# Amps and watts needed for this coil?

Discussion in 'General Electronics Chat' started by Nicholas, Jul 13, 2015.

1. ### Nicholas Thread Starter AAC Fanatic!

Mar 24, 2005
124
1
Hi guys

I have a coil(solenoid) that has a resistance of 4.4 ohm. I am supplying it with 40V DC. Is it correct
that ohms law apply, 40/4.4 = 9 amps and 40*9 = 360 watts?

Also, the coil will be on for half a second or so, does this 'alter' the real-world needs in any way? Or
do I need a 40V DC 360 watts 9 amp PSU? If I have three coils(and they energize at the same time),
would I need to triple the PSU?

Thanks a lot!

Nicholas

Last edited: Jul 13, 2015
2. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,070
Yes

Maybe, if you use a battery bank. Depending on the duty cycle, the average charging current delivered to recharge the battery could be much less than the peak current delivered by the battery.

Wouldn't that be a 40V supply?

Yes, unless you use the battery trick. Even though, that is a large battery bank...

Jul 18, 2013
12,913
3,547
That is rather low resistance/high wattage if just a small solenoid, are you sure it is not an AC type?
Is there no specs on the unit?
If using in an electronics environment use a BEMF diode across the coil.
Max.

4. ### DickCappels Moderator

Aug 21, 2008
3,820
1,038

"Also, the coil will be on for half a second or so, does this 'alter' the real-world needs in any way?"

The resistance of the solenoid will increase with temperature, so it will be drawing a little less current at the end of the on period than at the beginning. The current will be dependent upon the thermal characteristics of the solenoid, how long power is applied, and how long it has been since the prior application of power. In many cases this effect is insignificant.

The last question. At the top of your post you said you would drive the solenoid with 40 VDC then the question asks whether you need a 20V power supply. Please restate the question.

5. ### DC_Kid Distinguished Member

Feb 25, 2008
706
11
low Z fuel injectors come to mind.

the current may not peak to V/R, the mag field will restrict it depending in the Z of the coil and time on, etc.

6. ### AnalogKid AAC Fanatic!

Aug 1, 2013
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I think he was speculating that maybe there was a time component to Ohm's Law, that 40 V for 1 second might imply only 20 V for 1/2 second.

ak

7. ### Nicholas Thread Starter AAC Fanatic!

Mar 24, 2005
124
1
Hi, just corrected the original post. It is a 40V PSU, the 20 was a typo.

Thanks!

8. ### Nicholas Thread Starter AAC Fanatic!

Mar 24, 2005
124
1
So, I take it that I need a 27A 1080W psu, since they may need to kick at the same time. Sounds expensive

Thanks!

Jul 18, 2013
12,913
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You say on for half a sec, what is the duty cycle?
If low duty cycle and long periods between energising you may get away with a .5kva supply instead of 1kva.
A transformer and a just a bridge should work, nothing fancy.
Can you confirm if injector etc.
Max.

10. ### Nicholas Thread Starter AAC Fanatic!

Mar 24, 2005
124
1
Hi!

Sorry for the late answer. I don't have a duty cycle yet, but it is very low.

I think I will start with a 36V 20A 720W PSU. I seem to remember, that IF more coils
are energized at once, then the coils will just be weaker, yes? (to some extent?)

11. ### mcgyvr AAC Fanatic!

Oct 15, 2009
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Depending on the power supply.. It could go into shutdown mode to protect itself or overheat/fire,etc...
Start right.. Buy a supply greater than the maximum current demand of all combined solenoids..

Jul 18, 2013
12,913
3,547
Transformers are pretty rugged devices, they will tolerate over current for very short durations, also fused right will offer protection.
Max.

13. ### DC_Kid Distinguished Member

Feb 25, 2008
706
11
what exactly is the solenoid? and what exactly is the driving circuit? thus far this has been talk about the final power stage.