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Need help selecting a transistor for a large power load

meanEG

Jun 22, 2016
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I am having trouble figuring out what I need to complete the current project I am doing. I have a decent background in automotive dc systems and a little training in electronincs. I was originally going to use microswitch controlled relays to power 4 200w heater elements. I think I can use transistors instead.

Here is the current projects all circuits are 12-15 vdc
4 200w heater elements
4 microswitchs
1 switch controls 1 element, 1for 2, 1 for 3 and the last one is for all 4
I can draw the circuit so its easier to understand but I don't know the best way to load ir do it for you to see.

ill draw the system schematic and upload from photobucket for now
 

BobK

Jan 5, 2010
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What is wrong with the way you are doing it now (with relays?) Transistors would disapate a lot of power and need heat sinks.

Bob
 

meanEG

Jun 22, 2016
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I am still trying to upload pics, but the relays would require more wiring and cost I believe. then again just realized my new diagram would allow to use cheaper, normal relays
 

Bluejets

Oct 5, 2014
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As Bob says, heat ......... I would have thought the cost of heat sinks large enough for this would well exceed the cost of a couple of decent relays. Even mosfets with the correct parameters would be pricy in comparison I would imagine.
 

meanEG

Jun 22, 2016
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yea, after I redid the diagram as above, I now see I can use regular relays. if the relays have a 90 ohm coil what size diode should I use?
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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The 1N4002 are power rectifiers and are also commonly used for relay coil kickback suppression.

Chris
 

Sunnysky

Jul 15, 2016
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  • kickback diodes should be across each coil in reverse polarity to applied voltage.
  • coils may be wired to either common + or - then switches wired with common side to opposite polarity.
  • So either switch V+ or gnd switch.
  • Diodes only need to conduct coil current, so 14V/90Ω is low current for voltage surge suppresion when turned off.
  • Use twisted wires to load for less EMI pulse noise ( V+&V-)
  • Normally power relays are rated for DC current at 50% of AC current for same MTBF, if this is switched often >50k due to contact wear.
  • FWIW
  • applying wrong voltage polarity to diode protected coils will burn out diode
  • Tony
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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Is the intent to progressively turn on the four loads? Actuate SW1 and LOAD1 turns on; actuate SW2 and LOAD1 and LOAD2 turns on; actuate SW3 and LOAD1, LOAD2, and LOAD3 turns on; actuate SW4 and LOAD1, LOAD2, LOAD3, and LOAD4 all turn on? Is that what the three "steering" diodes are for?

If so, each diode will introduce a voltage drop of about 0.7 V as current flows through it to energize a relay coil. So, three diodes conducting means that the last relay coil gets 2.1 less volts for its coil than the first relay coil. Should not be a problem in an automotive system using automotive relays, but something to consider if you ever plan to add more heaters and more micro-switches.

An "easy" way around this, if it becomes a problem, is to use the diodes and micro-switches to steer signals to four small transistors that, when turned on, each energize a relay coil. An inexpensive NPN small-signal transistor (2N3904) works fine. You could also use an inexpensive MOSFET switch (2N7000). Since the currents needed to forward-bias the base-emitter junction of a transistor are small, inexpensive small-signal diodes (1N914 or 1N4148) will work fine too. You can build this on a strip-board with screw terminal blocks to accept the microswitch wires and provide connections to the coils of external automotive-type relays.

You will need either a base-current limiting resistor (about 10 kΩ) in series with the base of each transistor, or a gate-to-source terminating resistor of about the same value at each MOSFET. Relay coils connect between +12 V DC rail and the NPN transistor collector (or MOSFET drain). The transistor emitter (or MOSFET source) is connected to ground (- 12 V DC rail). If you decide to go this route, a 1N400x diode needs to be placed across each relay coil to protect the transistor when it turns off. Anode of diode toward transistor collector (or MOSFET drain), cathode of diode to +12 V DC rail. Any diode in the 1N4000 series will work.

It sounds like you have (re-)discovered diode/relay logic. I did a lot of that back in the 1960s before moving on to TTL (transistor-transistor logic) circuits and, later, microprocessors. Still has a place for simple applications like this one.
 
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