5V Logic Level with High Current

[Suraj143]

I need to drive a load of 12V/ 2-3A. I’m controlling it from an IC which gives only 20mA from its output. Also the IC needs a 5V supply not 12V.

Please tell about my circuit & tell what’s wrong. Also suggest a number for TR2 it must be a TO220 package.

Thanks

 

[Leftyretro]

I need to drive a load of 12V/ 2-3A. I’m controlling it from an IC which gives only 20mA from its output. Also the IC needs a 5V supply not 12V.

Please tell about my circuit & tell what’s wrong. Also suggest a number for TR2 it must be a TO220 package.

Thanks

Well your circuit won't work as designed because you have a hard ground connection from the first transister's emitter, how can it switch anything if it's hooked to ground? Lift that ground wire and then you have darlington connected pair that should work but I'll let others comment on that. To really nail a design or reccomendation one needs to know the current draw for the 12volt load.

Today there are much better devices to use for this kind of switching/driving funtion, mosfet logic level transistor or IC driver chips, etc. that require fewer components, probably cheaper, take up less space, yadda yadda.

Lefty

 

[Suraj143]

Hi Lefty thanks for your guide.I'll lift that ground terminal of that first transistor.
I want to drive two 12V /18Wbulbs.I want to do with minimum components.

Also its I'm not going to put a heat sink for that because I don't have space.I'm scared will that burn without a heat sink?

Can I use a IRFZ mosfet for that? or planing to put TIP31C.

 

[Leftyretro]

Hi Lefty thanks for your guide.I'll lift that ground terminal of that first transistor.
I want to drive two 12V /18Wbulbs.I want to do with minimum components.

Also its I'm not going to put a heat sink for that because I don't have space.I'm scared will that burn without a heat sink?

Can I use a IRFZ mosfet for that? or planing to put TIP31C.

18w/12v/X2 means a load current of 3 amps. Pretty sure any normal npn transistor would need a heat sink. However a power mosfet (logic level gate) with a low Ron spec might work without a heatsink. I'll let others help validate/select the best mosfet.

So use a mosfet with suitable specs for the load and the logic level gate input.

Lefty

 

[Suraj143]

So use a mosfet with suitable specs for the load and the logic level gate input.
Lefty

If I use IRFZ24N it needs 10V input gate voltage.I cannot supply this from controller side because the IC gives only 5V.

Also I cannot use level converters this must done with minimum components.

 

[Leftyretro]

If I use IRFZ24N it needs 10V input gate voltage.I cannot supply this from controller side because the IC gives only 5V.

Also I cannot use level converters this must done with minimum components.

That's why I stated a logic level Mosfet. There are mosfets that will switch full on with 5 volts. Not sure of type numbers but they are pretty common and avalible these days. Someone here will help with a specific recommendation I'm sure, if you can't source a suitable device number.

Lefty

 

[Tomble]

As Lefty says, if you use a bipolar power-transistor, you'll most likely need a heatsink. You say the load is 12V; is that the actual voltage-drop expected over the load, or just the voltage you intend to supply it from?

Power dissipated by the power transistor (if you hook it up like a darlington as Lefty first suggested) will be (Vsupply-Vloaddrop) x 3A. I'd hope that the load has a very constant voltage drop, if it does then you might be able to ensure there's not too much across the power transistor, if you can adjust the supply voltage.

Then you can work out the total thermal resistance needed (remember, resistance is opposite of conduction-a powerful heatsink has LOW resistance!), as (max junction temperature - ambient temperature) / max power dissipated (by transistor).

If you insist on running your transistor without ANY heatsink (ow) then this figure you've just calculated MUST be more than the figure the transistor has for "Rth(junction-to-ambient)". ("Rth" is Thermal Resistance; the "th" may be a "theta" character like (-) which we can't use here). Do not use the "junction-to-case" figure for this, that's just for if you use a heatsink!

