# 2N3904 + Relay

Discussion in 'General Electronics Chat' started by SteveYTI, Feb 18, 2004.

1. ### SteveYTI Thread Starter Member

Feb 9, 2004
23
0
I am using a 24 VDC Relay coil at about 70 Ohms. I am using a 2N3904 transistor to bring neutral to the other side of the coil to make it fire. I am thinking that I have enabled a breakdown of the 2N3904 although I am not sure. I am very green with transistors. I am thinking that it may be necessary to increase the resistance of the relay circuitry, ie. add a resistor in series to the coil of the relay perhaps in parallell not sure but mainly the idea is to decrease the current that moves through my transistor (Ice). If anyone out there is better at this math then me please let me know what I should do. Thanks

2. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
Whats the transistor pulling in terms of base current?

3. ### SteveYTI Thread Starter Member

Feb 9, 2004
23
0
I figured out what the problem was with the relay, it was bad the ones I'll be using are about 400 Ohm coil so there will be no problem there, the problem I have now is that out of my logic gate i need to drop about 3.1 volts to .850 or thereabouts so that i can make the transistor work without burning it up. I then toss on an RC circuit that is very archaic in design. So right now i drop on a 3:1 ratio about 300 Ohm to 100 Ohm, Ive played with it a bit to get more volts into my RC network. The RC i choose (quickly based solely on delay time) was 20K and a 200uF. On .7V input this takes about 8 seconds to charge so it was just right. However with all this combined together I find I am only able to put out about a half of a volt (.50 - .56) to the base of my transistor. I am absolutely aweful at circuit analysis, was never good at it and beyond that its very rusty as I haven't used it in about 2 years. In anycase I am toiling with math at the moment to make this all come together !! help =)