Servo PSU - drop a few volts; ~90 to 75 @ ~2 amps - Page 1
Forum section: electronics / projects

author: Kremmen
author_rank: Super Contributor
timestamp: November 08, 2013, 09:18:31 am
content: I promised to hammer together a quick & dirty(ish) PSU for a small test CNC mill for a friend. The mill will reuse 3 vintage Sanyo Denki brush DC servomotors, specced at nominal 75VDC, ~50W, with a 24VDC brake eating a couple hundred mA each. So the PSU needs to produce 75 VDC @ max ~2A continuous and 24 VDC @ ~500mA continuous.I am not going to make rocket science out of it, so rummaging through the parts bins i located this 230VAC to 30+30VAC ~180 VA iron transformer. This is definitely going to be the main step down component.A quick no-load measurement revealed the voltage to be 68 VAC, accordingly the Vp of the rectified voltage will idle at ~96V or so and ~85 V at nominal load. This i consider a bit too much for the servos (actually the servo amps) so i want to drop it back to 75V. This being a servo PSU the voltage does not have to be lab quality - in fact it really does not need to be regulated as such. It just needs to be closer to 75 volts than what i get directly from the transformer after rectification.Plan A: I could do the 3 component trick and use a resistor-zener-NPN circuit to drop the extra voltage. The dissipation however will be significant, on the close order of 30 W which is doable but a bit much. I would really like to avoid any special cooling arrangements in a smallish application like this.Plan B: i could cobble together a simple buck circuit to shed the extra voltage with very much smaller losses. The D however will be around 0.78 and this is in the region of subharmonic instability in a trivial buck. So i need to do something about it, like a ramp compensator in the oscillator part or something. That in turn has the potential of becoming a project, which i am trying to avoid if possible.Plan C: Locate a suitable buck IC and just apply it. I have spent a couple hours scanning DK, trying to find ms Right but so far with lukewarm success. Yes, there are chips that can be used and that will work, but my question really is, has someone done something like this recently, with a solution that a) works and b) needs a minimum amount of lab work to get there? The IC could be there but i just haven't spotted it...I will do it one way or the other, it is just a question if someone has implemented a working solution that i could replicate. Also bright ideas are welcome, of course

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 08, 2013, 09:40:48 am
content: Can you add windings to the primary? or remove them from the secondary?

author: Kremmen
author_rank: Super Contributor
timestamp: November 08, 2013, 10:04:45 am
content: I could, of course. Only i don't want to mess with the pristine transformer that is actually spot-on for this application. Lowering the idle voltage to not exceed 75V would cause the full load voltage to droop uncomfortably low, so i don't want to go there. If the transformer was significantly beefier then voltage droop would not be a problem but this xformer is actually just the right size and there it would be.Meanwhile, plan C is nudging ahead. IR has come up with an interesting chip intended for driving LEDs from a high voltage source - the IRS25401 and IRS25411. Those babies work with rails of 200Vmax or 600Vmax respectively and have an external switch so they are not inherently limited to any particular max. current. The only thing to watch out for is that the chip is intended as a constant current source instead of a constant voltage one. Yet, any buck relgulator works the same and if the control loop is closed appropriately then it will just do its thing. This one wants to see 500mV in the feedback input and won't know if that is produced by a voltage loss over a current sense resistor or, by a voltage divider across the output. The only thing is that the latter wont provide an inherent current limit so i need to look into that a bit more. But it appears promising anyway...