If instead you choose to use a small heatsink, then subtract about 6C/W from the figure calculated for required resistance, which would be a conservative estimate for the resistance of the case and the interface (between case and sink). The figure you're left with will be the maximum thermal resistance you can get away with for your heatsink. If you use a higher resistance, the transistor will get too hot.

Hope all that made sense...

If you can't use a power transistor with or without a heatsink, your other alternatives would seem to be (a) use a relay if you can fit one somewhere, (b) use a mosfet like Lefty said. Or (c) alter the specifications of the project so you can fit more in

 

[Suraj143]

Hi Tomble thanks for your great explanation.What about using a MOSFET (TO220) I don't know what number to use.I searched google but no progress.

I need to operate that from 5V.But most commonly available mosfets needs 10V for their gate.

Can anybody suggest me a MOSFET that can operate from 5V.

Also how can I find high power bipolar transistor (NPN) TO220.

 

[Tomble]

Hi Tomble thanks for your great explanation.What about using a MOSFET (TO220) I don't know what number to use.I searched google but no progress.

No problem! Sorry I can't help you re suitable power MOSFETs though, I don't know individual parts like the experts here. I'm sure someone else can though!

Also how can I find high power bipolar transistor (NPN) TO220.

AFAIK, the problem you will be running into with a bipolar transistor will mostly be the power one (although with the TIP31C, it looks like 3A will be the ABSOLUTE MAXIMUM you can put through it, which is a bad idea! TIP41C is 6A though ); the power dissipated won't really depend on the transistor (except in terms of how low its saturation voltage is perhaps??), but how much power you can get away with will depend on the transistor...

...except that I'd expect this figure to be relatively similar between TO220 transistors, I'm really not sure. Probably the biggest deal with power transistors would be getting a bit of a heatsink. FWIW 20C/W sinks often aren't too big, and PERHAPS it would be adequate for your project? It depends on the voltage across the transistor, as I say.

Meanwhile I'm frustrated to note that the TIPXX datasheets do not seem to mention thermal resistance at all, just dissipation at various temperatures Perhaps it's feasible to calculate the resistances from those anyway...

 

[Tomble]

Sorry I can't help you re suitable power MOSFETs

Sorry, I spoke too soon
How's this sound? http://www.fairchildsemi.com/pf/FD/FDS6690A.html
EDIT: Sorry, I'm half asleep That one's surface mount, not TO220...

THESE are TO-220 though.
http://www.fairchildsemi.com/ds/ND/NDP7050L.pdf
http://www.datasheetcatalog.com/datasheets_pdf/N/T/P/1/NTP18N06L.shtml

And this page has a selection of different things... http://www.nteinc.com/Web_pgs/LL_MOSFET.html but note that the references to "power dissipation at Tc=25C" mean when you've cooled the case down with a suitable heatsink...

Hope some of this helps; maybe other people will have better suggestions though.

 

[Suraj143]

Hi Tomble thanks for spending your time because of me.
I also wondering about that TIP31C but as you mentioned it will exceed the limit.But there is a solution in TIP41C I think it will suit for this matter.

I'll hook & see what will happens to the TIP31C or 41C.If it goes too much hot I'll have to add a heat sink & see.

BTW what you mean by FWIW 20C/W?

 

[audioguru]

Incandescent light bulbs draw 10 times their rated current when they are cold and first turned on. A TIP31 would blow up.
Also, its minimum current gain at 3A is only 10 and then it isn't saturated (4V loss).

A TIP41 might be able to pass 30A for a moment. Its minimum current gain is 15 at 3A but it needs a base current of 300mA to saturate well. It will need a heatsink.

Your circuit had a 4.7k resistor limiting the base current of the power transistor to only a few mA.
Your circuit had a 1k resistor in series with the base of the 2N2222 which limited its base current and its base voltage so it would have hardly any output.

My circuit supplies enough base current and base voltage to both transistors.

Use an IRF3711Z logic-level Mosfet. With a 4.5V gate voltage its max resistance is only 7.3 milli-ohms. Then it will have a voltage loss at 3A of only 22mV and will dissipate only 66mW.