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Looking for a small solid state switch/relay?

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rackley

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Hi, I'm looking for a small solid state switch that acts as kind of a relay. What I need to do is switch between circuit pathways to enable differing levels of illumination for day and night operation on an LCD and a couple of LEDs.

I looked into variable resistors but they don't offer the precision and simplicity I'm looking for.

I need to control 5 items and be able to switch the illumination circuit (300ma max) between two different pathways with no added resistance. Each pathway has a different resistor in it to control the illumination level.

Any ideas?

Thanks,
Ray
 
From what I read. Like 5+ times.

You want an LDR to switch (between night and day; a level of light) a solid state relay to two lines that have 5 loads on them that you have the resistors preset the way you need them?
 
EDIT: I think I know what's going on, so I rewrote my post.

You want to control current so you first looked at a variable resistor (I assume a manual one). However, for some reason it was not simple enough or accurate enough. I am not sure why it is not simple enough, because there is nothing simpler that will do the task. You want more precision AND simplicity? As far as the real world goes, you can have one but not both. A manual knob potentiometer would be the simplest way and you would have infintely variable intensity, but for whatever reason this is not good enough for you? Most likely you want it to be automated.

So anyways, what you wanted to do is build a digitally controlled potetiometer. You built several resistive paths and now you need a way to switch between them. In commercial digital potentiometer ICs, they pretty much do the same thing (some get more flexible than just choosing resistive paths by placing many small resistors in series where each resistor has a MOSFET in parallel with it. By switching on the MOSFET you essentially short that particular resistor out and it's resistance is not seen on the output. Switching the MOSFET off allows that particular resistor to be seen on the output). The disadvantage is more complexity and you now only have discrete steps in intensity.

So basically, do not use an solid-state relay (SSR). They are expensive and complicated. A MOSFET is all you need (assuming it is a unipolar DC application).

However, you have to relax your requirement of "no additional resistance" to "negligible additional resistance" since all solid state devices have a noticeable resistance on them. The smaller the resistors are, the more significant this silicon resistance becomes in the grand scheme of things. Small MOSFETs around 300mA capability can have on-resistances of 300ohms or so, but you can also get ones that are a lot less. If your resistors within a magnitude of this, you might have to account for the on-resistance.
 
Last edited:
dynguyen:
Yeah, I read it over and over. And still not sure... But you are right if I decoded it right.

I think they are after an LDR to SSR and they already have the resistors worked out and working.
 
dknguyen said:
EDIT: I think I know what's going on, so I rewrote my post.

You want to control current so you first looked at a variable resistor (I assume a manual one). However, for some reason it was not simple enough or accurate enough. I am not sure why it is not simple enough, because there is nothing simpler that will do the task. You want more precision AND simplicity? As far as the real world goes, you can have one but not both. A manual knob potentiometer would be the simplest way and you would have infintely variable intensity, but for whatever reason this is not good enough for you? Most likely you want it to be automated.

So anyways, what you wanted to do is build a digitally controlled potetiometer. You built several resistive paths and now you need a way to switch between them. In commercial digital potentiometer ICs, they pretty much do the same thing (some get more flexible than just choosing resistive paths by placing many small resistors in series where each resistor has a MOSFET in parallel with it. By switching on the MOSFET you essentially short that particular resistor out and it's resistance is not seen on the output. Switching the MOSFET off allows that particular resistor to be seen on the output). The disadvantage is more complexity and you now only have discrete steps in intensity.

So basically, do not use an solid-state relay (SSR). They are expensive and complicated. A MOSFET is all you need (assuming it is a unipolar DC application).

However, you have to relax your requirement of "no additional resistance" to "negligible additional resistance" since all solid state devices have a noticeable resistance on them. The smaller the resistors are, the more significant this silicon resistance becomes in the grand scheme of things. Small MOSFETs around 300mA capability can have on-resistances of 300ohms or so, but you can also get ones that are a lot less. If your resistors within a magnitude of this, you might have to account for the on-resistance.

Hi all, first off my apologies for not being more clear in my original request. And thanks for helping despite not being able to decipher my mumblings :) Let me see if I can elaborate/clarify:

I have a display "keypad" that I use in my car that has self-illuminated buttons (LEDs embedded in the buttons) and an LCD. It's all driven by an 18F4550 PIC. The LCD is either too bright at night or not visible enough during the daytime, so my primary issue is controlling the backlight for the LCD.

Unfortunately the resistor used to control the backlight intensity is a relatively small value, around the 450-550 ohm range. Small variances of 10-20 ohms in either direction make a noticable difference in brightness.

I explored digital potentiometers but basically could not find one that could control resistance in fine enough increments in that resistance range to work. In addition to that it just seems easier to have two fixed resistors and have a switching device of some sort (controlled via a simple HIGH or LOW output from the PIC) switch the voltage bus through either resistor A or resistor B, both of which terminate at the LCD backlight terminal.

And once that's figured out I can easily replicate it to turn off the button backlights during the day since it will use the same power bus and signal line from the PIC. The only difference would be for the button LEDs there would only be one actual pathway and the other would simply open the circuit.

The day/night determination is already handled, the PIC will use a voltage divider and ADC to look at the illum voltage input from the car's illum circuit and output a clean +5v or 0v signal to turn on or off whatever device I wind up using for switching.

Perhaps this crude diagram will help conveyed my ideas :)

Code:
High/Low Sgnl -\
               \/        /->  Resistor A \
+5v -> (switching device)                -> LCD Backlight
                         \->  Resistor B /
I was thinking about using a MOSFET (and admittedly I don't have much experience with them although I understand them in principal) to my knowledge they're either on/off devices - so I would need two of them to control the above circuit with one having an inverse trigger from the other (activates on low signal?)

Regarding the resistance, as long as it is a constant 300 ohms it's not be an issue because I can adjust the resistor values accordingly. But if it's "300 ohms typical" that can be 285 in one MOSFET but the next mosfet I pick up is 320 then that creates quality control issues. Know what I mean?

Thanks all!
Ray
 
As far as relays go, you could probably use the following or one similar to it:

RELAY,DIP,DPDT,5VDC,2A CONT

Code:
             _______________
+0v --> | Normally Closed |--> Resistor A                            (default)
            |  Relay             |                  --> LCD Backlight
+5v --> | Normally Open   |--> Resistor B                             (brighter)
            _______________
 
Well if the resistances you are working with are that low...a mechanical relay might be the only thing that has a on resistance low enough to have a negligble impact.

Or, you could get a big, expensive, very low resistance 180A power MOSFET...but search around and you will probably find small MOSFETs like at www.irf.com and you will probably find on-resistances much lower...just make the on-resistance your deciding characteristic...you don't really need ultra fast switching times for your application so...
 
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