Homebrew_PCBs

I'm making my first PCB for a thermostat for
an electric heater.   I need to connect the
electronics to several outside components:

  - a thermistor that goes in the oven
  - an thermal fuse to shut off power if it gets too hot
  - battery power & ground
  - 3-conductor connector to an external solid-state relay

I plan to use pin headers and matching sockets for each of
these four connections.    For each of these four connections,
what criteria should I use to determine whether to put the
pin header on the PCB and the matching socket on the connect-
ing cable, or vice versa ?

The thermostat itself and the SSR will fit into a plastic box that
also contains the battery.    So my question pertains to orient-
ing the headers/sockets inside the plastic box.   None of
the connections will ever be disconnected without first removing
four retaining screws from the lid of the plastic box.

On 6/28/2016 5:20 PM, alan00463@... [Homebrew_PCBs] wrote:

>
> I'm making my first PCB for a thermostat for
> an electric heater. I need to connect the
> electronics to several outside components:
>
> - a thermistor that goes in the oven
> - an thermal fuse to shut off power if it gets too hot
> - battery power & ground
> - 3-conductor connector to an external solid-state relay
>
> I plan to use pin headers and matching sockets for each of
> these four connections. For each of these four connections,
> what criteria should I use to determine whether to put the
> pin header on the PCB and the matching socket on the connect-
> ing cable, or vice versa ?
>
> The thermostat itself and the SSR will fit into a plastic box that
> also contains the battery. So my question pertains to orient-
> ing the headers/sockets inside the plastic box. None of
> the connections will ever be disconnected without first removing
> four retaining screws from the lid of the plastic box.
>
>

Hello, Alan--

Standard procedure is to put sockets on any group of wires that carry
hazardous voltages
and thus could deliver a shock to an unwary person if the connector's
pins were exposed.

Also, when unplugged, exposed pins carrying voltage will inevitably
short-circuit themselves on any
nearby metal (that's one of Murphy's Laws)<g>.

Also, if you're using identical connectors, if possible you should
provide a means of keying
or otherwise identifying which plug mates with which socket. If the
connectors you're
using don't have provisions for keying, label the plugs and sockets.

HTH, and 73--

Brad AA1IP

That's good advise, but in practice it is wishful thinking.
In 99% of board-to-wire connectors the male pin side is on the PCB and
the female socket side is on the cable.

Inside an enclosure that is not a problem. The PCB has many other
components anyway that carry exposed voltage, so the pin header adds
no danger. The cables are actually more important to shroud, since
they can come adrift and contact areas they are not supposed to
contact.
User accessible connectors are another story, but we are talking
inside the box here. Your project is low voltage anyway up to the SSR,
so there is no great danger.

As for polarization / keying the connectors. Again, nice if you can,
not the end of the world if you can't. There is no requirement if
these aren't user accessible. Just label them.

Personally, for something like a thermostat I would go with plug-in
PCB pcb screw terminals like this:
<http://i.ebayimg.com/00/s/MTIwMFgxMjAw/z/zRsAAOSwHaBWlNS-/$_57.JPG>
There are good reasons why they are standard on temperature controllers.

ST


On Tue, Jun 28, 2016 at 11:48 PM, Brad Thompson
brad.thompson@... [Homebrew_PCBs]
<Homebrew_PCBs@yahoogroups.com> wrote:

>
>
> On 6/28/2016 5:20 PM, alan00463@... [Homebrew_PCBs] wrote:
>>
>> I'm making my first PCB for a thermostat for
>> an electric heater. I need to connect the
>> electronics to several outside components:
>>
>> - a thermistor that goes in the oven
>> - an thermal fuse to shut off power if it gets too hot
>> - battery power & ground
>> - 3-conductor connector to an external solid-state relay
>>
>> I plan to use pin headers and matching sockets for each of
>> these four connections. For each of these four connections,
>> what criteria should I use to determine whether to put the
>> pin header on the PCB and the matching socket on the connect-
>> ing cable, or vice versa ?
>>
>> The thermostat itself and the SSR will fit into a plastic box that
>> also contains the battery. So my question pertains to orient-
>> ing the headers/sockets inside the plastic box. None of
>> the connections will ever be disconnected without first removing
>> four retaining screws from the lid of the plastic box.
>>
>>
> Hello, Alan--
>
> Standard procedure is to put sockets on any group of wires that carry
> hazardous voltages
> and thus could deliver a shock to an unwary person if the connector's
> pins were exposed.
>
> Also, when unplugged, exposed pins carrying voltage will inevitably
> short-circuit themselves on any
> nearby metal (that's one of Murphy's Laws)<g>.
>
> Also, if you're using identical connectors, if possible you should
> provide a means of keying
> or otherwise identifying which plug mates with which socket. If the
> connectors you're
> using don't have provisions for keying, label the plugs and sockets.
>
> HTH, and 73--
>
> Brad AA1IP
>
>
> ------------------------------------
> Posted by: Brad Thompson <brad.thompson@...>
> ------------------------------------
>
> Be sure to visit the group home and check for new Links, Files, and Photos:
> http://groups.yahoo.com/group/Homebrew_PCBs
> ------------------------------------
>
> Yahoo Groups Links
>
>
>

Thank you to all who replied.     Your advice helps.   I had not considered  "polarized" pin/header socket connectors.   However, I will for future projects, at least for power & ground.   Since they are standard for PC's, I should probably make a habit of using them also for more-than-two-conductor connectors.    They would certainly make connecting/disconnecting quicker.

For external DC power connectors, I like to use coaxial (+ on the pin, - on the shield) jacks and matching plugs, because they are quick to disconnect and because you can't accidentally reverse the leads if you wired them correctly in the first place.    These are also called "DC barrel jack" connectors.

On 30 Jun 2016 06:21:35 -0700, you wrote:

>Thank you to all who replied. Your advice helps. I had not considered "polarized" pin/header socket connectors. However, I will for future projects, at least for power & ground. Since they are standard for PC's, I should probably make a habit of using them also for more-than-two-conductor connectors. They would certainly make connecting/disconnecting quicker.

On some of my projects, the power connector is symmetric (pin header).
Center pin is ground, then the remaining pins duplicate the functions,
so +5 gnd +5. Doesn't matter if you plug it in backwards. Even found
it to be very useful with adjustable regulators. Destroy the symmetry
by cutting a pin, plug it in one way and the output floats (and can be
adjusted), plug it in normally and you have a working (and adjusted)
regulator.

Harvey

>
>For external DC power connectors, I like to use coaxial (+ on the pin, - on the shield) jacks and matching plugs, because they are quick to disconnect and because you can't accidentally reverse the leads if you wired them correctly in the first place. These are also called "DC barrel jack" connectors.