Problem with current when 8051 is driving a relay

• Dec 27, 2004

• #1

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chardbiggie

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i am using a ds89c420 Ucontroller, using the ports as output to drive a relay. I know the Ucontroller has low current output so i use 2n3904 transistor but the problem is when the relay is connected the output of the ports, goes down from 5v to 0v.

 

• Dec 27, 2004

• #2

flatulent

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darlington pair to operate 12 relay

Are you using a resistor from your IC to the base of the transistor? You should size this resistor for the current drive capacity of the chip. Try 1 mA as a first guess if you have no other information. The transistor beta should be about 100 so it can draw 100 mA with the collector.

 

• Dec 27, 2004

• #3

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CMOS

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bc517 relay

I use 4.7k resistor between 8051 port and base of transistor and also 4.7k pullup from port pin to Vcc. I generally use BC547. Even 2N3904 will work.

 

• Dec 29, 2004

• #4

klystron

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8051 relay

Try to use a uln2803 chip. It includes a EMI diode.
If the relay requires more than 5 VDC you will require a separate power supply

Added after 33 seconds:

Try to use a uln2803 chip. It includes a EMI diode.
If the relay requires more than 5 VDC you will require a separate power supply

 

• Dec 29, 2004

• #5

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Vonn

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uln2003 bistable relay driver maxim

as klystron said , using the relay driver chip (uln2003) is a very good solution but if you want to use transistors to drive your relay you should put diode as a reverse connection in parallel with the relay coil to avoid the effect of coil charging in the transition time when driving the relay ..
BR, Vonn

 

• Dec 29, 2004

• #6

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barrybear

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driving multiple latching relays

Another solution would be to use a fet off the port pin on to the fet`s gate then you will only need the drain connected to your realy with a diode to remove the back emf as the relay coil energises/deenegises.

Barrybear

 

• Dec 29, 2004

• #7

glenjoy

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bjt+relay+driver

Hi, compute your base current that is enough to generate the proper collector current for your relay current.

On this basis you will decide what base resistor you will be using.

 

• Dec 29, 2004

• #8

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felix1200

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8051 drive relay

as far as I remember the 8051 has a weak pull-up current source of about 100uA at his ports. It is not able to drive a ULN2803! You will need an external pull-up resistor. The current consumption is to high for more channels.
An external FET is only a solution if it is a logic-level FET ! You need a flyback diode.
The best way is to use a intelligent switch (HITFET) like BTS117 or smaller.
Or look for a driver like ULN2803 with CMOS inputs.

 

• Dec 29, 2004

• #9

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chardbiggie

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br-500 relay schematic

thanks for the replies, it work already but i just used 2 2n3904 in a darlington pair, thanks anyways

 

• Dec 29, 2004

• #10

ilker

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connections of 8051 and relay using uln2803

if you will drive multiple relay use uln2x03 they sources 500 ma per collector

 

• Jan 3, 2005

• #11

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ricpax

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uln2803a latch relay

you must take into consideration the base current of the 2n3904..if still the relay doesn't trigger,try to use darlington pair instead..

 

• Jan 6, 2005

• #12

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irfansyah

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driving a relay using 8051

chardbiggie said:

thanks for the replies, it work already but i just used 2 2n3904 in a darlington pair, thanks anyways

Click to expand...

I usually use one BC517 npn darlinton transistor for driving relays. I noticed that if i change this to a single normal BJT, the relay won't work. Same like yours .

 

• Jan 6, 2005

• #13

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khach

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8051 how to drive a relay

And what about driving single coil bistable relay? Bistable relay is wery good in power saving application. But single coil bistable relay is driving (switching On an OFF) by reverse polarity pulse. Any good schematic solution to this driver?

 

• Jan 9, 2005

• #14

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e_eja

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8051 and port relays

what do you mean by this "reverse polarity pulse"

 

• Jan 11, 2005

• #15

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khach

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bistable relay schematic pulse

At the single coil latch relay coil is usually deenergized. Only for switching some short pulse is applied to coil. One polarity pulse is switch relay to On state and reverse polarity pulse is switch to Off state. Good appnote from Maxim for driving this relays is here
**broken link removed**