Discussion Of Latching A Momentary Switch To Control A Power Supply

idbruce

Hello Everyone

I am in desperate need of some assistance.

Adafruit has a breakout board that I pretty much would like to duplicate, with a through hole design, and it is not because I am too cheap to buy it, but because I will be working in close quarters and I do not have the room for attaching the board. Believe me there is a lot headache associated with this task. I want to duplicate their circuitry because I want a tact switch to control the 7.5VDC/1.5A power supply going to a Propeller Proto Board.

Anyhow, the product that I would like to duplicate is here: http://www.adafruit.com/blog/2013/06/07/new-product-adafruit-push-button-power-switch-breakout/ and here is the schematic: http://www.adafruit.com/datasheets/1400sch.png

First off, I want to rip Q1 and R4 out of the schematic, because I do not want or need that kind of shut down. I don't foresee this as a problem, but I could be wrong.

Secondly, is it acceptable to use 1/4W resistors for R2 and R3?

And thirdly, I need a drop in through hole replacement part for this MOSFET: http://www.adafruit.com/datasheets/DMG3415U.pdf, is there any MOSFET available that would do the job?

I would not know what to look for when trying to find a through hole MOSFET replacement part, so any guidance you might be able to provide, would be greatly appreciated.

jmg

idbruce wrote: »

Secondly, is it acceptable to use 1/4W resistors for R2 and R3?

Sure, but making both 100k is marginal, as a closed switch gives 50% to the CMOS in.
I would make R2 ~ 1/2 R3

idbruce wrote: »

And thirdly, I need a drop in through hole replacement part for this MOSFET: http://www.adafruit.com/datasheets/DMG3415U.pdf, is there any MOSFET available that would do the job?

That FET has a VGSS ±8V, but is driven with 18V (!) = Bad design.

With an 18V supply, you can use almost P-CH FET, with enough margin,
A 20V FET, with 8V VGS, on a 18V supply I would not call enough margin.

So a search for P-FET, cheapest, thu-hole on somewhere like Digikey, suggests FQU11P06TU* (IPAK), and that is a few cents more than TO-92, but gives 60V/9.4A/185 mOhm @ 10V

You can solder SMD parts to pin-headers for bread board work ?

* FQU11P06TU shows as
1 $0.2549 @ Avnet
1 $0.381 @ Element14
1 $0.7600 @ Mouser & Digikey

kwinn

Almost any P channel mosfet with with Vdss over 18V and a current rating over 2.5A would work here. The problem is they are almost all SMT. The only thru hole part I found at digikey was the ZXM64P035L3-ND. It's Vdss is 50V and continuous current 3.3A. Price $2.04

idbruce

jmg and kwinn

I thank you guys both so much. I would have responded much quicker, but I had to leave for a while.

We all have are strengths and weaknesses, good thing for me, there are helpful people like you two. Once again, thank you very much.

Oh and

You can solder SMD parts to pin-headers for bread board work ?

I have never messed around with SMT or SMD parts. I know I will have to sooner or later, with all electronics heading for the smallest possible design.

idbruce

Here is another schematic to accomplish the same task: http://www.mosaic-industries.com/embedded-systems/_media/microcontroller-projects/electronic-circuits/push-button-switch-turn-on/nand-gate-latching-press-on-hold-off-logic-toggle-switch-circuit.png

EDIT: After desoldering some components from some old electrical junk, I finally had enough components to test the circuit linked to above, and I must say it works very well. I did not have any 5M resistors lying around, so I slapped in a 10M resistor and tested it with a 12VDC nominal power supply. One quick button press and voltage appeared on my DVM and to shut the circuit off, you must hold the tact switch down for about 3-1/2 seconds. Taking it a step further, I replaced the 10M resistor with a 1M resistor and tested again. One quick button press and voltage appeared on my DVM and to shut the circuit off, you must hold the tact switch down for about 1-1/2 seconds. So the lower the resistance, the faster the circuit can shut down and vice-versa. 1-1/2 second time delay is just about perfect for my needs. All I need now is a worthy MOSFET or transistor to add onto the end of it for controlling the main supply.

Mark_T

jmg wrote: »

Sure, but making both 100k is marginal, as a closed switch gives 50% to the CMOS in.
I would make R2 ~ 1/2 R3

Although if the capacitor were larger so that the time constant was longer than a normal button
push it would not matter.

That's a first, a bad circuit from LadyAda.... Make R2 10k

[ I take that back, the schmitt-trigger inputs save it from failing. However
there's no need for schmitt-triggers if the resistors have the right values ]

jmg

Mark_T wrote: »

That's a first, a bad circuit from LadyAda.... Make R2 10k

[ I take that back, the schmitt-trigger inputs save it from failing. However
there's no need for schmitt-triggers if the resistors have the right values ]

The schmitt threshold is still very close to 50%,
See fig 9
http://www.nxp.com/documents/data_sheet/HEF4093B.pdf

The other issue with ~ 50%, is the Icc on CMOS is very high, the data above shows ~ 2mA of peak Icc at this level.(fig 8)

For a circuit that is supposed to be a power-saving thrifty toggle, that's a lot of power.

