Lost in the amperes

[poutpout]

Hi!
I am sorry asking always basic questions, I am really new to electronic but I am totally into it.

Now...
How to calculate the resistance of a DC motor or electromagnet?
How to calculate the amp or a DC motor or electromagnet?

I have 5 solenoids like that PED|67-120-610-620|SOLENOID, LATCH, 12VDC | Farnell UK
I have a MIDI controller like that **broken link removed**

1- I'd like to know if I can run those 5 solenoids with the controller without burning anything?
2- If I have a DC motor but no specifications at all except it's a 12V DC, how can I know if I can use it without exceed the amp or voltage max of the controller?

Electrical specifications:
● Contact Rating: 10W
● Maximum Switching Voltage: 24 VAC
● Maximum Switching Current: 500 mA
● Typical "On" Resistance: 0.2 Ω
● Approximate Switching Delay: 1 ms

Thanks a lot!
one day, it's me who will answer your questions...

Cheers,
Julie

 

[picbits]

That solenoid rating is 5 watt at 12 volt so just under 0.5 amps (5/12)

 

[poutpout]

That solenoid rating is 5 watt at 12 volt so just under 0.5 amps (5/12)

Oh thank you!

But how do you calculate it if you don't know the Watt?

 

[Nigel Goodwin]

Oh thank you!

But how do you calculate it if you don't know the Watt?

You always need to know TWO values, to calculate the third. The choices are volts, amps, ohms, or watts - any two will do.

 

[picbits]

Oh thank you!

But how do you calculate it if you don't know the Watt?

I looked at the spec from the link you posted from the solenoid.

It was listed at 5 watts at 12 volts which using the formula P=IV (watts)

P=IV
I=P/V
V=P/I

P=5 watts
V=12 volts

therefore I=5/12 amps (0.42amps)

 

[poutpout]

You always need to know TWO values, to calculate the third.

ok but when you don't have two values?
I have 4 DC motors 12V that's all I know. I'd like to use them with the MIDI controller. Can I calculate one of the other value with a multi-meter? If yes, how to do it?

therefore I=5/12 amps (0.42amps)

Thanks for the correction and thanks for the formula!EDIT: Does that mean I can't use 5 solenoids with that controller because it have a max of 500ma?

 

[flat5]

If you only have the voltage rating, apply the voltage and measure the current.

"Lost in the amperes"
Great book title.

 

[BeeBop]

ok but when you don't have two values?
I have 4 DC motors 12V that's all I know. I'd like to use them with the MIDI controller. Can I calculate one of the other value with a multi-meter? If yes, how to do it?

You can put an ammeter in series with the motor to find out how much current it draws. Motors tend to take more current when they start up than when running. That said, there may be a spec on the motor; have you looked closely? You posted the spec for the solenoids do you have a part number for the motors?

Does that mean I can't use 5 solenoids with that controller because it have a max of 500ma?

Can't is a strong word. Will more than one solenoid be on at the same time? You could use your controller to switch on a transistor or relay which powers your motors from a higher current supply.

EDIT: Looking at your midi controller, the 500 mA rating is for each of the relays on the board. You use a separate supply for the motors or solenoids, so you should be ok with this. You do want to put a diode across each solenoid though to deal with the inductive spikes when your motors switch off.

 

[poutpout]

You can put an ammeter in series with the motor to find out how much current it draws

Thanks!

EDIT: Looking at your midi controller, the 500 mA rating is for each of the relays on the board. You use a separate supply for the motors or solenoids, so you should be ok with this. You do want to put a diode across each solenoid though to deal with the inductive spikes when your motors switch off.

Thanks, thanks, thanks!

"Lost in the amperes"
Great book title.

Isn't it?