Beefing up an existing circuit - simple way to do this?

[Al Borowski]

Hi all,

I have a low-volume project that has several digital outputs. These
outputs were designed to source 12V at up to an amp. They don't turn on
every often - perhaps 2 or 3 times a day, for 20 minutes at a time max.

The output circuit can be seen at http://alpage.ath.cx/transistors.gif
- it's about as simple as it gets. Q7 is just a BC338, and Q1 is either
a TIP32 or MJE2955.

Now I need to modify these outputs to supply up to 3 amps instead. Even
without crunching the numbers the MJE2955 gets far too hot at 2 amps,
let alone 3. Heatsinking isn't really an option since the design is in
an enclosed box in a fairly hot environment. The PCB itself can handle
the 3 amps, just not the transistor.

I know I could use a FET or something - but can anyone think of a
better solution that doesn't involve hacking up existing PCBs too much?
The magic bullet would be a PNP transistor in the same pinout that can
handle 3A without needing a heatsink and tons of base current.

The project is fairly low volume so labour is more of an issue then
parts cost. I have about 30 of these devices to upgrade.

I'm sure there is an elegant solution to this. Does anyone have any
ideas?

Thanks a ton,

Al

 

[Jan Panteltje]

Hi all,

I have a low-volume project that has several digital outputs. These
outputs were designed to source 12V at up to an amp. They don't turn on
every often - perhaps 2 or 3 times a day, for 20 minutes at a time max.

The output circuit can be seen at http://alpage.ath.cx/transistors.gif
- it's about as simple as it gets. Q7 is just a BC338, and Q1 is either
a TIP32 or MJE2955.

Google PNP power darlington TO220

There are many of those, for example the MJE700T:
http://www.digchip.com/datasheets/parts/datasheet/343/MJE700T.php
Dunno about the pinout...

 

[John B]

Hi all,

I have a low-volume project that has several digital outputs. These
outputs were designed to source 12V at up to an amp. They don't turn
on every often - perhaps 2 or 3 times a day, for 20 minutes at a time
max.

The output circuit can be seen at http://alpage.ath.cx/transistors.gif
- it's about as simple as it gets. Q7 is just a BC338, and Q1 is
either a TIP32 or MJE2955.

Now I need to modify these outputs to supply up to 3 amps instead.
Even without crunching the numbers the MJE2955 gets far too hot at 2
amps, let alone 3. Heatsinking isn't really an option since the
design is in an enclosed box in a fairly hot environment. The PCB
itself can handle the 3 amps, just not the transistor.

I know I could use a FET or something - but can anyone think of a
better solution that doesn't involve hacking up existing PCBs too
much? The magic bullet would be a PNP transistor in the same pinout
that can handle 3A without needing a heatsink and tons of base
current.

The project is fairly low volume so labour is more of an issue then
parts cost. I have about 30 of these devices to upgrade.

I'm sure there is an elegant solution to this. Does anyone have any
ideas?

Thanks a ton,

Al

A good old-fashioned relay? There's plenty to choose from here:


http://www.omroncomponents.eu/home/products/Relays/PCBPowerRelays/upto5A
/default.asp

 

[Andy]

Hi:

Hi all,

...
The output circuit can be seen at http://alpage.ath.cx/transistors.gif
- it's about as simple as it gets. Q7 is just a BC338, and Q1 is either
a TIP32 or MJE2955.

Now I need to modify these outputs to supply up to 3 amps instead. Even
without crunching the numbers the MJE2955 gets far too hot at 2 amps,
let alone 3. Heatsinking isn't really an option since the design is in
an enclosed box in a fairly hot environment. The PCB itself can handle
the 3 amps, just not the transistor.

...

Can you tell the value of R2 and of how big power ("wattage") R2 can be
mounted?

Currently, Q1 does not have a heatsink, correct ?

-- Andy

 

[Jan Panteltje]

Google PNP power darlington TO220

There are many of those, for example the MJE700T:
http://www.digchip.com/datasheets/parts/datasheet/343/MJE700T.php
Dunno about the pinout...

Actually with 1V drop at 3A = 3W, and no heatsink, maybe you should at
least consider some small heatsinks on each transistor.

 

[nospam]

I know I could use a FET or something - but can anyone think of a
better solution that doesn't involve hacking up existing PCBs too much?

Use a FET or something.

You can get P channel MOSFETs in S08 that will hardly get warm at 3A
(Fairchild FDS6675 for example).

If you add a pull up resistor on the gate you could drive it with the
existing circuit.

Make a little TO220 sized PCB with the MOSFET, pull up, and 3 legs and
solder it in the existing PCB instead of the TIP32.
--

 

[Al Borowski]

Hi,

Thanks for the replies so far everyone. To answer a few questions:

R2 can (barely) fit a 1W resistor. My current mock-up has R2 as 150
ohms, 1W (assuming 12V drop, handles just under a watt). This gives a
base current of roughly 80 milliamps for the PNP transistor. So,
without stacking additional resistors on top of each other, I need a
PNP with a minimum DC gain of 40, with a drop-out voltage of roughly
..25 volts. Does such a beast exist?