Any design where the button many be pressed/held by accident will surprise the user with higher drain.
(R2 = 10 k is better.)

Mark_T

[QUOTE=jmg;1275043

The other issue with ~ 50%, is the Icc on CMOS is very high, the data above shows ~ 2mA of peak Icc at this level.(fig 8)

Any design where the button many be pressed/held by accident will surprise the user with higher drain.
(R2 = 10 k is better.)[/QUOTE]

Well I doubt that's the use-case that was considered.

I suspect the part number for the MOSFET in the schematic was just whatever they had in their
Eagle library at the time, hopefully the actual MOSFET has a higher Vgs rating...

idbruce

So what do you gentlemen think of the second schematic I linked to? It seems to work well.

Mark_T

idbruce wrote: »

So what do you gentlemen think of the second schematic I linked to? It seems to work well.

Ideally the NANDs would be schmitt-trigger, but its likely to be reliable anyway as its pure set-reset flipflop. I would add a
schottky diode from the junction of the 1uF and 5M to the supply voltage to prevent the capacitor discharging through the
input-protection diode of the gate. anode to RC junction, cathode to V+.

idbruce

Mark_T

I would add a schottky diode from the junction of the 1uF and 5M to the supply voltage to prevent the capacitor discharging through the
input-protection diode of the gate. anode to RC junction, cathode to V+.

I just got finished with a schematic and board, and I was getting ready to post them both, until I saw your post. Now you tell me Any particular diode that you would recommend?

This is what I have so far, JP1 goes off board to the desired switch:

EDIT: I just saw a big mistake
EDIT: On second thought, I believe it is correct

idbruce

My intent was to mount a vertical pcb/panel mount power jack to the center of the board, which would have been a clean and neat way to bring power to the inside of my enclosure, but I could not find a suitable combination of a jack and power adapter to match my needs. The closest I came was a device from Switchcraft, which is shown below:

idbruce

@jmg, kwinn, and Mark_T

Even though I opted against the MOSFET design, because of readily available parts, I truly do appreciate the time and effort that you put forth in trying to help me, in selecting a proper MOSFET. I just hope that someone will make good use of the information that you provided and I apologize for changing directions. Please accept my apology.

@All Others

I wrote a fair description of my situation yesterday, but then IE shut down and I lost my post. So instead of attempting to rewrite that post, I will just tell you about the current situation.

Referring to my previous post, I wanted a panel mount jack for my power adapter to supply power my Propeller Proto Board, but I also wanted the circuitry for turning on and off the power with a simple tact switch. The Proto Board will reside inside of a NEMA 1 enclosure, the jack will be mounted to the cover of the enclosure, and the tact switch for the power supply will be grouped with seven other tact switches for a user interface, which will all be mounted to a seperate PCB, which will also reside inside of the enclosure, and extend out for user access.

To make a long story short, since I was unable to find a suitable jack, I modified a Radio Shack DC Power Jack (Cat. #: 274-1565) to become a SMD. I started by sanding off the mounting ears with a disc sander and I then bent the solder lugs outward to allow attachment to a PCB. On this PCB, all components will be mounted to the top layer of the board, with the exception of the DC Power Jack, which will be mounted to the bottom layer. To mount the board, three holes will be drilled in the enclosure cover and two standoffs equaling the height of the DC Power Jack will be utilized to appropriately space the PCB from the inside surface of the enclosure.

As for the shown schematic, I am unsure if D1 is an appropriate component or if R4 has the appropriate resistance, but I will try these. Although I mentioned top and bottom layer, this design is actually a single sided board. The attached PDF file contains the bottom layer for all necessary connections.

• JP1 is utilized for connecting a remote tact switch to the PCB
• DCJ1 is the DC Power Jack for the power adapter
• X1 is for attaching the plug which goes to the Proto Board

tonyp12

Many solution to a momentary to latching toggle here:
http://electronics.stackexchange.com/questions/4060/make-a-momentary-switch-control-a-toggle

or a IC that does it:
http://www.mouser.com/ProductDetail/Maxim-Integrated/MAX16054AZT+T/?qs=sGAEpiMZZMuuBt6TL7D%2f6CD7ytZT54k%252b

idbruce

Tony

I forgot to say thanks for your input in the other thread, so thank you.

Many solution to a momentary to latching toggle here:
http://electronics.stackexchange.com...ntrol-a-toggle

That is where I found the circuit that I am using, but there are other options than the one I am using.

or a IC that does it:

Yea that chips seems nice depending upon available voltage. However that chip and a voltage regulator would probably work very nicely for many different applications, including mine.

CuriousOne

LTC2950 also does the same as MAXIM:

http://www.linear.com/product/LTC2950