ballpark inrush current
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Posts: 46,084
Hello everyone,
Beautiful day outside, but I have a question.
Is there a rule of thumb for figuring inrush current?
I have a Jameco p/n145314 12vdc, 89ohm solenoid,
so by ohms law I get about 128 mA current draw.
But I keep hearing about inrush current, as it is right
now I ought to be able to switch it on and off with a
Zetex 455 transistor, but wonder if the inrush will burn out the '455 or
lessen its life considerably.
Also, how much over design do you usually allow for
transistors, if you have to deliver 125mA, do you like
select a transistor that would easily handle twice that.
In taking apart various power handling devices it seems like they really
over design the heck out of things, just wondered if there was a rule of
thumb here as well.
Mark
Beautiful day outside, but I have a question.
Is there a rule of thumb for figuring inrush current?
I have a Jameco p/n145314 12vdc, 89ohm solenoid,
so by ohms law I get about 128 mA current draw.
But I keep hearing about inrush current, as it is right
now I ought to be able to switch it on and off with a
Zetex 455 transistor, but wonder if the inrush will burn out the '455 or
lessen its life considerably.
Also, how much over design do you usually allow for
transistors, if you have to deliver 125mA, do you like
select a transistor that would easily handle twice that.
In taking apart various power handling devices it seems like they really
over design the heck out of things, just wondered if there was a rule of
thumb here as well.
Mark
Comments
A solenoid is an inductive load. Inductors do not have inrush current; in
fact, they resist current changes, so the current builds gradually according
to L/R when they are turned on. However, because of this property, they
also resist current change when turned off, which causes the turn-off
voltage spike necessitating a protection diode.
Incandescent lamps have inrush current as do capacitive loads.
Your ZTX455 should be fine as long as you use a protective diode across the
solenoid.
Ray McArthur
> I have a Jameco p/n145314 12vdc, 89ohm solenoid,
> so by ohms law I get about 128 mA current draw.
>
> But I keep hearing about inrush current, as it is right
> now I ought to be able to switch it on and off with a
> Zetex 455 transistor, but wonder if the inrush will burn out the '455 or
> lessen its life considerably.
solenoids. The coil starts off as air core, but when energised, as
the steel plunger gets drawn into the coil it becomes an iron core
coil, and inductance increases, reactance increases and current
decreases. This is why AC coils heat up and burn out if the coils
ever get detached from the solenoid. With DC though there is no
reactance, current flow is always limited by the ohms resistance of
the copper winding.
techno_masai wrote:
>
> Hello everyone,
>
> Beautiful day outside, but I have a question.
>
> Is there a rule of thumb for figuring inrush current?
>
> I have a Jameco p/n145314 12vdc, 89ohm solenoid,
> so by ohms law I get about 128 mA current draw.
>
> But I keep hearing about inrush current, as it is right
> now I ought to be able to switch it on and off with a
> Zetex 455 transistor, but wonder if the inrush will burn out the '455 or
> lessen its life considerably.
>
> Mark
Thank you both for the info.
Guess I must have heard about inrush current on AC motors.
So with DC operated coils like solenoids and motors inrush current doesn't
seem to be an issue.
Too bad you can't see current on a scope.
Mark
Original Message
From: rjmca <rjmca@w...>
To: basicstamps@egroups.com <basicstamps@egroups.com>
Date: Saturday, August 05, 2000 6:38 PM
Subject: Re: [noparse][[/noparse]basicstamps] ballpark inrush current
>Hi Mark:
>
>A solenoid is an inductive load. Inductors do not have inrush current; in
>fact, they resist current changes, so the current builds gradually
according
>to L/R when they are turned on. However, because of this property, they
>also resist current change when turned off, which causes the turn-off
>voltage spike necessitating a protection diode.
>
>Incandescent lamps have inrush current as do capacitive loads.
>Your ZTX455 should be fine as long as you use a protective diode across the
>solenoid.
>
>Ray McArthur
>
>> I have a Jameco p/n145314 12vdc, 89ohm solenoid,
>> so by ohms law I get about 128 mA current draw.
>>
>> But I keep hearing about inrush current, as it is right
>> now I ought to be able to switch it on and off with a
>> Zetex 455 transistor, but wonder if the inrush will burn out the '455 or
>> lessen its life considerably.
>
>
>
>
>
>
>
> Mark
>
> >
HI Mark,,,
Well you can " see " current on a scope . In just about any circuit that
has a resistor . If you connect your scope across the resistor . The voltage
drop
represents the current in the circuit .
Good Luck
RAVEN
> Guess I must have heard about inrush current on AC motors.
Well, it does apply there...sorta. Read below.
> So with DC operated coils like solenoids and motors inrush current doesn't
> seem to be an issue.
Not really. Inrush applies anywhere you have a system with a changing dynamic
load, most often inductors/coils and caps.
The nice thing about small DC relays is that their resistance (~90 ohms) is
large enough to effectively swamp their inductance. You can say it's not an
issue, but it's for the wrong reason.
Sorry, just pet peeve of mine.
> Too bad you can't see current on a scope.
Yes you can. Get a low value series resistor (5W, 0.5 ohm or so for your
application) and read the voltage across it. Of course you'll need to place it
in a manner such that you can ground one end and not affect the circuit. I
sugest placing it betwen the 'bottom' of the coil/snubber circuit and ground.
Attach your scope probe and watch! It's even better if you have a scope that can
display things with a very slow time scale and presistent phosphors.
Using this setup, it's also easy to see why the snubbing network is almost
universally used (diode/cap).
-dave
Next week's topic: Faster gardening through dynamite
dave paton http://www.dpaton.net dave at ieee dot org