Searching \ for '[EE]: Protecting a BJT' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/index.htm?key=protecting+bjt
Search entire site for: 'Protecting a BJT'.

Exact match. Not showing close matches.
PICList Thread
'[EE]: Protecting a BJT'
2000\11\02@061924 by dvanced Technika

flavicon
face
Hello all.

I am driving a 24VDC motor, draws < 1A, with a TIP121 that pulls down the
motor's negative side. I switch this motor on and off regularly (about 10
seconds on, 30 seconds off). I have found that the transistor becomes short
circuit after some time (days or weeks) and then self destructs due to the
continuous conduction and overheating. Has anyone experienced this or know a
way to protect the BJT from this?

Regards, Kresho

--
http://www.piclist.com hint: To leave the PICList
spam_OUTpiclist-unsubscribe-requestTakeThisOuTspammitvma.mit.edu




2000\11\02@065404 by Vasile Surducan

flavicon
face
1N4007 , on the motor pins will supress high spikes on transistor.
anode to colector, cathode to V+
****************************************
Surducan Vasile
email:.....vasileKILLspamspam@spam@l30.itim-cj.ro
URL:http://www.geocities.com/vsurducan
****************************************

--
http://www.piclist.com hint: To leave the PICList
piclist-unsubscribe-requestspamKILLspammitvma.mit.edu




2000\11\02@070435 by staff

flavicon
face
Vasile Surducan wrote:
>
> 1N4007 , on the motor pins will supress high spikes on transistor.
> anode to colector, cathode to V+

Also put a 0.1uF ceramic/greencap across the motor terminals,
and since it is under 1 amp @ 24v, put a 1 ohm or 0.68ohm
series resistor between the motor and collector of the transistor.
-Roman

--
http://www.piclist.com hint: To leave the PICList
.....piclist-unsubscribe-requestKILLspamspam.....mitvma.mit.edu




2000\11\02@070849 by Russell McMahon

picon face
>I am driving a 24VDC motor, draws < 1A, with a TIP121 that pulls down the
>motor's negative side. I switch this motor on and off regularly (about 10
>seconds on, 30 seconds off). I have found that the transistor becomes short
>circuit after some time (days or weeks) and then self destructs due to the
>continuous conduction and overheating. Has anyone experienced this or know
a
>way to protect the BJT from this?


Kresho,

A little more information would be useful but lets see what we can
establish..

- There should be a diode connected with its anode to the transistor
collector and its cathode to the positive power supply rail.
Is there?

- The transistor should be driven hard enough to ensure it is being
"saturated" when on.
How is it driven? (voltage, base resistor)
What is the voltage across the transistor when it is on?
Is the transistor on a heatsink?
How hot does it get after running for an extended period?
(Hot enough that you can just bear to hold on to it is about 55 degrees
Celsius).

The TIP121 is rated at 80V, 5A and is a Darlington so -

- Drive should be very easy to provide so the transistor should be being
saturated.
Darlingtons do not saturate as well as a single transistor so Von is
probably around 0.8V - say 1 volt.
Ion of <1A should give under 1 watt on dissipation or only 1/4 Watt average
dissipation at your 25% duty cycle so a heatsink should not be needed.
Is this a TO220 package? (the equivalents book I used did not list case but
this series are mainly TO220 AFAIK)

Voltage and current rating are adequate.

I would therefore pick the most liklely problem as lack of a flyback diode
as noted above. This diode stops a large inductive spike occuring when the
transistor is turned off and the motor current continues to flow.
Use a 1N4004 or similar or better here.

If you have a diode here already then we need more information.



regards,




     Russell McMahon
_____________________________

What can one man* do?   Help the hungry at no cost to yourself!
at  http://www.thehungersite.com/
(* - or woman, child or internet enabled intelligent entity :-))
From other worlds:
http://www.changingourworld.com    http://www.easttimor.com   http://www.sudan.com

--
http://www.piclist.com hint: To leave the PICList
EraseMEpiclist-unsubscribe-requestspam_OUTspamTakeThisOuTmitvma.mit.edu




2000\11\02@071513 by Simon Nield

flavicon
face
Hi Kresho,
when switching an inductive load you need to provide a path through which the current can decay -
otherwise the a large voltage will be produced across the inductor as it tries to keep conducting;
v=l.di/dt, and you have just made a big di/dt by switching off the bjt so you get a big voltage.

depending on how fast you want the motor field to collapse you can use one of a number of circuits;

a diode connected back across the motor coil such that it's anode is connected to the collector of
the bjt and the cathode is connected to the +24v rail will allow the current to collapse slowly. the
diode needs to have a similar current rating to the bjt... a standard 3A diode would do the trick
here.

another option is to use a snubber circuit, which is an R and a C in series... this involves a
little more thought as to component values and ratings, and I have personally never had to use one
of these - I tend to use the diode method instead... especially as I seem to always need to use a
full bridge drive circuit. but I digress...

use a diode.

