looking for BJT that inludes protection diode. - Electronic Circuits Projects Diagrams Free

alphadog · 23rd February 2009, 11:46 PM

I have a BJT that switches on & off a relay, and i couldnt find a low cost, not through hole, BJT with protection diode included.

Does anyone know about some?

Thank you.

Addition:
I forgot to mention that i'm interested in an NPN BJT.
Thanks.

mneary · 24th February 2009, 07:01 AM

Check in Digikey for transistors, and sort on transistors, single bjt. Scroll to the right and find the selection for NPN+diode. (and check 'in-stock').

Looks like there are a few to choose from starting at 56 cents.

alphadog · 24th February 2009, 10:12 AM

Thank you.
Everything thats on stock there is transistors array, not single.

**Nigel Goodwin** · 24th February 2009, 10:20 AM

Why on earth do you want one?, protection diodes need to really go across the relay, NOT across the transistor - just use a separate diode across the relay.

blueroomelectronics · 24th February 2009, 02:18 PM

Since it's probably for a 4 transistor h-bridge the diodes will work.

**Nigel Goodwin** · 24th February 2009, 03:17 PM

Quote:

Originally Posted by blueroomelectronics

Since it's probably for a 4 transistor h-bridge the diodes will work.

Except he specified an NPN transistor (singular) and a relay.

blueroomelectronics · 24th February 2009, 03:21 PM

Ah yes, silly me thought it was a continuation of the latching coil thread.

alphadog · 24th February 2009, 09:17 PM

I have a question please, not related to BJT and relay.
I found a N-channel MOSFET that has a diode across its Drain-Source channel.
If the VGS is lower than VTO, then IDS should be zero and therefore VDS becomes VCC (assuming its source's connected to VCC).
In my case, VCC = 5V which is larger than the VF of the Drain-Source diode.
How come the diode doesnt start coduncting current and therefore increasing IDS from 0A to IF of the diode?

Thanks.

mneary · 24th February 2009, 10:39 PM

Because the diode is reverse biased.

alphadog · 24th February 2009, 11:33 PM

Quote:

Originally Posted by mneary

Because the diode is reverse biased.

Thats the thing, the anode is connected to drain, not to source, so its forward biased.

Take a look please:

mneary · 25th February 2009, 01:38 AM

That's a dual transistor, one N-channel and the other P-channel. The drain of the P-channel device (Q2) is connected to the anode of its diode.

The drain of the N-channel device (Q1) is connected to the cathode of its diode.

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23rd February 2009, 11:46 PM	#1
alphadog	looking for BJT that inludes protection diode. I have a BJT that switches on & off a relay, and i couldnt find a low cost, not through hole, BJT with protection diode included. Does anyone know about some? Thank you. Addition: I forgot to mention that i'm interested in an NPN BJT. Thanks. Last edited by alphadog; 24th February 2009 at 12:22 AM.

24th February 2009, 07:01 AM	#2
mneary	Check in Digikey for transistors, and sort on transistors, single bjt. Scroll to the right and find the selection for NPN+diode. (and check 'in-stock'). Looks like there are a few to choose from starting at 56 cents.

24th February 2009, 10:12 AM	#3
alphadog	Thank you. Everything thats on stock there is transistors array, not single.

24th February 2009, 10:20 AM	#4
Nigel Goodwin	Why on earth do you want one?, protection diodes need to really go across the relay, NOT across the transistor - just use a separate diode across the relay. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

24th February 2009, 02:18 PM	#5
blueroomelectronics	Since it's probably for a 4 transistor h-bridge the diodes will work. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com/

24th February 2009, 03:21 PM	#7
blueroomelectronics	Ah yes, silly me thought it was a continuation of the latching coil thread. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com/

24th February 2009, 09:17 PM	#8
alphadog	I have a question please, not related to BJT and relay. I found a N-channel MOSFET that has a diode across its Drain-Source channel. If the VGS is lower than VTO, then IDS should be zero and therefore VDS becomes VCC (assuming its source's connected to VCC). In my case, VCC = 5V which is larger than the VF of the Drain-Source diode. How come the diode doesnt start coduncting current and therefore increasing IDS from 0A to IF of the diode? Thanks.

24th February 2009, 10:39 PM	#9
mneary	Because the diode is reverse biased.

25th February 2009, 01:38 AM	#11
mneary	That's a dual transistor, one N-channel and the other P-channel. The drain of the P-channel device (Q2) is connected to the anode of its diode. The drain of the N-channel device (Q1) is connected to the cathode of its diode.