Relay Question

[AMDKilla]

I'm building a cosplay minigun, and I have a 5v battery bank powering the 6v motor I have to spin the barrel. I have a Adafruit soundboard (a little pcb that plays sounds when a pin is connected to ground). I have a switch going from the battery to the motor, but I'm assuming the current pulled by the motor will likely fry the soundboard as it only expects a ground connection. I have a relay that I bought from amazon, it says the control current is 15-20mA, but I would imagine the motor is probably pulling closer to 1A. Would I fry the relay if I used the motor circuit as the control? I just need to get a signal from the motor circuit to the soundboard without frying anything.

Heres a like to the relay:

https://www.amazon.co.uk/gp/product/B01H2D2RI0/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1

[Hackentosher]

That relay is rated to 10 amps at 120v ac. It'll be fine.

 

Might I suggest using a MOSFET like this one as it wont have the clicking of a relay and will be much more compact.

[AMDKilla]

3 minutes ago, Hackentosher said:

That relay is rated to 10 amps at 120v ac. It'll be fine.

 

Might I suggest using a MOSFET like this one as it wont have the clicking of a relay and will be much more compact.

The clicking won't bother me. I know it's rated for 10A, but I assumed that was for the circuit it was controlling, rather than the circuit controlling it (I hope that makes sense)

[AMDKilla]

33 minutes ago, Pangea2017 said:

you need 15-20mA to control the relay. This mean the current is only needed to keep the relay closed and not used for the output ports. You might want to checkout motor controller ICs.

I know the 20mA isn't used for the output ports, I was wondering if running 1A through the 20mA inputs would likely damage anything. I don't want to control the motor with the relay, I want the relay to close when the motor is running.

[AMDKilla]

1 hour ago, Pangea2017 said:

You want to pull all the amp trough the controlling port of the relay?  it would be better to redesign it. Connect the switch to the low amp part which activate the relay for the motor. Keep in mind that with the original design also the switch need to be build for 1A at 6v,

That's a much better way of doing things, thanks for that. The switches I wanted to use are only rated for 50mA, so that would have been a bad idea to push 1A through them. Now all I need to do is find out if the soundboard that I'm using pushes out 15-20mA when connecting the pins to ground...

[AMDKilla]

Someone on the soundboard forums said they believed the current on the GPIO pins was around 0.3uA, so nowhere near enough to trigger the relay

[mariushm]

A relay has a coil of wire inside. When there's current going through that coil of wire inside the relay, the coil of wire basically becomes a magnet and connects the two other contacts inside the relay together, so the relay becomes closed. 

When there's no more current going through the coil, the magnetic properties go away, so the relay disconnects.

 

The relay is rated for some current at DC voltage and some other current at AC voltage.... that rating is for the secondary (load) terminals  So your relay could be rated for 120v AC 10A but may only be rated for something like 30v DC 10A. The reason for these two different ratings is because when the magnet activates and gets the two load terminals together inside, when the contact is made there are some sparks, especially if there's a lot of current going between the two terminals. These sparks can damage the surface of the terminals inside the relay so in time, the surfaces corrode and the contact inside the relay becomes weaker and the relay heats more and eventually it can break down.

With AC current, since the current goes back and forth, it takes very little time for the sparks to be made and extinguish by themselves, for just a few ms every time the relay is closed.  But with DC current, since the current goes only one way, there's a longer time in which sparks can be created inside the relay... so the relay is more sensitive to DC voltage.

 

Anyway... in order for the relay to turn on and close the load contacts, it must receive the rated voltage and there must be enough current flow through the coil pins for the magnetic field to be created. That's why you see that 10-15mA value. The battery or whatever you use must be able to provide that much current to the relay to turn on or off.

 

The pins on the sound card are likely to only provide less than 1mA or something around that value, so it's not a good idea to connect the relay coil directly to the sound card. BUT, you can connect the relay to the trigger on your toy or you can use workarounds.

Like for example, you can use a transistor between the sound card and the relay.... the transistor also works like a switch, with just a tiny signal on one of its pins, it creates a connection between its other pins, so you'll have  sound card -> turn on transistor - > turn on relay -> power goes to motor through relay.

 

Note that when the motor starts, it pulls so much power all of the sudden from the battery that for a brief moment the voltage may go down, for example from 5v to 4.5v ... if you use a relay rated for 5v, then the relay may disconnect all of the sudden when the voltage drops briefly, or your sound card may reset. So it would be best to add a capacitor very close to the sound card input and the relay to hold some charge and reduce this effect.

