best transistor for relay driving

hi,

I know in the instructions they list the BCX38C as a driver for a relay but upon looking on ebay for a pack of 10 they seem very expensive. so the question is, is there any other "cheaper" ways that people have used?

thanks
 

westaust55

Moderator
It will depend upon the current draw of the relay however for many applications a small signal transistor such as the BC548 or 2N2222 will suffice.
The BC548 is rated to 100 mA and the 2N2222 is rated to 500 mA.
Operation in saturation keeps the Vce down and the heat dissipation down.
 

geoff07

Senior Member
Pretty well any transistor will work provided it is the right polarity for your circuit (most likely npn with a + vcc) and will carry the required current. If you have a few transistors already then you may already have something suitable to experiment with. Don't forget the back-emf diode, and make sure there is enough base current when on for it to saturate.

The BCX35C is a very high gain (darlington) transistor but it takes over 0.6 volts even when saturated, so will get warm if the current is appreciable. Techsupplies have the BCX38C for 46 pence.

If you have a free hard with components then a mosfet would be better (less resistance when on and thus heat but a fiddle to drive), and even better a logic-level mosfet that you can drive directly from a Picaxe into saturation. Sparkfun have FQP30N06L logic level mosfets for 57 pence. As mosfets are voltage driven (unlike junction transistors that are current driven) you don't need a gate resistor.
 

Goeytex

Senior Member
First, tell us what relay you want to drive and then you can get a specific recommendation. Otherwise you will get general recommendations and/or guesses.

A link or datasheet for the relay would be most helpful.
 
hi

thanks for the replys, I have some small relays just lying around. im unsure of make etc so will have a fiddle.. the only reason id asked was a search on ebay turned up the BCX38C for £1.50 each !!!!
 

neiltechspec

Senior Member
Why not buy them from Rev-Ed @ 46p ea, or even CPC-Farnell @ 32.4p ea.

As Rev-Ed take paypal as well & they won't be fakes !!, it's a no-brainer surely.

Neil.
 

inglewoodpete

Senior Member
While the BCX38C is documented and available from Rev-Ed, I have never used one to drive a relay. Almost any small signal transistor driven into saturation will do for most small relays.
 

AllyCat

Senior Member
Hi,

Yes, the BCX38C is not an "ordinary" transistor but a "Darlington" (two transistors in one package giving very high current gain) and I agree with IP that it's rarely needed to drive a relay. Indeed it can be a bad choice for a 5 volt rail/relay because the extra transistor "loses" at least half a volt (as mentioned earlier in this thread). But "relay" is a very vague description, what coil resistance or current is needed?

However, in most cases I would use a BC337 (or the BC327 PNP equivalent if required, typically 7 - 8p each) with between 330 ohms and 1k from PICaxe pin to base. If you really do need a Darlington, it could be "made" with two BC337s, or a BC548 + BC337, etc.. But do check the data sheet(s) carefully, because these "TO92" packages do NOT have a standardised pinout. IIRC the BC337 and most similar devices appear to have their "flat" on the opposite side to the BCX38C. :eek:

Cheers, Alan.
 
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westaust55

Moderator
hi

thanks for the replys, I have some small relays just lying around. im unsure of make etc so will have a fiddle.. the only reason id asked was a search on ebay turned up the BCX38C for £1.50 each !!!!
Measure the relay coil resistance.
Then volts / resistance = amps
Will give an indication of the normal current.
There may be a brief peak currents so the relay operates until the magnetic iron circuit closes.

That will give a guide for transistor selection. Then you can select a BC548 or 2N2222 / BC337 to suit.

As already mentioned, don't forget the freewheeling diode across the relay coil.
 

westaust55

Moderator
hi

thanks for the replys, I have some small relays just lying around. im unsure of make etc so will have a fiddle.. the only reason id asked was a search on ebay turned up the BCX38C for £1.50 each !!!!
Measure the relay coil resistance.
Then volts / resistance = amps
Will give an indication of the normal current.
There may be a brief peak currents so the relay operates until the magnetic iron circuit closes.

That will give a guide for transistor selection. Then you can select a BC548 or 2N2222 / BC337 to suit.

As already mentioned, don't forget the freewheeling diode across the relay coil.
 

john2051

New Member
Hi I've seen earlier mentioned that the bcx38c has more of a voltage drop than a standard transistor,
Please could someone explain this, as when we were making darlingtons in the 70s, there was still only one
output transistor. The two collectors were connected, and the emitter of the first went to the base
of the second. How does this increase the voltage drop?
 

Roman505

Member
The emitter of the first transistor feeds the base of the second, as you say. The collector of that first transistor will be Vce above its emitter, and its emitter will be at least the base-emitter voltage of the second transistor above ground. The sum of those two is pretty much the voltage drop you will see across the output, I expect.
 

