you just need to do the switching downstream of the control system. I would pull the control voltage from a separate feed, then that would allow you to select 24 or 12 V for the magnet. A battery isolator might do the trick also.

It makes sense - I'll set up a test on the bench tonight and use diodes on the two 12v legs
Thanks !