Polarity Ambiguos Connector - Page 1
Forum section: electronics / projects

author: sambran
author_rank: Contributor
timestamp: March 06, 2016, 03:57:31 pm
content: I am designing two connectable modules, with one of them supplying DC power to the other. Ideally I'd like the connection to not have a specific orientation, meaning that the load module can either accept +Vdc of -Vdc. Due to mechanical restraints this needs to be implemented electronically.My first instinct is to use a rectifier at the input, I've never done that or seen it done so I am not sure of the implications when used to reverse DC voltage. Any options or suggestions would be appreciated.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: March 06, 2016, 04:10:23 pm
content: a diode bridge will do the job, works just fine with DC - but diodes do drop some voltagea bridge could get some circuitry working to then turn on lower V drop transistors or relays

author: amyk
author_rank: Super Contributor
timestamp: March 06, 2016, 04:25:13 pm
content: You can get "ideal diode bridge rectifiers"http://www.linear.com/docs/43273not cheap though.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: March 06, 2016, 04:26:34 pm
content: And there is the old trick to use the body diode of a P-FET on the supply side (or N-FET on the ground side) as protection and to switch on the FET in case of correct polarity.-> very little voltage drop (depending on Rds).http://www.ti.com/lit/an/slva139/slva139.pdf

author: Kilrah
author_rank: Unknown
timestamp: March 06, 2016, 04:27:51 pm
content: A product I worked on has a diode bridge on the charge input (standard DC barrel plug) because one of the long-standing players in the market we're dealing with was using the center negative convention meaning that people getting our product were pretty likely to have a few cables wired "backwards" that they might use either on purpose or by mistake.

author: Knuckx
author_rank: Newbie
timestamp: March 06, 2016, 04:33:50 pm
content: Use a 3-pin reversible connector, put the supply on the center pin, and ground on the two outer pins - AKA the Apple MagSafe method.

author: macboy
author_rank: Super Contributor
timestamp: March 06, 2016, 07:28:16 pm
content: Quote from: Knuckx on March 06, 2016, 04:33:50 pmClassic KISS solution. Everyone else is automatically thinking about an electrical solution when the mechanical one is the most elegant and efficient one.

author: ajb
author_rank: Super Contributor
timestamp: March 06, 2016, 08:42:13 pm
content: Quote from: macboy on March 06, 2016, 07:28:16 pmWell, the OP did specify that they needed an electronic solution:Quote from: sambran on March 06, 2016, 03:57:31 pm

author: sambran
author_rank: Contributor
timestamp: March 07, 2016, 06:54:38 pm
content: Thanks for the help.A mechanical solution would be ideal but unfortunately it is not possible.

author: Delta
author_rank: Super Contributor
timestamp: March 08, 2016, 03:19:15 am
content: How much power are we talking? I diode bridge will be fine if you can live with a bit of volts drop and wasted power...

author: sambran
author_rank: Contributor
timestamp: March 08, 2016, 05:05:57 pm
content: At this point I'm thinking 24V for up to 30W.The losses due to a diode bridge are affordable but obviously not ideal.My other thought was to somehow identify the orientation on the supply side and reverse the polarity using mosfets. This obviously makes things more complicated design wise but it would be more efficient and it would allow the contacts to be disabled when disconnected.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: March 08, 2016, 05:41:44 pm
content: You haven't told us enough about your requirements for us to provide a whole lot of useful suggestions. For example, you said there are limitations that prevent you from solving the problem mechanically, but you never told us what those limitations are. How many pins are in this connector? Do you have any pins available? Do you have any flexibility with the pinout? Do you have an MCU on the host that could monitor current draw or voltages on some of the pins and flip the polarity on demand? Since we don't know what you can do, we can't really tell you what you should do.If you only have two pins, then I see two options:1) diode bridge (ideal or not) so the device will work with either polarity2) Pch mosfet for reverse voltage protection on the device, and an MCU on the host that monitors the current draw and flips the polarity back and forth until it gets a reasonable valueIf you have more than two pins, then you can get fancy with a feedback voltage on a spare pin to let the host know whether or not the device is plugged in and its orientation, so it can control the output accordingly.

author: dom0
author_rank: Super Contributor
timestamp: March 08, 2016, 06:09:53 pm
content: The simple active MOSFET bridge rectifier will work here, assuming that only one side can supply power.By the way,no bridge will work, if the power supply and signal connections share a ground upstream of the connector- if you have signal connections...

author: sambran
author_rank: Contributor
timestamp: March 08, 2016, 06:22:19 pm
content: Quote from: suicidaleggroll on March 08, 2016, 05:41:44 pmYou are correct, I'll add some details.The base/supply does have a MCU and I'd prefer to increase the cost of parts on the supply side.As for as electrical connections, I have zero flexibility and I am limited to two connections. However I might be able to detect the orientation using some other more mechanical method (button or a sensor).

author: suicidaleggroll
author_rank: Super Contributor
timestamp: March 08, 2016, 06:41:59 pm
content: In that case I'd say either a bridge on the device (up to you whether you want ideal or not), or a Pch reverse voltage protection circuit on the device and a current monitor (if you don't have one already) and an H-bridge on the supply to measure current draw and flip the polarity on the connector as necessary.

author: sambran
author_rank: Contributor
timestamp: March 20, 2016, 03:41:17 am
content: Quote from: suicidaleggroll on March 08, 2016, 06:41:59 pmThanks, that's what I'll do.