Circuit

Senior Member
My favourite switcher is NTD5867NL-1G, N-Channel MOSFET Transistor 20A 60V
http://australia.rs-online.com/web/p/mosfet-transistors/7192907/
It is cheap (40 cents each in 10s) and only needs <3v to switch 5A and has flyback diode built in.

Fred
Wow! Thanks for that... I have been using STB55NF06 "logic level MOSFETs" which have a somewhat higher current capacity than the ones you mention, but I only need to run less than 10 amps so I am very interested in the devices you specify. Can I just check out my understanding of "logic level MOSFETs" with the grown-ups please? If the Vgs threshold voltage of the STB55NF06 is 1.8 volts and the Vgs of the NTD5867NL-1G is specified as the same, then the latter can be regarded as a "logic level MOSFET" and, for current switching around ten amps or so, I can use it as a direct substitution for the STB55NF06. My enthusiasm to do so is that the former costs about a third of the price of the latter! I would be firing it up through a 1K resistor from a PICAXE chip running on 5V.

Also, could someone advise on any possibility of "logic level MOSFETs" that can be switched by a PICAXE running on 3.3 Volts? I know that there are low current devices that do this, but I am looking for something that will switch 15 volts and 10 amps or so.
 
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Colinpc

New Member
Circuit, check the output current vs gate voltage and you will see what voltage you need to get 10A. There is also more to it than just looking at the current. You need to look at power dissipation as well. I would not be using this at 10A.

I suggested this as a switch instead of the transistors being discussed. I would be happy using it to switch a 12v relay or any load drawing a couple of Amps in free air.

I vaguely recall there was post with instruction on using FETs but I don't know where it was.

Fred
 
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Colinpc

New Member
The NTD5867NL-1G looks like a nice part, but at what current (with what series resistor) do you drive it at 3V? I read the datasheet as showing the Rds(on) graph going near vertical below about 3.5V gate to source.

Datasheet: http://www.onsemi.com/pub_link/Collateral/NTD5867NL-D.PDF
You are of course quite right. I had only has a quick look at the specs when I replied above.

I had used it to switch 12 v using picaxe at 5v.

So, Circuit, it is not suitable for high current with only 3v on the gate.

Fred
 

noelnelson

New Member
My quick comment on the use of a Mosfet. (a bit off track, but...) Also fit a parallel gate to ground resistor of 100k to 1M.
If for some reason your program assigns the output driving the gate, as an input, without a parallel resistor there, the floating high impedance input can pick up stray voltage and turn the Mosfet on, with possible unexpected results.
(I have fixed many of a particular brand of commercial power inverter because of such a simple omission)
 

Goeytex

Senior Member
Also, could someone advise on any possibility of "logic level MOSFETs" that can be switched by a PICAXE running on 3.3 Volts? I know that there are low current devices that do this, but I am looking for something that will switch 15 volts and 10 amps or so.
Almost all Logic level FET's that can be efficiently driven at 3.3 volts will be SMD parts. A parametric search at Digikey shows about 10 possibilities. One of these is the FDS6570A which I can tell you works very well at 3.3v. It is in SOIC-8 which can be tricky to hand solder, but doable. I use SOIC-8 to DIP-8 adapter boards from Futurlec or EBAY for these. Don't try to solder these without a fine tip soldering iron and solder flux.

MOSFET DATASHEET

SOIC ADAPTER
 

jojojo

Senior Member
If you like low cost, logic command level, high Vds, high Ids and raisonably RdsOn, you can use IRLR120. One of my favorites.
Cheers.
 

Circuit

Senior Member
Just taken delivery of 100 units of NTD5867NL from Farnell. These I shall be switching with 5v PICAXE chips, so no real problem with the sort of currents I am working with - I now realise that the STB55NF06Ls that I have been using are rather overspecified for most of the applications I have been putting them to. The FDS6570A looks interesting, but I don't seem to see it in the Farnell inventory. It is listed at RS but costs £1.88 per unit - a little steep; but £1.10 per unit at Digikey. I think that I shall stick with 5volt PICAXE operation and keep my costs down with the NTD587L at 17 pence each - a bargain, I think, given the spec.

So, to all those who provided the pointers above, I echo vectraboyv6 whose thread I somewhat hijacked - Thanks for the replies!
 

The bear

Senior Member
Hi circuit,
Would you consider selling five or ten of your NTD5867NL's.You can double or triple the price. No way could I cope with 100 units.
Not used the PM system yet.

Regards, Bear..
 

Colinpc

New Member
With them being so cheap, no need to buy more than 10 if you don't need them. RS sell them so cheap and free post if ordered online (In Australia at least)

Fred
 

The bear

Senior Member
@ freddagg,
RS in the UK;
Your order is under £20.00 and will incur a handling fee of £4.95

Regards, Bear..
 
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