Regards,
Simon

--
http://www.piclist.com hint: To leave the PICList
piclist-unsubscribe-requestspamspam_OUTmitvma.mit.edu




2000\11\02@073034 by w. v. ooijen / f. hanneman

picon face
Do you limit the induction-cause voltage accross the TIP? (for instance a
35V zener)
Wouter

----------
> From: Kresho @ Advanced Technika <@spam@kreshoKILLspamspamADVANCEDTECHNIKA.COM.AU>
> To: KILLspamPICLISTKILLspamspamMITVMA.MIT.EDU
> Subject: [EE]: Protecting a BJT
> Date: Thursday, November 02, 2000 12:07
>
> Hello all.
>
> I am driving a 24VDC motor, draws < 1A, with a TIP121 that pulls down the
> motor's negative side. I switch this motor on and off regularly (about 10
> seconds on, 30 seconds off). I have found that the transistor becomes
short
> circuit after some time (days or weeks) and then self destructs due to
the
> continuous conduction and overheating. Has anyone experienced this or
know a
{Quote hidden}

--
http://www.piclist.com hint: To leave the PICList
spamBeGonepiclist-unsubscribe-requestspamBeGonespammitvma.mit.edu




2000\11\02@100300 by staff

flavicon
face
Russell McMahon wrote:
{Quote hidden}

I'm gonna stick my "TV repairer" oar in here. I fix VCRs every day that
use
the cheapest possible electronics to drive motors etc. Always use a
small
series resistor, a fusible one if possible (although all 1 ohm resistors
work good as fusible haha!) All the manufacturers do this.

The series resistor helps a lot in reducing spike intensity to the semi,
especially very short duration HV spikes like brush arcing which happens
fairly constantly. (Always use the diode too though!!)

Also the resistor reduces start current a lot. A 24v motor that draws
1 amp (as mentioned) may draw 10+ amps for a very short duration at
start
up. The series resistor will provide a much gentler start up with the
added bonus of spike protection and will burn out before your expensive
semi in the case of motor seize etc.

Now, re the transistor, a darlington SUX for this purpose. Most
darlingtons in the TO220 pack have about 1.2 to 1.5v C-E at 1 amp,
believe it or not. The TIP121 is very close to this I remember.
It will run real hot at 1 amp 1.5 volt. Big heatsink.

My favorite transistor of choice for this 12v/24v motor/solenoid/
lightbulb type of dc load driving is the BD203, 60v, 8amp TO220.
It is NOT darlington, but has beta of about 150 on most I tested.
The ON resistance is fet-like, 0.25v C-E at 1 amp or better!
ie, NO heatsink for switching 1 amp loads, they run stone cold.

So where do you get these super transistors? Mine are free, they
were popular vertical output transistor in 20yr old televisions,
so I have hundreds (and at 1 to 2 amps they DON'T fail).
I have seen them new for 50c US and under.

Perfect example of good 20yr old technology that is seemingly
ignored now. Why use a $3 fet that is ESD sensitive when you
can use a 50c/free transistor that has a huge chunky silicon
die in the same package size?? Ahhh, (sigh) nostalgia grips me
again.

Anyone have favorite transistors for uses like this??
-Roman

--
http://www.piclist.com hint: To leave the PICList
TakeThisOuTpiclist-unsubscribe-requestEraseMEspamspam_OUTmitvma.mit.edu




2000\11\02@101224 by Olin Lathrop

flavicon
face
> I am driving a 24VDC motor, draws < 1A, with a TIP121 that pulls down the
> motor's negative side. I switch this motor on and off regularly (about 10
> seconds on, 30 seconds off). I have found that the transistor becomes
short
> circuit after some time (days or weeks) and then self destructs due to the
> continuous conduction and overheating. Has anyone experienced this or know
a
> way to protect the BJT from this?

