Dual transistors

Use two singles and tie their bases together.

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According to the designer, two singles won't work well because it would be difficult to keep the base temperatures precisely the same. But he does suggest that as an alternative, using power transistors clamped together with thermal paste.

BTW, I have sent e-mail to him, but no reply. He may have lost interest, or doesn't check e-mail very often.

Thanks.

Ed

"Ed"

** The 2N4044 is a dual - however an NPN one ! ** No, the emitters are commoned - so no good for you.

....... Phil

What country are you in? it may help us to recommend suppliers.

Do you know how much current the transistor needs to handle?

There are quite a few dual PNP transistors in very small surface mount packages, but I expect they would be quite fiddly to solder?

see and select "PNP (Dual)" under "transistor polarity" on the page below:

The only dual PNP I can think of right now in a leaded package is the MAT-03. This is a precision audio part (that means expensive). I don't know if it is suitable though.

Given the errors you have discovered so far with this design, I'm not sure I would want to fit it to my car.

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gareth.harris

The emitters are commoned. Useless.

You know what I'd do ?

I'd get maybe 6 - 10 low cost, general purpose, low-to-medium power pnp transistors but with decent current gain, such as the type you'd get as the driver in a modest audio amp.

I'd find the 2 devices with the best match for Vbe and then glue them together back-to-back (ensuring that the collector tabs are insulated) . That'll ensure good thermal tracking.

Job done.

Graham

It's the *die* temperatire that matters.

Thermal paste is messy and not required here. Since there's no power involved ordinary epoxy glue would do the job just fine.

Duals are rare as hen's teeth btw and *expensive*.

Use the 2 power transistor method.

Graham

I'm in California

Very low. I've measured the current through the sensor to be less than

100 mA.

Thanks, Gareth.

Ed

I guess the definition of "very low" depends on who is asking. 100 mA from a 12 volt source can produce enough heat to put a blister on your finger (1.2 watts). But that is pretty low, compared to the current a cranking motor draws.

I think you can make the matching a lot less important if you add a couple ohms in series with each emitter. I think that mounting two TO-220 power tab transistors on the same small heat sink (with insulated mounting) would make this work just fine. Any PNP jellybean power tab should work, like TIP30 or TIP32.

Beware the low beta for those devices. I forget how beta dependent the circuit was or wasn't.

Graham

I think the circuit is a crude current mirror that boosts the current that passes through the sensor by an adjustable factor. Nothing precise about it. It is just a simple way to expand the central part of the meter scale.

"Eeysore Idiot "

** Bollocks.

A typical TIP31 /32 has a beta of * 150 *

with Vce = 2 volts & Ic =100mA.

........ Phil

Yes, you're right. Now I notice that the gauge has a 2W 50 ohm resistor at the sender input. Also, my calcs show the sender current can be up to about 0.2A.

OK, once I settle on the transistors I'll do that.

When I search for TIP30 at

(easily available locally) I am led to NTE153, a PNP power transistor in the TO-220 package. However, I notice that h_fe1 can range from 40 to 200. It might be hard to get a pair with "matched" h_fe, if that is what is called for, ideally. I also notice that the NPN version, NTE152, is available as a matched pair, NTE152MP. A quick Google tells me there are current mirror circuits based on NPN transistors. So, I'm wondering if I could use NPN in my application. Is that possible?

Ed

(snip)

Actually, what needs to be matched is the relationship between base to emitter voltage and collector current. But adding 2 ohms in series with each emitter adds an additional resistove drop to each effective base to emitter (plus) resistor drop, that swamps out any mismatch in this current range, so unmatched transistors can be used. These resistors do not reduce the current gain of the transistors, but increase the voltage required to turn them on (by 0.2 ams * 2 ohms =

0.4 volts, which totally reduces any millivolt difference in the ~.6 volt base emitter drop to insignificance, even if the two transistors are not at the same temperature. You might be able to use 1 ohm resistors (0.2 volts drop, max) and do almost as well, since this is really not a high precision application.

John Popelish

to improve the

So we have amateurs fixing Jag electronics ? Well I wont be buying a Jag !