transistor heat up
Hi Dave, Hi Daniel:
Thanks for the o-scope current measuring tip, a 5Wx0.5ohm resistor will be
on my next order to Digikey, I don't have anything even close, it
will be interesting to get an idea of current flow
from my old HP 1202A, it goes up to 5 sec/div.
I did hook up a Zetex 455 to the 90 ohm, 12vdc coil
with an IN4007 in parallel across the relay with the
banded end toward positive 12v. I had an 840 ohm resistor between the base
and the stamp.
The '455 got too hot to touch after maybe 15 seconds
in cycling it every 50 milliseconds, (pause 50).
Mark
Original Message
From: Dave Paton <dave@d...>
To: basicstamps@egroups.com <basicstamps@egroups.com>
Date: Sunday, August 06, 2000 8:01 AM
Subject: Re: [noparse][[/noparse]basicstamps] ballpark inrush current
>On 8/6/00 at 6:27 AM, techno_masai <plunkettm@e...> wrote:
>
>> 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
>
>
>
>
>
>
Thanks for the o-scope current measuring tip, a 5Wx0.5ohm resistor will be
on my next order to Digikey, I don't have anything even close, it
will be interesting to get an idea of current flow
from my old HP 1202A, it goes up to 5 sec/div.
I did hook up a Zetex 455 to the 90 ohm, 12vdc coil
with an IN4007 in parallel across the relay with the
banded end toward positive 12v. I had an 840 ohm resistor between the base
and the stamp.
The '455 got too hot to touch after maybe 15 seconds
in cycling it every 50 milliseconds, (pause 50).
Mark
Original Message
From: Dave Paton <dave@d...>
To: basicstamps@egroups.com <basicstamps@egroups.com>
Date: Sunday, August 06, 2000 8:01 AM
Subject: Re: [noparse][[/noparse]basicstamps] ballpark inrush current
>On 8/6/00 at 6:27 AM, techno_masai <plunkettm@e...> wrote:
>
>> 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
>
>
>
>
>
>
Comments
>
> Thanks for the o-scope current measuring tip, a 5Wx0.5ohm resistor will be
> on my next order to Digikey, I don't have anything even close, it
> will be interesting to get an idea of current flow
> from my old HP 1202A, it goes up to 5 sec/div.
Just be careful. Many high wattage resistors are wire wound and exhibit
inductance. This can lead to nasty circuit effects you didn't plan on. They
do make resistors with low inductance where the winds of the wire go
opposite directions on each turn but not all of them are like this.
This can be a big problem in places like emitter circuits where early effect
(AKA base width modulation) can cause rapid change in Ve which causes large
swings in back EMF.
Regards,
Al Williams
AWC
*Stamp project of the month: http://www.al-williams.com/awce/som.htm
The ZTX455 is rated for 200 deg C junction, so it can run hot to the touch.
It should saturate at 0.7 volts collector-emitter with a forced beta of 10.
I would use:
12V/90 ohms=133 ma.
133ma/10=13.3 ma into base.
approx 0.7 volts base-emitter.
almost 5 volts from Stamp, so
Rbase= (5V-0.7V)/13.3ma=323 ohms from Stamp to base.
use 333 ohms, 1/8 watt.
The ZTX should dissipate about 0.7Vce*133ma=0.1 watt.
I like to use MOSFETS for drivers. We are driving a 1.4 amp stepper
with an IRL530N (TO220 case). The stepper gets warm, but you can barely
feel any temp rise on the MOSFET. However, your ZTX should be OK in your
ckt.
Ray McArthur
> I did hook up a Zetex 455 to the 90 ohm, 12vdc coil
> with an IN4007 in parallel across the relay with the
> banded end toward positive 12v. I had an 840 ohm resistor between the base
> and the stamp.
>
> The '455 got too hot to touch after maybe 15 seconds
> in cycling it every 50 milliseconds, (pause 50).