Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

555 -> transistor driving problems

Status
Not open for further replies.

cool-agent

New Member
Hi,

I'm building a solid-state flasher relay for my bike as the standard one is load dependant - ie if a bulb is out it flashes at twice the rate. I don't want this as I'm using LED's. I've used a 555 in astable mode.

I've got the cct built up on veroboard at the mo, and am playing with a few transistors. Basically the emitter will be the 'flash' wire so this will be wired to a flylead to connect to the wiring loom. +ve 12V is supplied to the cct upon left or right indication.

My problem is this. If I connect pin 3 to the base of the transistor via a 1K resistor (like I was taught) then it will flash a bunch of leds, but not a 21W bulb.

If I directly connect pin 3 to to base (shorting the resistor) then the bulb WILL flash, but the transistor gets scarily hot very quick.

If I connect pin 3 to the coil of a relay (with protective diode) it will pick and drop it fine, so as a last resort I can use a relay. I would really like to use a transistor though.

I've tried a BFY51 and a high power NPN transistor (who's part no I have forgotten at the mo!) which I robbed out of a PC PSU. The datasheet for this states Ic to be 6A or more, so that should suffice.

The maximum load will be around 15W in thoery. Practice should be much lower. I'd like it to be able to handle similar to the original 47W (21W+21W+5W).

Anything I'm missing?

Cheers.
 
cool-agent said:
Hi,

I'm building a solid-state flasher relay for my bike as the standard one is load dependant - ie if a bulb is out it flashes at twice the rate. I don't want this as I'm using LED's. I've used a 555 in astable mode.

I've got the cct built up on veroboard at the mo, and am playing with a few transistors. Basically the emitter will be the 'flash' wire so this will be wired to a flylead to connect to the wiring loom. +ve 12V is supplied to the cct upon left or right indication.

My problem is this. If I connect pin 3 to the base of the transistor via a 1K resistor (like I was taught) then it will flash a bunch of leds, but not a 21W bulb.

If I directly connect pin 3 to to base (shorting the resistor) then the bulb WILL flash, but the transistor gets scarily hot very quick.

If I connect pin 3 to the coil of a relay (with protective diode) it will pick and drop it fine, so as a last resort I can use a relay. I would really like to use a transistor though.

I've tried a BFY51 and a high power NPN transistor (who's part no I have forgotten at the mo!) which I robbed out of a PC PSU. The datasheet for this states Ic to be 6A or more, so that should suffice.

The maximum load will be around 15W in thoery. Practice should be much lower. I'd like it to be able to handle similar to the original 47W (21W+21W+5W).

Anything I'm missing?

Cheers.

Well how hot does it get? I'm sure it feels hot but why don't you calculate the temperature rise on the Junction and see if that exceeds the Max Tj allowed. If it does, put your transistor on a heat sink and re-do the Trise calculations.

What is the transisistor package?

If you can't reconcile with the power needed to be dissipated, consider paralleling multiple transistors or throw out the idea and use a solid state relay if possible.
 
optikon - Too hot to touch after 10 secs or so. Bear in mind it's 50% load as well due to it flashing.

The 2 power transistors I have here are a 2SC2625 (10A) and a C5027 (6A). From the datasheets I found they both have relatively low hFE (50-200) but that shouldn't matter as I'm only using it as a switch.

Got a K972 MOSFET I'm going to try in place.

I'll be back!
 
If you're not supplying enough base current, the transistor will not saturate, and there will be a high amount of power dissipated in the transistor. You need to determine your load, design for worst case (lowest hfe and then some), and determine the required base current. You need to make sure the transistor is rated for the current (and that the 555 is capable of supplying it). You'll probably be looking for a darlington. A TIP122 should be plenty good.

BTW, I've always thought that the faster blinking rate when a bulb is "feature," to let you know that you have a bad bulb.

j.
 
USE MOSFET TRANSISTORS W? A SMALL HEAT SINK!!!they can be easylly driven with 555's and they handle high currents 8)
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top