This is probably happening because the transistor is getting abused.  Do you
have proper flyback protection, since the motor is an inductive load?  Is
the flyback diode fast enough?  In this case it seems a FET is more
appropriate.  At 1A and 24V max you can easily get a FET with a very small
on resistance which will result in a much smaller voltage drop than the BJT.
The right FET can do this without even a heat sink  -  assuming you drive it
properly.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, RemoveMEolinspamTakeThisOuTcognivis.com, http://www.cognivis.com

--
http://www.piclist.com hint: To leave the PICList
piclist-unsubscribe-requestEraseMEspam.....mitvma.mit.edu




2000\11\02@102445 by J.Feldhaar

flavicon
face
Roman Black schrieb:

{Quote hidden}

Hello Roman,

I favor the D44H11 and D45 H11 series, they are even cheaper, and have better
(lower!!) Uce-sat , they also come in TO220 case.

Greetings
Jochen Feldhaar

--
http://www.piclist.com hint: To leave the PICList
RemoveMEpiclist-unsubscribe-requestEraseMEspamEraseMEmitvma.mit.edu




2000\11\02@110302 by Chris Eddy

flavicon
face
Hmm.. if the problem is a shorted load, and thus high overcurrent, try puting a
polyswitch device in series with the supply.  Three people make this type of
device, run to your nearest Digikey catalog.

Chris~

"Kresho @ Advanced Technika" wrote:

{Quote hidden}

--
http://www.piclist.com hint: To leave the PICList
RemoveMEpiclist-unsubscribe-requestTakeThisOuTspamspammitvma.mit.edu




2000\11\02@115649 by David VanHorn

flavicon
face
>> I am driving a 24VDC motor, draws < 1A, with a TIP121 that pulls down the
>> motor's negative side. I switch this motor on and off regularly (about 10
>> seconds on, 30 seconds off). I have found that the transistor becomes short
>> circuit after some time (days or weeks) and then self destructs due to the
>> continuous conduction and overheating. Has anyone experienced this or know


Do you have a protection diode from the collector of the transistor back to
the motor's supply?

You are trying to instantly stop the current through an inductor, which
will get you in trouble every time.

--
Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9

--
http://www.piclist.com hint: To leave the PICList
EraseMEpiclist-unsubscribe-requestspamspamspamBeGonemitvma.mit.edu




2000\11\02@141008 by hard Prosser

flavicon
face
If you already have adequate spike protection, Look into an effect known as
"second breakdown". Effectively it will mean you have to change the way you
turn the BJT off - probably involving an inductor to "suck" charge out of
the base. Alternative possibilities are using a baker clamp to prevent
total saturation of the transistor - but this will increase ON dissipation.
The best solution may be to use a MOSFET as these devices don't suffer from
this.
I got bitten hard by this effect a while ago and it can be a real akward
one to solve.

Richard P




Hello all.

I am driving a 24VDC motor, draws < 1A, with a TIP121 that pulls down the
motor's negative side. I switch this motor on and off regularly (about 10
seconds on, 30 seconds off). I have found that the transistor becomes short
circuit after some time (days or weeks) and then self destructs due to the
continuous conduction and overheating. Has anyone experienced this or know
a
way to protect the BJT from this?

Regards, Kresho

--
http://www.piclist.com hint: To leave the PICList
RemoveMEpiclist-unsubscribe-requestKILLspamspammitvma.mit.edu

--
http://www.piclist.com hint: To leave the PICList
piclist-unsubscribe-requestSTOPspamspamspam_OUTmitvma.mit.edu




2000\11\02@145753 by Spehro Pefhany

picon face
At 08:08 AM 11/3/00 +1300, you wrote:
>If you already have adequate spike protection, Look into an effect known as
>"second breakdown". Effectively it will mean you have to change the way you
>turn the BJT off - probably involving an inductor to "suck" charge out of
>the base. Alternative possibilities are using a baker clamp to prevent
>total saturation of the transistor - but this will increase ON dissipation.
>The best solution may be to use a MOSFET as these devices don't suffer from
>this.
>I got bitten hard by this effect a while ago and it can be a real akward
>one to solve.

SOA (Safe Operating Area) should be ok on a TIP121 at 1A/24V, but it's always
a good idea to refer to the chart on the data sheet to be sure.

Best regards,


=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spamBeGonespeffSTOPspamspamEraseMEinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

--
http://www.piclist.com hint: To leave the PICList
KILLspampiclist-unsubscribe-requestspamBeGonespammitvma.mit.edu




2000\11\03@024928 by dvanced Technika

flavicon
face
Thanks for all the replies.

