Hi

I wonder what is needed to protect a 5v circuit whilst powering a motor.

I initially thought that if something such as a motor was used, that needed some extra voltage from batteries, then a relay would be required.

Since then I have seen a lot of circuits that just use transistors.

In some examples transistors are used as relay switches with batteries and in other circuits transistors are used without batteries because of their amplification capabilities. (I don't know if this is correct I think I read this, but I may have read it wrong or misunderstood)

I am wondering how big a voltage a transistor can be used to switch in stead of a relay whilst protecting the 5v circuit that switches it.

Also how much power can it provide as an amplifier(if this is true). Could it power a small motor?

Thanks

 

I am wondering how big a voltage a transistor can be used to switch in stead of a relay

Check the datasheet for a given transistor to see its maximum Vce rating. Some go to hundreds of Volts.

Also how much power can it provide as an amplifier(if this is true)

A transistor doesn't provide power, it only controls the power (from batteries etc) used by a load such as a motor. In fact, the transistor actually wastes some power and gets warm/hot in the process.

 

Thanks

Hundreds of volts.

Why would a relay be used if higher voltages can be handled by transistors?

 

What do you mean by protecting ?

 

Why would a relay be used if higher voltages can be handled by transistors?

A relay can provide ciruit isolation and may have multiple sets of contacts, for greater versatility. Any relay must have a reverse-biased diode across its coil to protect the device/circuit which switches the coil voltage from the back-emf generated when the coil is switched off..

 

That's where I am confused. It seems that circuit isolation can be performed by transistors.

I presume it depends on the voltage?

I want to use 9v and switch it with 3.3v but I don't want to damage the gpio.

 

It seems that circuit isolation can be performed by transistors.

I don't see how. Perhaps we understand different things by 'isolation'. Can you give an example?

 

That's where I am confused. It seems that circuit isolation can be performed by transistors.

I presume it depends on the voltage?

I want to use 9v and switch it with 3.3v but I don't want to damage the gpio.

If you experience a collector-base breakdown in the transistor, for whatever reason, you could be applying 9v to you GPIO terminal. There is no galvanic path from the relay terminals back to the GPIO terminal.

Ken

 

Ok so relays are the only real solution here.

Transistors offer no isolation and should be avoided at 9v?

Thanks

 

There are opto-isolators...Solid State Relays. Optocoupler driving a transistor that switches your motor. Good isolation.

Ken

 

I was going to try this chaps circuit

the thing being my local supplier only has:
http://cpc.farnell.com/imo-precision-controls/srm-1c-sl-5vdc/minature-power-spco-5vdc/dp/SW02552

well its not all they have, but spdt and 10A 5vDC relay, this is all I can really see. But obviously novice here.

I wonder if this is ok compared to 1:34 on the youtube clip?

 

The above-mentioned opto-isolator (aka coupler) would be my choice for optimum protection of the micro port. Cheaper than an SSR. The opto output would then drive a transistor (BJT or MOSFET) to switch the motor power.

 

You never mentioned what your 9V(?) motor current is. Depending on the gain of the transistor, you can provide some protection to your controller's output with a transistor. Add a Schottky diode from the output pin to the +3.3V supply (cathode to +3.3V). The transistor's base resistor then becomes a voltage dropping and current limiting device, should 9V ever appear at the transistor's base.

Ken

 

I am wondering how big a voltage a transistor can be used to switch in stead of a relay whilst protecting the 5v circuit that switches it.

Thanks

A relay will only offer ON/OFF control, Transistor/Mosfet can offer RPM control.
What do you need?
Max.

 

Probably on off for now

 

That's where I am confused. It seems that circuit isolation can be performed by transistors.

I presume it depends on the voltage?

I want to use 9v and switch it with 3.3v but I don't want to damage the gpio.

Just in case your not clear, a relay provides isolation between the control circuit and the "load". The isolation is, basically, an air gap between the control circuit (the relay coil) and the load (the circuit connected thru the relay contacts).

A transistor doesn't, by itself, provide isolation. There is a silicon conductor between the control circuit (the transistor base) and the load (transistor collector-emitter junction).