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

On a sunny day (11 Dec 2006 04:17:08 -0800) it happened "Al Borowski" wrote in :

Google PNP power darlington TO220

There are many of those, for example the MJE700T:

On 11/12/2006 the venerable Al Borowski etched in runes:

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

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

On a sunny day (Mon, 11 Dec 2006 12:52:41 GMT) it happened Jan Panteltje wrote in :

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

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.

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

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 =^.^=

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.

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

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

Cheers

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

What about ZTX1149A then?

-- Andy

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

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

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

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.

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

Have the transistors drive headlamp relays?

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

Bye. Jasen

note the VCE when fully on,

avoid darlingtons where heat is an issue.

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

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