Hi everyone. I have a project which needs to pull power from a DB25 cable. The power coming from this supply is unregulated up to 80V, and I need to step this down and regulate the power to a clean 5V signal. Can anyone recommend a good breakout boards or circuit setup for something like this? I found a board like this Voltage Regulator but it says it only allows 0.75A of output current and I may need closer to 2A output. Any thoughts?
andyopayne:
Hi everyone. I have a project which needs to pull power from a DB25 cable. The power coming from this supply is unregulated up to 80V, and I need to step this down and regulate the power to a clean 5V signal. Can anyone recommend a good breakout boards or circuit setup for something like this? I found a board like this Voltage Regulator but it says it only allows 0.75A of output current and I may need closer to 2A output. Any thoughts?
Any of the DB series of connectors are designed for signal voltages, not current. Before you go any further, please test your connection at 2 amps for several days and see if you encounter problems.
Paul
Thanks @Paul_KD7HB. According to the spec I've received, the DB25 port will provide voltage range of 35-80V with a max current of 3A/pin. I'm actually only driving two servos and a couple of sensors, etc... so my current draw should be well within a 2A range... but probably more than 0.75A. I did find this step down regulator which provides for more output current. Does this seem reasonable for my application?
Don't unplug the DB25 while under power!
Paul
At 80v input, the current will be much lower than the current you're drawing from the output at 5v. (1/16th, divided by the efficiency of the buck converter, probably 0.85~0.95). So for 2A output, you'd be looking at like 150mA input - if the 5v at higher current isn't going back through the DB25 cable for some reason, the current through the cable is not going to be problematic...
I don't suppose you can find a step-down/buck converter that accepts a little more than 80v max - you never want to use something (especially a cheap made in china module) right at the limit of it's specs.
That cable will hurt if you touched the pins while it's powered, though, that's for sure. For safety, I hope that the cable which is supplying the 80v is a female cable (so you can't accidentally touch the live pins - when you design your own cables for things, you always want the side that can be supplying power to be female, so you can't short the power against debris or electrocute yourself). Put the buck converter in a grounded or non-conductive box too, as you also need to worry about the pins on the underside of the board, and the exposed pins, tops of screws, etc on top of board.
Thanks. That's all super helpful. I hadn't really considered the grounding of my enclosure stuff... so that's something I need to consider.
Andyopayne,
I needed a DC DC converter to go from 50VDC to 12VDC and was struggling to find one. Then I had the idea of trying a mains to 12VDC wall wort to see if it would work on 50VDC. If you take one apart you should see that the mains input is fed through a rectifier. Connect your 80VDC to the rectifier instead of the mains. Every one I tried worked fine at 50VDC, I don't imagine you will have much trouble finding one that works from 80VDC.
If you are new to this then get into the habit of collecting wall worts from old equipment that is being thrown away. I've collected loads of them; most I will never use, but I always have a power supply if I need one.
PerryBebbington:
Andyopayne,I needed a DC DC converter to go from 50VDC to 12VDC and was struggling to find one. Then I had the idea of trying a mains to 12VDC wall wort to see if it would work on 50VDC. If you take one apart you should see that the mains input is fed through a rectifier. Connect your 80VDC to the rectifier instead of the mains. Every one I tried worked fine at 50VDC, I don't imagine you will have much trouble finding one that works from 80VDC.
If you are new to this then get into the habit of collecting wall worts from old equipment that is being thrown away. I've collected loads of them; most I will never use, but I always have a power supply if I need one.
blink why didn't I think of this? (oh, yeah, I have never had to work with an awkwardly high DC voltage)
Yeah, a switchmode supply designed to work off mains power should typically work (you're undervolting it, but if you're not pushing the limit of it's output, it probably doesn't care). Obviously, don't try this with an old-style (transformer -> rectifier) type wallwart. That would not be a good thing to do.
andyopayne:
I'm actually only driving two HS-785HB servos and a couple of sensors, etc... so my current draw should be well within a 2A range... but probably more than 0.75A.
