First circuit design needs to be checked please?

[EasyGoing1]

Re: First circuit design needs to be checked please?

By the way, the solenoid is rated at 12 volts and 1 amp (thats what the sticker says anyways)

[EasyGoing1]

Re: First circuit design needs to be checked please?

That means the solenoid is only getting 12V - 7.3V = 4.7V.
Since DCR of the solenoid is 7 Ohms, that means the current through the Transistor/solenoid is 0.671 A.
Transistor dissipation = 0.671A x 7.3V = 4.89W! It is wasting lots of power.
Is the solenoid actually working?

YES, its working fine, but the transistor does heat up pretty quickly if I leave the solenoid engaged for more than a few seconds.

[budm]

Re: First circuit design needs to be checked please?

7.3 Volts

That means the solenoid is only getting 12V - 7.3V = 4.7V.
Since DCR of the solenoid is 7 Ohms, that means the current through the Transistor/solenoid is 0.671 A.
Transistor dissipation = 0.671A x 7.3V = 4.89W! It is wasting lots of power.
Is the solenoid actually working?

[budm]

Re: First circuit design needs to be checked please?

I measured 7 ohms with my Fluke from the solenoid. Is that the right way to do it or should I find the datasheet on it? From what I remember, coil resistance isn't linear ONLY when AC is involved...???

DCR is 7 Ohms with 12Vdc applied = 1.71A of current.
That means the Transistor has to be bias on enough the let 1.7A of current flow.
Beta 20 @ Ic of 5A and Vce of 4V. Max beta is 150.
I see that Vbe is 1.3 or 3.5 based on those parameter on the left column.

[EasyGoing1]

Re: First circuit design needs to be checked please?

Just check the DCV between E and C to see what you have when the solenoid is operating.

7.3 Volts

[EasyGoing1]

Re: First circuit design needs to be checked please?

What is the resistance of the solenoid?

I measured 7 ohms with my Fluke from the solenoid. Is that the right way to do it or should I find the datasheet on it? From what I remember, coil resistance isn't linear ONLY when AC is involved...???

[EasyGoing1]

Re: First circuit design needs to be checked please?

the Value of R1 based on what the beta of the transistor is

See the clip I attached from the datasheet ... hfe is beta, right? But is it 20? Or is it 5? ...

R2 should have about 10% of the base current, so the current through R1 will be Ir2 + Ibe.

Ibe - being the emitter base saturation current when Vbe is 1.3 Volts??

[budm]

Re: First circuit design needs to be checked please?

Are you saying that if the transistor is not fully saturated, it heats up, where as if it were fully saturated, and the current passing from collector to emitter is within spec, it won't heat up?

If you look at the page 2 of the spec sheet if the transistor, you will see Vsat rating of the transistor:
@5A of Ic and Base current of 0.5A the Vsat (Voltage between Emitter and Collector) of the transistor is 1.3V. so if you have 5A flowing through the transistor, the power dissipation will be 1.3V x 5A = 6.5W!
So the more Voltage drops between E and C the more power dissipation onn the transistor will occur = more power dissipation. You want to have the lowest possible Vce when transistor is turned on. Just check the DCV between E and C to see what you have when the solenoid is operating.
What is the resistance of the solenoid?

[budm]

Re: First circuit design needs to be checked please?

No need for that added Rb.
R1 is the main one that supply the Base current for that transistor.
As REDWIRE explained, R1 resistance needs to go down to provide bias to put the transistor in saturation mode, the Value of R1 based on what the beta of the transistor is, R2 should have about 10% of the base current, so the current through R1 will be Ir2 + Ibe.

[EasyGoing1]

Re: First circuit design needs to be checked please?

but the transistor is probably partially "on" losing 1-2V and burn your finger if you keep the solenoid on.

Are you saying that if the transistor is not fully saturated, it heats up, where as if it were fully saturated, and the current passing from collector to emitter is within spec, it won't heat up?