I can probably live without a compatible pinout, or even a TO-220
package. I have half a dozen outputs per board, but really only need 2
of them - if I remove the unused ones I free up a little space.

Q1 does not have a heatsink.

Darlingtons would certainly have enough gain but the VCE saturation
voltage is too high for my liking. Relays are an option, but it might
be a a bit labour-intensive to fit them.

Currently I'm leaning towards trying to wire in a P channel FET. If I
find one with the right pinout it shouldn't be hugely difficult to do.

Thanks a ton,

Al

 

[colin]

Hi all,

I have a low-volume project that has several digital outputs. These
outputs were designed to source 12V at up to an amp. They don't turn on
every often - perhaps 2 or 3 times a day, for 20 minutes at a time max.

The output circuit can be seen at http://alpage.ath.cx/transistors.gif
- it's about as simple as it gets. Q7 is just a BC338, and Q1 is either
a TIP32 or MJE2955.

Now I need to modify these outputs to supply up to 3 amps instead. Even
without crunching the numbers the MJE2955 gets far too hot at 2 amps,
let alone 3. Heatsinking isn't really an option since the design is in
an enclosed box in a fairly hot environment. The PCB itself can handle
the 3 amps, just not the transistor.

I know I could use a FET or something - but can anyone think of a
better solution that doesn't involve hacking up existing PCBs too much?
The magic bullet would be a PNP transistor in the same pinout that can
handle 3A without needing a heatsink and tons of base current.

The project is fairly low volume so labour is more of an issue then
parts cost. I have about 30 of these devices to upgrade.

I'm sure there is an elegant solution to this. Does anyone have any
ideas?

You can use a pch FET without hacking up the pcb too much
just put a beefy enough device in place of Q1,
al you need to do extra is put a resistor from gate to source,
a smd type would be an easy fit acros the terminals.

Colin =^.^=

 

[TuT]

Google PNP power darlington TO220

There are many of those, for example the MJE700T:
http://www.digchip.com/datasheets/parts/datasheet/343/MJE700T.php
Dunno about the pinout...

The problem with a Darlington in an application like this is their poor
saturation characteristic, Vsat being the Vce-sat of the drive transistor
plus the Vbe of the output device. Unless there are some parts out there
that bring out the driver collector on a separate pin, so it can be
resistively driven from 0V instead of connecting to the output transistor's
collector, you will be stuck with dissipation approaching 3W in each
Darlington.

 

[PeteS]

Use a FET or something.

You can get P channel MOSFETs in S08 that will hardly get warm at 3A
(Fairchild FDS6675 for example).

If you add a pull up resistor on the gate you could drive it with the
existing circuit.

Make a little TO220 sized PCB with the MOSFET, pull up, and 3 legs and
solder it in the existing PCB instead of the TIP32.
--

The venerable IRF9530 comes in TO220 and won't even break a sweat at
3A.

https://ec.irf.com/v6/en/US/adirect/ir?cmd=catProductDetailFrame&productID=IRF9530

There are lots of others of course; Siliconix (now part of the Vishay
empire) also comes to mind.

Cheers

PeteS

 

[Al Borowski]

Hi,

The venerable IRF9530 comes in TO220 and won't even break a sweat at
3A.

The pinout matches, and I love the word "venerable". But with rds(on)
at 300 milliohms, I'll have almost 3 watts to get rid of However
you've convinced me that a FET is the way to go.

Our local supplier has a heap of 2SJ602
(http://www.ortodoxism.ro/datasheets2/6/0ryf8hgd6p7je6xjfag0agc7kq7y.pdf)
in stock for a very cheap price. The pin-out is the same and the
rds(on) is about 50 milli-ohm worst case, giving less then half a watt
output. I don't think we'll have any trouble making this sucker fit.

Thanks a lot everyone. I really appreciate your help.

Best regards,

Al

 

[Andy]

Hi,

Thanks for the replies so far everyone. To answer a few questions:

R2 can (barely) fit a 1W resistor. My current mock-up has R2 as 150
ohms, 1W (assuming 12V drop, handles just under a watt). This gives a
base current of roughly 80 milliamps for the PNP transistor. So,
without stacking additional resistors on top of each other, I need a
PNP with a minimum DC gain of 40, with a drop-out voltage of roughly
.25 volts. Does such a beast exist?

I can probably live without a compatible pinout, or even a TO-220
package. I have half a dozen outputs per board, but really only need 2
of them - if I remove the unused ones I free up a little space.

What about ZTX1149A then?

-- Andy

 

[TuT]

Hi,

Thanks for the replies so far everyone. To answer a few questions:

R2 can (barely) fit a 1W resistor. My current mock-up has R2 as 150
ohms, 1W (assuming 12V drop, handles just under a watt). This gives a
base current of roughly 80 milliamps for the PNP transistor. So,
without stacking additional resistors on top of each other, I need a
PNP with a minimum DC gain of 40, with a drop-out voltage of roughly
.25 volts. Does such a beast exist?