Yes, I've got a flyback diode across the transistor. Just did some tests.
Hooked the motor up through a 1R resistor to measure current. When the motor
starts up i see a short peak of about 6V across the resistor. I am only
using a simple analog cro so maybe the voltage is even higher but i can't
see it. Also, i am using a current regulated 3A power supply with a large
cap so this could be affecting the results as well, because in the actual
unit it is run off an unregulated supply. When the motor settles down in
about 1/4 sec then the waveform is a saw-tooth as expected from the
commutator, about 1V peaks, average about .4V. So the startup may be what is
killing the device - 6A surge. I have been running the motor, pulsing on and
off at 2 sec intervals, and it's survived so far (over an hour) but i could
see how over time this could and would kill the bjt.

Funny thing is when the motor switches off i see no reverse surge through
the diode - again maybe i just don't have the right instrument to see this.

Damn semis. If the thing blew open circuit it would be a much nicer
situation. I also used the l298n dual h-bridge for this application
previously and it also blew short circuit till the whole device went up in
smoke.

Kresho.


{Original Message removed}

2000\11\03@034455 by staff

flavicon
face
Kresho @ Advanced Technika wrote:
{Quote hidden}

Kresho, you do realise that the 6amp peak was with the 1 ohm
resistor *in* circuit, the peak would have been much higher
without the resistor there (and with the other supply).
I have some 24v servomotors here that run at about 1amp on
light loads but draw 25amps+ at stall from a battery.

Also the flyback diode should be across the MOTOR. I still
suggest the 0.1uF cap across the motor terminals.

Was startup performance still good with the 1 ohm resistor in
circuit? If so, maybe even use a 2 ohm, will really protect the
transistor.

You said h-bridge in previous design, do you need direction
reversal or is the motor always on/off??

I think moving the diode to the right spot, adding the resistor
and capactior will fix your problem permanently. I also recommend
using a transistor that has 10x the rated current, why not
use a larger non-darlington transistor? Darlingtons are always
less reliable for switching inductive and high power loads, not
many commercial manufacturers use them.

If you really want to bulletproof it add a snubber network
consisting of a 10 ohm 1 watt R and 0.1uF HV polyester cap
in series from C-E. This is common on many of the "better"
commercial motor driver circuits. :o)

-Roman

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
"[PIC]:","[SX]:","[AVR]:" =uP ONLY! "[EE]:","[OT]:" =Other "[BUY]:","[AD]:" =Ads




2000\11\03@040819 by David VanHorn

flavicon
face
>Funny thing is when the motor switches off i see no reverse surge through
>the diode - again maybe i just don't have the right instrument to see this.

You probably won't without a current probe.
This sort of thing can happen so fast that it's unobservable on a 350 MHz
scope.

The protection diode should be at the transistor(the wiring has inductance
too) but in paralell with the motor.

--
Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
"[PIC]:","[SX]:","[AVR]:" =uP ONLY! "[EE]:","[OT]:" =Other "[BUY]:","[AD]:" =Ads




2000\11\03@043818 by David VanHorn

flavicon
face
At 07:45 PM 11/3/00 +1100, Roman Black wrote:
>Kresho @ Advanced Technika wrote:
>>
>> Thanks for all the replies.
>>
>> Yes, I've got a flyback diode across the transistor.

I missed this comment previously.

This won't cut it, unless the diode is a high power zener, with a knee
somewhat lower than VCEmax.   For a simple diode, it needs to be in
paralell with the motor.

Picture what happens at turn-off. The voltage on the bottom of the motor is
going to rise as the magnetic field collapses. Then it's going to force
current through the easiest path, which is likely putting your transistor
into avalanche mode. Depending on the energy stored in the rotor and the
magnetic field, the transistor barbecues.


--
Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
"[PIC]:","[SX]:","[AVR]:" =uP ONLY! "[EE]:","[OT]:" =Other "[BUY]:","[AD]:" =Ads




2000\11\03@135230 by w. v. ooijen / f. hanneman

picon face
> Yes, I've got a flyback diode across the transistor.

That protects against revese voltages, but not against overvoltage!
Wouter

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
"[PIC]:","[SX]:","[AVR]:" =uP ONLY! "[EE]:","[OT]:" =Other "[BUY]:","[AD]:" =Ads




More... (looser matching)
- Last day of these posts
- In 2000 , 2001 only
- Today
- New search...