[EasyGoing1]

Re: First circuit design needs to be checked please?

in this case you want the transistor to be in saturation condition (fully on).
http://www.dummies.com/programming/e...r-as-a-switch/
http://www.petervis.com/GCSE_Design_...h_Biasing.html
http://www.electronics-tutorials.ws/...or/tran_4.html
http://www.rason.org/Projects/transwit/transwit.htm

After reading the first two links, I came up with this:
(I edited this because of an error in the math png file)

[redwire]

Re: First circuit design needs to be checked please?

No I think this is what you're saying? (see attachment)...

Yes that looks better. R1 is still 10x too big to saturate the transistor. It works now but the transistor is probably partially "on" losing 1-2V and burn your finger if you keep the solenoid on.

That car battery sitting there doing nothing, self-discharge estimate 3.5%/month or 3.7mA due to impurities in the acid alone.

Please add a fuse close to the car battery for safety. Say 5A.

[budm]

Re: First circuit design needs to be checked please?

No I think this is what you're saying? (see attachment)

That is one way of doing it. But R1 has to be able to provide enough bias current to turn the transistor fully on. R2 can be higher value too.

[budm]

Re: First circuit design needs to be checked please?

Just put the switch on the high side feeding the R1 so when switch is not closed there will be current draw through the bias resistors. R1 needs to be able to give enough bias to put the transistor in saturation mode.

"are you saying that R2 should connect from the Emitter to the Collector?" That will give the path for the current to flow through the solenoid and through the resisto even when the transistor is not on, just follow the current flow and you will see why.
Did you read any of the links I gave?

[EasyGoing1]

Re: First circuit design needs to be checked please?

No I think this is what you're saying? (see attachment)

It's bloody brilliant ... I was trying to think of a way to incorporate the switch so that the circuit was not completed at all until the switch was actuated, and this configuration had not even occurred to me, but now that I look at it ... its simple and its perfect.

[EasyGoing1]

Re: First circuit design needs to be checked please?

Make sure you can access this thing if the battery goes dead.

I had thought about 'idle' current draw of the voltage divider, so I put my Fluke in there in current mode and its drawing around 3mA when not in use. Now, a car battery is roughly 75 AH??? I think when I did the math, it would take something like 10 years to drain the battery.

I think if I lowered the resistor values to give the transistor more of what it wants, that might increase my idle current draw... but this suggestion of moving the resistor to the other side ... are you saying that R2 should connect from the Emitter to the Collector?

[redwire]

Re: First circuit design needs to be checked please?

It will not work so great, a few suggestions.

R2 should be across the E-B junction of the transistor (move it from other side of reed switch). This to ensure R1+R2 don't drain the battery dead (they are always wasting current as drawn), and to ensure the transistor leakage currents keep it off. I would set R2 to 10k-100k ohm, not critical.
You are looking at R1+R2 as a voltage divider, dropping 12V down but a transistor is a current-controlled device. Input current (base) controls output (collector) current.

This means R1 sets the base-current. 2N6487 is a low gain part, as low as hFE=20. For 1A solenoid, theory says R1 = (Vcc-VBE)/IB or (12V-0.6V)/50mA =228ohms. In practice you'd use a 150-220ohm resistor to give the transistor extra base drive. Since R2 is so much bigger than R1, it is ignored in the calculation for convenience.

Make sure you can access this thing if the battery goes dead.

[stj]

Re: First circuit design needs to be checked please?

a fet may be better, as it's only on or off
or an scr - if they even make those anymore!

[budm]

Re: First circuit design needs to be checked please?

That is not how you do it, The R2 = 0.6V because it is connected in parallel with B-E junction of the transistor, so VR1 = 11.64V, and then you can calculate what the Bias current will need tobe for the transistor to be turn on to allow enough current to flow through the relay, in this case you want the transistor to be in saturation condition (fully on).
http://www.dummies.com/programming/e...r-as-a-switch/
http://www.petervis.com/GCSE_Design_...h_Biasing.html
http://www.electronics-tutorials.ws/...or/tran_4.html
http://www.rason.org/Projects/transwit/transwit.htm

[EasyGoing1]

Re: First circuit design needs to be checked please?

Updated the schematic