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SPDT push button switch under 8mm

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Tizzo

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Has anyone ever come across a push button switch that is under 8mm tall? I'm looking for something surface mounted, requiring very little throw. It should handle 5A or so. When looking for switches, it seems all the manufactures are using the same style case. I've come across some that are close, but too tall by a mm or so. They have legs as well, and I can't help but think they'd be perfect if they were made in a SMD configuration instead.
 

Sceadwian

Banned
Any particular reason you don't use a micro switch which you can get VERY small and simply use a pair of transistors to drive the actual loads?
 

Boncuk

New Member
Has anyone ever come across a push button switch that is under 8mm tall? I'm looking for something surface mounted, requiring very little throw. It should handle 5A or so. When looking for switches, it seems all the manufactures are using the same style case. I've come across some that are close, but too tall by a mm or so. They have legs as well, and I can't help but think they'd be perfect if they were made in a SMD configuration instead.
Switching 5A with a pushbutton requires a fairly big package because of the necessary contactor strength. You could find a small size pushbutton but only capable of switching a few mA. Use a transistor to switch the load.

Boncuk
 

Tizzo

New Member
Right. I've given up on trying to find a tact switch sized SPDT that actually handles 12A and decided on a MOSFET. One part of my circuit that I'm not sure about is the diode. V1 is not always connected. But when it is, I want to make sure no current flows from v2 into it. V1 should be outbound only...to charge v2. Also not sure how large the gate controlling resistor should be.
 

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Boncuk

New Member
Hi Tizzo,

if V2 is greater than V1 you might as well omit V1. V2 should supply the load without being connected to V1 in any way.

The diode just prevents current flow into V1.

Further to control a MosFet transistor with a positive voltage you must use an N-Channel transistor.

The load is then connected between the positive rail and the drain of the transistor.

With the schematic I provided BAT1 (V1) will control the gate via the pushbutton (S1). You can use a low value resistor to control the gate since the gate current is very low anyway. When T1 becomes conductive (min. 4V) it will connect the load resistor to ground.

Use D1 when supplying an inductive load.

If the transistor won't turn off fast enough (because the gate remains charged after control voltage interruption) use a 1 to 10K resistor tied between the gate and ground to quickly discharge the gate.

Hope that makes sense to you.

Boncuk
 

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Tizzo

New Member
The mosfet is depicted as N channel in the diagram, isn't it? Arrow pointing in? I choose it from Microcap.

V1 is not always connected. It's just there to charge V2 when connected.

Your schematic is quite different in that one battery is dedicated to controlling the gate, and the other provides current through the load.
 
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Boncuk

New Member
The mosfet is depicted as N channel in the diagram, isn't it? Arrow pointing in? I choose it from Microcap.

V1 is not always connected. It's just there to charge V2 when connected.

Your schematic is quite different in that one battery is dedicated to controlling the gate, and the other provides current through the load.
Yes, it is depicted as N-channel with the load connected to it's source which is wrong!

V1 and V2 are drawn as voltage sources in your circuit. How should one know which one is the "real thing"?

To control a MosFet transistor you won't require two voltage sources anyway. Just wire the circuit the way that only one voltage source is necessary. Control voltage will be interrupted by the switch anyway.

So why bother with V1 and V2?

Boncuk
 

Boncuk

New Member
Here's the modified circuit.

One question: If you are sure about what you are doing why post questions about it in the forum?

Additionally your schematic doesn't show any removable voltage source. If V2 is present at all times what is it? A battery, a capacitor or anything else?

Boncuk
 

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Tizzo

New Member
Yes, let's ignore v1 and just consider them as one.

I'm not sure about what I'm doing. How do you know from my schematic which is source and which is drain? I don't see anything on the mosfet symbol (provided by Microcap) that would indicate which is which. Yours is different and seems to more clearly indicate all of the leads.

On your modified circuit, could you explain why R3 is necessary? It seems that even without R3 you'd still have voltage to your gate. It's unclear why you need to provide a current path down past the gate back to the voltage source.
 
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Boncuk

New Member
Hi Tizzo,

Even if the symbol of a MosFet transistor isn't drawn the proper way the only conclusion to be met is the source connected to ground and the drain via the load to +VDD. Therefor this particular transistor can only be an N-Channel with the other two connections wired a way for proper function.

Just compare it with a bipolar NPN transistor where the source is the emitter and the drain is the collector, gate for base.

The gate of the Fet is charged with +VDD via S1. If S1 is open there is no more current flow into the gate, but the gate is still charged (positive), hence the Fet will still conduct until the charge is removed by losses within the transistor.

R3 takes care of discharging the gate to ground for fast switch off. Since it is connected to R1 there is absolutely no current flow through R3 as soon as the gate is discharged since S1 is open.

Of course there will be voltage at the gate, but this voltage is ground potential taking care of a reliable interruption of current flow through the load resistor.

Boncuk
 
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Tizzo

New Member
Oh ok. I've reviewed the theory on IGFETs. I was thinking the channel was interchangeable like JFETs.

I've put a new circuit together using a JFET to control V1 rather than a diode which wasn't going to work.

The idea is that when the main load (R3) is on with the switch on, V1 won't be active. From what I've understood, the negative voltage applied to the JFET should cut OFF the connection from V1 to V2 the way I've set it up.

If the switch is open, V1 is free to charge V2. Hopefully there is little to no voltage drop through the JFET.
 

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