The servos have a stall (startup) current of 1.5Amp@4.8volt (1.8A@6volt) each.
Power supply must be able to provide that.
Leo..
"Unregulated up to 80V"... What is the minimum? If 50V or higher then the wall wart idea seems best.
If it's really 12V with an 80V fault condition then decide on how often 80V will occur and if you even need to make your device survive for that fault.
MorganS:
"Unregulated up to 80V"... What is the minimum? If 50V or higher then the wall wart idea seems best.If it's really 12V with an 80V fault condition then decide on how often 80V will occur and if you even need to make your device survive for that fault.
The specs state that the power voltage ranges from 35V to 58.8V. There is a clamping circuit provided to ensure that the power never exceeds 72V... although it then goes on to say that they recommend being able to handle maximum voltages up to 80V. My assumption is that it would rarely if ever get to that 80V range, so hopefully the step down regulator linked above would be appropriate.
Wawa:
The servos have a stall (startup) current of 1.5Amp@4.8volt (1.8A@6volt) each.
Power supply must be able to provide that.
Leo..
Good eyes! So, yes I likely need to provide more than 2A current given that there are 2 servos and some additional sensors/microcontrollers involved. In the specs, the DB25 port has 4 pins that are capable of providing up to 3A each. I'm assuming I could use an individual pin for each servo connection (just to be safe) and then a third pin to power the rest of the electronics. Would this be ok? I just assume that my ground connections for everything need to be connected together so they're all working from the same reference?
Remember the same current flows in the ground. If you have multiple power pins due to current limitations, you need the same number of ground pins.
andyopayne:
The specs state that the power voltage ranges from 35V to 58.8V. There is a clamping circuit provided to ensure that the power never exceeds 72V... although it then goes on to say that they recommend being able to handle maximum voltages up to 80V. My assumption is that it would rarely if ever get to that 80V range, so hopefully the step down regulator linked above would be appropriate.
Okay.
So within that voltage range, if you're stepping it down to 5v, it would need like 1/6th to 1/9th the current it's outputting at 5v, at the HV input to the step-down converter (though you've still got the 2.1A limit on output of the converter you linked).
It seems like a lot of the above discussion about current through the cable seems to be acting on the assumption that you're drawing up to 1.5A (stall current) from that highish voltage line on the cable, ie, that you're stepping it down before the cable, when my interpretation of what you've said is that the step-down converter and electronics will be after the cable, and the cable won't be carrying the 5v to the servos.
DrAzzy:
Okay.So within that voltage range, if you're stepping it down to 5v, it would need like 1/6th to 1/9th the current it's outputting at 5v, at the HV input to the step-down converter (though you've still got the 2.1A limit on output of the converter you linked).
It seems like a lot of the above discussion about current through the cable seems to be acting on the assumption that you're drawing up to 1.5A (stall current) from that highish voltage line on the cable, ie, that you're stepping it down before the cable, when my interpretation of what you've said is that the step-down converter and electronics will be after the cable, and the cable won't be carrying the 5v to the servos.
Yes, this is correct. I'm planning on stepping down the voltage right after the DB25 cable. So, basically I've got a DB25 breakout which I'm going to take the power pins and connect those to these step down converters. From there, I'm then going to control the rest of my circuits (ie. servos, arduino, etc.) So, I'm planning on stepping down the voltage after the DB25 cable, so the cable shouldn't be taking any sort of large current.
Then I would say that those step-down adapters that you linked would work. Your plan generally seems sound.
You would need one for each servo due to the stall current, though - naturally, the current ratings on those cheap chinese dc-dc converters are always the least favorable one (output for step down, input for boost). And probably a third to provide clean power for the electronics - sometimes you can get away without this by use of some filtering, but it's best to start with a separate supply, to make sure everything else works, then see if you can get it to run and not crash/reset when the motors run).