I can probably live without a compatible pinout, or even a TO-220
package. I have half a dozen outputs per board, but really only need 2
of them - if I remove the unused ones I free up a little space.

Q1 does not have a heatsink.

Darlingtons would certainly have enough gain but the VCE saturation
voltage is too high for my liking. Relays are an option, but it might
be a a bit labour-intensive to fit them.

Currently I'm leaning towards trying to wire in a P channel FET. If I
find one with the right pinout it shouldn't be hugely difficult to do.

Thanks a ton,

Al

The BD132 has a pretty flat current gain to well over the current you
require. Take a look at:

http://www.datasheetcatalog.com/datasheets_pdf/B/D/1/3/BD132.shtml

 

[budgie]

A good old-fashioned relay? There's plenty to choose from here:


http://www.omroncomponents.eu/home/products/Relays/PCBPowerRelays/upto5A
/default.asp

That was my thought too, assuming one with a decent (aka low) coil wattage is
used.

 

[John Popelish]

Hi,

Thanks for the replies so far everyone. To answer a few questions:

R2 can (barely) fit a 1W resistor. My current mock-up has R2 as 150
ohms, 1W (assuming 12V drop, handles just under a watt). This gives a
base current of roughly 80 milliamps for the PNP transistor. So,
without stacking additional resistors on top of each other, I need a
PNP with a minimum DC gain of 40, with a drop-out voltage of roughly
.25 volts. Does such a beast exist?

Probably, but I can't put my finger on a candidate.

Currently I'm leaning towards trying to wire in a P channel FET. If I
find one with the right pinout it shouldn't be hugely difficult to do.

The 12 volts for base drive is a bad mismatch to the base
voltage needed, so you have to waste all that extra voltage
with a hot resistor. The mosfet would have a much better
match to a 12 volt drive, and use power only during the
transients. Something like an IRF5305, for less than $2,
with 60 milliohms on-resistance, would carry 4 amperes with
..24 volts drop.
http://www.irf.com/product-info/datasheets/data/irf5305.pdf

 

[jasen]

Hi all,

I have a low-volume project that has several digital outputs. These
outputs were designed to source 12V at up to an amp. They don't turn on
every often - perhaps 2 or 3 times a day, for 20 minutes at a time max.

The output circuit can be seen at http://alpage.ath.cx/transistors.gif
- it's about as simple as it gets. Q7 is just a BC338, and Q1 is either
a TIP32 or MJE2955.

Now I need to modify these outputs to supply up to 3 amps instead. Even
without crunching the numbers the MJE2955 gets far too hot at 2 amps,
let alone 3. Heatsinking isn't really an option since the design is in
an enclosed box in a fairly hot environment. The PCB itself can handle
the 3 amps, just not the transistor.

have you got enough base drive on the transistors, describe the circuit in
more detail.

I know I could use a FET or something - but can anyone think of a
better solution that doesn't involve hacking up existing PCBs too much?

The magic bullet would be a PNP transistor in the same pinout that can
handle 3A without needing a heatsink and tons of base current.

The project is fairly low volume so labour is more of an issue then
parts cost. I have about 30 of these devices to upgrade.

Have the transistors drive headlamp relays?

I'm sure there is an elegant solution to this. Does anyone have any
ideas?

maybe there's some sort of mosfet you could drop-in with a pull-up resistor
on the gate.

Bye.
Jasen

 

[jasen]

Google PNP power darlington TO220

There are many of those, for example the MJE700T:

note the VCE when fully on,

avoid darlingtons where heat is an issue.

 

[jasen]

Hi,



The pinout matches, and I love the word "venerable". But with rds(on)
at 300 milliohms, I'll have almost 3 watts to get rid of However
you've convinced me that a FET is the way to go.

you can parallel them, seems like you've got room for three on each of your
two outputs

possibly you could short the old base terminal to the +12 and jumper the
gate pin to the other end of the base resistor.

probably a smaller resistor could be used instead,

Bye.
Jasen

 

[PeteS]

Hi,



The pinout matches, and I love the word "venerable". But with rds(on)
at 300 milliohms, I'll have almost 3 watts to get rid of However
you've convinced me that a FET is the way to go.

Our local supplier has a heap of 2SJ602
(http://www.ortodoxism.ro/datasheets2/6/0ryf8hgd6p7je6xjfag0agc7kq7y.pdf)
in stock for a very cheap price. The pin-out is the same and the
rds(on) is about 50 milli-ohm worst case, giving less then half a watt
output. I don't think we'll have any trouble making this sucker fit.

Thanks a lot everyone. I really appreciate your help.

Best regards,

Al

There's another advantage; you won't have to buy those expensive 1W base
drive resistors, although you need to calculate the instantaneous peak
gate charge currents - you could probably replace them safely with 1/4W
or 1/2W devices

Cheers

PeteS