RS232 parasitic power - Page 1
Forum section: electronics / projects

author: westfw
author_rank: Super Contributor
timestamp: July 25, 2017, 07:35:58 am
content: Does anyone have a clever “phantom power” circuit for getting the most out of an rs232 port?Ideally it would have as inputs multiple ~10mA @+10V or -10V (at least DTR and RTS) (~200mW) and provide 50mA at 3.3V (150mW) or thereabouts…The nominal goal would be to buffer and send “occasional” serial traffic over BTLE or one of the other “low power radio” standards. I figure with the advances in radios, this should be approaching feasibility.

author: fcb
author_rank: Super Contributor
timestamp: July 25, 2017, 08:46:54 am
content: This used to be a common way of powering dongles.Problem will be that most RS232 ports don't actually follow the original standard, so you'll probably have to test on the actual RS232 port you are planning on using. I guess this is a legacy machine?I think the idle condition of most RS232 outputs is -ve, so probably just diode them together and use a high-efficiency buck-boost circuit?

author: capt bullshot
author_rank: Super Contributor
timestamp: July 25, 2017, 08:53:42 am
content: I don't remember all details, but a quite simple and effective way to get +5V from a RS232 port was to abuse the charge pumps of a MAX232 type driver.Use some diodes to get +10V and -10V from the three available lines, connect them to the +/-10V pins of the MAX232, with a resistor and a zener diode create a little startup helper voltage at the +5V input of the chip - once the charge pumps run, they also work in reverse, supplying 5V from the +10V or -10V, whichever is available.

author: nowlan
author_rank: Frequent Contributor
timestamp: July 25, 2017, 10:08:55 am
content: Been meaning to ask a similar question for a project idea.I had people suggest I look at old school mice, which run off comms port.

author: westfw
author_rank: Super Contributor
timestamp: July 25, 2017, 11:19:00 pm
content: A lot of the old stuff just runs off of the <10mA you can get from running an unused signal through linear regulation of some sort (frequently resistor + zener.) I'd like to do better than that.(and yeah, the new hardware that's barely providing the low end of the rs232 spec (+/- 3V? Or just inverted TTL?) would be a problem. That's Ok, in my case. I think.)The max232 trick sounds interesting...

author: Andreas
author_rank: Super Contributor
timestamp: July 26, 2017, 08:05:49 pm
content: Quote from: westfw on July 25, 2017, 07:35:58 amNo way: the RS232 only guarantees +/-5V at the transmitter with a minimum 3kOhm load.The original MC1488/89 devices deliver typically up to 7-8 mA (Current limitation) at 5-7V maximum.Among the chargepump devices (MAX23xxx) there are some with "mouse drivability" which guarantee up to 5mA at +/-5V.So you should not expect to get more than 75mW out of 3 lines.And do not forget that the receivers also want minimum 3V at 3-7kOhms.With best regardsAndreas

author: westfw
author_rank: Super Contributor
timestamp: July 27, 2017, 06:01:50 am
content: QuoteUm, I'm looking at the ON Semi datasheet:Quote

author: fcb
author_rank: Super Contributor
timestamp: July 27, 2017, 09:49:45 am
content: As you said you have 'occasional' traffic, you could use the RS232 lines to charge a small battery or perhaps a super cap?

author: Andreas
author_rank: Super Contributor
timestamp: July 27, 2017, 06:44:38 pm
content: Quote from: westfw on July 27, 2017, 06:01:50 amlook harder: (not only the "advertising" on page 1)Who really needs 10 mA at 0V for a power supply? ( = 0.000 mW)There is a figure with a 3 kOhms working resistor.If you adjust the slope * 3 for a 1 kOhm you will see there is not much room above 25 mWwith best regardsAndreas