 

Oh... here's the datasheet for the relays on that Amazon page: Songle SRD-5VDC-SL-C: http://www.circuitbasics.com/wp-content/uploads/2015/11/SRD-05VDC-SL-C-Datasheet.pdf

 

If you check the table in the datasheet, you'll notice that those particular relays aren't that great, they require around 70-90mA for the coil to energize and close the contacts - the 15mA for the relay is with the 24v version of the relay probably :

You can see the high sensitivity estimates 71mA and 90mA for the standard version of the relay.

 

 

If you don't need relays that carry so much current on their load terminals, you can buy standalone 5v relays that don't consume so much power to stay closed.

See stores like Digikey.com or Farnell .

 

For example, here's a bunch of 5v relays, with 5A or less current on the load terminals (enough for a motor using up to 1A): https://www.digikey.com/products/en/relays/power-relays-over-2-amps/188?FV=1200001%2C120006f%2Cffe000bc%2Cmu2A|1410%2Cmu3A|1410%2Cmu4A|1410%2Cmu5A|1410&quantity=&ColumnSort=1000011&page=1&stock=1&pageSize=25

 

You can see for example this one needs at least 3.5v to work, and uses up to 22mA to work : https://www.digikey.com/product-detail/en/omron-electronics-inc-emc-div/G6DN-1A-DC5/Z5439-ND/5864613

 

This one uses up to 24mA and up to 3A of current on load terminals: https://www.digikey.com/product-detail/en/te-connectivity-potter-brumfield-relays/PCN-105D3MHZ000/PCN-105D3MHZ000-ND/1427537

 

 

[AMDKilla]

2 hours ago, mariushm said:

A relay has a coil of wire inside. When there's current going through that coil of wire inside the relay, the coil of wire basically becomes a magnet and connects the two other contacts inside the relay together, so the relay becomes closed. 

When there's no more current going through the coil, the magnetic properties go away, so the relay disconnects.

 

The relay is rated for some current at DC voltage and some other current at AC voltage.... that rating is for the secondary (load) terminals  So your relay could be rated for 120v AC 10A but may only be rated for something like 30v DC 10A. The reason for these two different ratings is because when the magnet activates and gets the two load terminals together inside, when the contact is made there are some sparks, especially if there's a lot of current going between the two terminals. These sparks can damage the surface of the terminals inside the relay so in time, the surfaces corrode and the contact inside the relay becomes weaker and the relay heats more and eventually it can break down.

With AC current, since the current goes back and forth, it takes very little time for the sparks to be made and extinguish by themselves, for just a few ms every time the relay is closed.  But with DC current, since the current goes only one way, there's a longer time in which sparks can be created inside the relay... so the relay is more sensitive to DC voltage.

 

Anyway... in order for the relay to turn on and close the load contacts, it must receive the rated voltage and there must be enough current flow through the coil pins for the magnetic field to be created. That's why you see that 10-15mA value. The battery or whatever you use must be able to provide that much current to the relay to turn on or off.

 

The pins on the sound card are likely to only provide less than 1mA or something around that value, so it's not a good idea to connect the relay coil directly to the sound card. BUT, you can connect the relay to the trigger on your toy or you can use workarounds.

Like for example, you can use a transistor between the sound card and the relay.... the transistor also works like a switch, with just a tiny signal on one of its pins, it creates a connection between its other pins, so you'll have  sound card -> turn on transistor - > turn on relay -> power goes to motor through relay.

 

Note that when the motor starts, it pulls so much power all of the sudden from the battery that for a brief moment the voltage may go down, for example from 5v to 4.5v ... if you use a relay rated for 5v, then the relay may disconnect all of the sudden when the voltage drops briefly, or your sound card may reset. So it would be best to add a capacitor very close to the sound card input and the relay to hold some charge and reduce this effect.

 

Oh... here's the datasheet for the relays on that Amazon page: Songle SRD-5VDC-SL-C: http://www.circuitbasics.com/wp-content/uploads/2015/11/SRD-05VDC-SL-C-Datasheet.pdf

 

If you check the table in the datasheet, you'll notice that those particular relays aren't that great, they require around 70-90mA for the coil to energize and close the contacts - the 15mA for the relay is with the 24v version of the relay probably :

You can see the high sensitivity estimates 71mA and 90mA for the standard version of the relay.

 

 

If you don't need relays that carry so much current on their load terminals, you can buy standalone 5v relays that don't consume so much power to stay closed.

See stores like Digikey.com or Farnell .

