Need Advice On The Best Type of Switch For This Application...

rs14smith · May 14, 2011

Hi all,

I need to know what type of switch I can use that will do the following:

Basically it would act like a transistor or MOSFET, and that may be what I need for this but just want to double check to see what else I can maybe use. When I send 5v to the gate/switch for example, the switch will OPEN and allow current (25A max) to flow to my load. When the voltage going to the gate/switch is less than 5v, the switch will CLOSE, and prevent any current to flow to my load.

So basically, this switch needs to be able to handle 25A at least, and trigger from 5v.

Thanks

carbonzit · May 14, 2011

Just want to point out that you're using "open" and "close" backwards. Normally we consider a switch closed when current flows through it, and open when current cannot. (Think of what a physical electric switch looks like. You're apparently thinking more of a water valve, which lets water flow when it's open.)

Other than that, sorry, can't help you, but hopefully an expert will be along shortly ...

rs14smith · May 14, 2011

carbonzit said:
Just want to point out that you're using "open" and "close" backwards. Normally we consider a switch closed when current flows through it, and open when current cannot. (Think of what a physical electric switch looks like. You're apparently thinking more of a water valve, which lets water flow when it's open.)

Other than that, sorry, can't help you, but hopefully an expert will be along shortly ...

lol Wow what was I thinking when I wrote that, but yes you are correct. I was completely thinking backwards at the time of writing that.

Jaguarjoe · May 14, 2011

At first blush, a MOSFET sounds like a good candidate. Can you post a sketch of what you want to do including what the load will be and if it is connected to ground or to the high side of the power for it? What voltage does it run at? Where is the 5 volts coming from? How fast does it switch on and off?

rs14smith · May 14, 2011

I think I'm going to go with the MOSFET idea since I think I figured out how to hook it up now

The motor at full load can draw 25A, and it requires 36v. The 5 volts is coming from another power source separate from the main power source.

I'd like the switch to switch between On-Off fairly quickly, almost instantly.

I posted a simple schematic of the circuit, let me know if it's missing something important.

crutschow · May 15, 2011

Make sure you add a protection diode (cathode to plus) across the motor to suppress turn-off spikes that could zap the transistor.

Reloadron · May 15, 2011

Not a good choice for the MOSFET. You want a logic level MOSFET since you are using 5 volts on the gate to turn it on. Give this a read as an example.

Ron

rs14smith · May 15, 2011

crutschow said:
Make sure you add a protection diode (cathode to plus) across the motor to suppress turn-off spikes that could zap the transistor.

What type of diode would I probably need for this application? Do you have the part number by any chance?

Reloadron · May 15, 2011

1N4002 would do just fine. I also suggest you note what I posted as to the choice of MOSFET if you plan on 5 volt logic to turn the MOSFET on.

Ron

rs14smith · May 15, 2011

Right I saw that and will look into it after I figure out how I just destroyed my microprocessor lol.

Attached is how I have my system currently setup, and for some reason, I'm guessing the 36v's ground and the Microprocessor's ground cannot be connected together, as when I did this, my micro-controller just stopped working.

But if I don't do this, the MOSFET doesn't get it's gate triggered at all, so I'm confused on how I'm suppose to implement this basic system. What am I doing wrong?

crutschow · May 15, 2011

The two grounds must be connected so that the MOSFET has a ground reference to the Microprocessor signal.

So how are you connecting them together? Make sure the the motor current doesn't somehow run through the uP ground. You want to connect the grounds together near the MOSFET, not at the power supplies.

If grounding is a problem, then you could use a solid-state relay, which has and opto-coupler isolator built in.

rs14smith · May 15, 2011

crutschow said:
The two grounds must be connected so that the MOSFET has a ground reference to the Microprocessor signal.

So how are you connecting them together? Make sure the the motor current doesn't somehow run through the uP ground. You want to connect the grounds together near the MOSFET, not at the power supplies.

If grounding is a problem, then you could use a solid-state relay, which has and opto-coupler isolator built in.

Could you point me to where I could get the right solid-state relay (for my particular application), as now I have to spend another $100 for a new microprocessor and don't want to risk this happening again lol?

Glady appreciated if you can.

Thanks!

rs14smith · May 15, 2011

Yeah, a quick Google search is giving me a lot of results for, DC Input and AC ouput for the Solid-State Relay, but I can't seem to find one that is DC Input and DC Output, or maybe it doesn't matter?

Here's an example of one I found on Amazon: https://www.amazon.com/25A-SSR-Solid-State-Relay/dp/B004HZN628

For the most part, it seems simple to hookup, but rather be safe than sorry this time around

audioguru · May 15, 2011

A door or a window opens and closes.
But a transistor or a switch turns on or turns off.

rs14smith · May 15, 2011

audioguru said:
A door or a window opens and closes.
But a transistor or a switch turns on or turns off.

K thx...

crutschow · May 15, 2011

rs14smith said:
Yeah, a quick Google search is giving me a lot of results for, DC Input and AC output for the Solid-State Relay, but I can't seem to find one that is DC Input and DC Output, or maybe it doesn't matter?

Yes it matters. AC output relays typically use TRIACs or SCRs which are unsuitable for DC. Google "DC output solid state relay".

rs14smith · May 15, 2011

crutschow said:
Yes it matters. AC output relays typically use TRIACs or SCRs which are unsuitable for DC. Google "DC output solid state relay".

Yeah I tried that as well, and I'm having little luck finding DC input/DC output. I guess these just aren't as popular?

Let the search continue...

rs14smith · May 15, 2011

Wow! Are all DC output solid state relays this expensive that allow output current over 20A: D1D40: CRYDOM: Electromechanical

Most that I've ran into that seem to be what I need are around $60 on up. Are there any other cheaper solutions than solid state relays for DC output?

crutschow · May 15, 2011

You could use an opto-isolator such as **broken link removed** to drive the MOSFET.

Make sure you add a resistor on the input to limit the current to less than the 50mA max. rating.

KMoffett · May 15, 2011

audioguru said:
A door or a window opens and closes.
But a transistor or a switch turns on or turns off.

Standard switch nomenclature: N.O. = normally "open" N.C. = normally "closed"

Ken

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