 

For example, here's a bunch of 5v relays, with 5A or less current on the load terminals (enough for a motor using up to 1A): https://www.digikey.com/products/en/relays/power-relays-over-2-amps/188?FV=1200001%2C120006f%2Cffe000bc%2Cmu2A|1410%2Cmu3A|1410%2Cmu4A|1410%2Cmu5A|1410&quantity=&ColumnSort=1000011&page=1&stock=1&pageSize=25

 

You can see for example this one needs at least 3.5v to work, and uses up to 22mA to work : https://www.digikey.com/product-detail/en/omron-electronics-inc-emc-div/G6DN-1A-DC5/Z5439-ND/5864613

 

This one uses up to 24mA and up to 3A of current on load terminals: https://www.digikey.com/product-detail/en/te-connectivity-potter-brumfield-relays/PCN-105D3MHZ000/PCN-105D3MHZ000-ND/1427537

 

 

So basically passing up to 2A through something designed for 0.2A is just obviously a bad idea. Not only would it damage to coil contacts, but it would likely overheat too. I literally just need a way of having the soundboard trigger when the motor runs without frying anything.

 

So I could use a transistor like a switch to take the 0.3uA current from the soundboard to close a 20mA circuit to trigger the relay to run the motor? In which case, surely I then need something that draws 20mA on the circuit between the transistor and the relay to make the relay trigger?

[mariushm]

The relay coil needs that amount of current to stay energized and keep the contacts closed and let energy flow through and keep the motor working.  The relay won't use more than it needs to. If coil needs 20-30 mA, then it will only use that much amount of energy even if the battery can provide even 1000mA for short periods of time.

If the sound card can only provide a very low amount of current like let's say 0.5 mA, then the coil will never have enough current flowing through to create the magnetic field.  So you need to power that coil directly from battery.

 

You can however use a transistor like an on/off switch..

 

A transistor has 3 pins , 3 wires ... base, collector and emitter  ... when there's some voltage and current on the base pin (above some thresholds), there's a connection created between the emitter and collector, just like a relay.

The amount of current that can flow between the collector and emitter pins will depend on the amount of current that can flow into the base pin.

 

So for example, let's say you pick a transistor that has a hFe between 100 and 300 (the transistors can't be manufactured to an exact value, and this value also depends on the temperature of the transistor and the voltage on the base pin and other things) and you go with the worst case scenario and just assume hFe would be 100.

In that case, if the voltage on the base pin of the transistor goes above some level, let's say 2v, and the current is 1mA, then a connection is made between the collector and emitter pins which allows up to 100 x 1mA = 100mA to flow between those two pins.  So if you connect the relay right after the transistor and that relay consumes the amount it needs, 20-30mA or whatever.

 

In your particular case, I don't think it's a good idea to activate the motor from the sound card. I think it would be best to have a trigger or something, and connect the "turn on sound card" and "turn on motor" separately on that trigger , like a double pole double throw switch or push button.

 

Watch this video to learn how to use transistors, it's very easy to understand:

 

 

[AMDKilla]

47 minutes ago, mariushm said:

The relay coil needs that amount of current to stay energized and keep the contacts closed and let energy flow through and keep the motor working.  The relay won't use more than it needs to. If coil needs 20-30 mA, then it will only use that much amount of energy even if the battery can provide even 1000mA for short periods of time.

If the sound card can only provide a very low amount of current like let's say 0.5 mA, then the coil will never have enough current flowing through to create the magnetic field.  So you need to power that coil directly from battery.

 

You can however use a transistor like an on/off switch..

 

A transistor has 3 pins , 3 wires ... base, collector and emitter  ... when there's some voltage and current on the base pin (above some thresholds), there's a connection created between the emitter and collector, just like a relay.

The amount of current that can flow between the collector and emitter pins will depend on the amount of current that can flow into the base pin.

 

So for example, let's say you pick a transistor that has a hFe between 100 and 300 (the transistors can't be manufactured to an exact value, and this value also depends on the temperature of the transistor and the voltage on the base pin and other things) and you go with the worst case scenario and just assume hFe would be 100.

In that case, if the voltage on the base pin of the transistor goes above some level, let's say 2v, and the current is 1mA, then a connection is made between the collector and emitter pins which allows up to 100 x 1mA = 100mA to flow between those two pins.  So if you connect the relay right after the transistor and that relay consumes the amount it needs, 20-30mA or whatever.

 

In your particular case, I don't think it's a good idea to activate the motor from the sound card. I think it would be best to have a trigger or something, and connect the "turn on sound card" and "turn on motor" separately on that trigger , like a double pole double throw switch or push button.

 

Watch this video to learn how to use transistors, it's very easy to understand:

 

 

Thanks for all your advice, I think transistors are a little out of my league for now, so I've ordered a DPDT switch. I chose a 3 position (on)off(on) rocker switch as it means I can trigger another sound without having to add an extra button.

 

Thanks again for taking the time to explain everything