First circuit design needs to be checked please?

[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?

[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.

[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?

[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??

[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?

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

7.3 Volts

[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.

[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?

[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.

[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)

[budm]

Re: First circuit design needs to be checked please?

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

That is happening because the transistor is not fully turned on as explained. R1 needs to be lower value to provide bias to put that transistor in saturation mode, or you can use Darlington Transistor which has a lot more hfe.

[budm]

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)

You can easily verify that by using DCA to measure the current draw of the solenoid when 12VDC is applied to the solenoid through the DC AMP meter.

[eccerr0r]

Re: First circuit design needs to be checked please?

You're running as a saturated switch, more base current is needed to saturate! Reduce R1 to around 150R to 220R and no need for R2 (remove it) or Rb (short it out). Don't worry about the base voltage limit, the base voltage won't go past 5V just like an LED won't go past its characteristic voltage as long as the current is limited. You should connect the resistor directly to the base though, just for safety.

Ideally you want to completely saturate the switch and thus the voltage across Vce will be as low as possible, decreasing the dissipation. For your transistor it's a max of 1.3V at 5A - but I suspect it will be typically lower, and even lower for 1-2 amps - so at an amp of collector current it'll still dissipate up to 1W or so, which can get warm but it's fine for an unheatsinked TO220.

Increasing Ib is can be a problem too beyond the transistor's base current limit: what's the maximum current of the reed switch? I suspect it will take 100mA just fine so perhaps this isn't a problem after all.

[EasyGoing1]

Re: First circuit design needs to be checked please?

You're running as a saturated switch, more base current is needed to saturate! Reduce R1 to around 150R to 220R and no need for R2 (remove it) or Rb (short it out). Don't worry about the base voltage limit, the base voltage won't go past 5V just like an LED won't go past its characteristic voltage as long as the current is limited. You should connect the resistor directly to the base though, just for safety.

Ideally you want to completely saturate the switch and thus the voltage across Vce will be as low as possible, decreasing the dissipation. For your transistor it's a max of 1.3V at 5A - but I suspect it will be typically lower, and even lower for 1-2 amps - so at an amp of collector current it'll still dissipate up to 1W or so, which can get warm but it's fine for an unheatsinked TO220.

Increasing Ib is can be a problem too beyond the transistor's base current limit: what's the maximum current of the reed switch? I suspect it will take 100mA just fine so perhaps this isn't a problem after all.

So is this what you're saying I should be using as the design?

[budm]

Re: First circuit design needs to be checked please?

That will give about 50mA of Bias current, try it and see what Voltage you will get between E and C of the transistor to see if it is in saturation mode or not. I would put a 2.2K resistor between GND and Base so the Base circuit will not be floating when the switch is not closed

[EasyGoing1]

Re: First circuit design needs to be checked please?

That will give about 50mA of Bias current, try it and see what Voltage you will get between E and C of the transistor to see if it is in saturation mode or not. I would put a 2.2K resistor between GND and Base so the Base circuit will not be floating when the switch is not closed

So then you recommend doing it like this ... won't that 2.2k reduce our Vbe current?

[stj]

Re: First circuit design needs to be checked please?

that wont work, the 2k2 will kill the current to the solenoid

[budm]

Re: First circuit design needs to be checked please?

So then you recommend doing it like this ... won't that 2.2k reduce our Vbe current?

No, connect one end of the 2.2K to negative, another end to the Base of the Transistor.
What you show is Emitter and GND (negative).

[eccerr0r]

Re: First circuit design needs to be checked please?

Bipolar transistors don't care about floating base, all the charge on the base will bleed off very quickly. People must be thinking about FETs which should have them, but the input impedance of BJTs are fairly low in common emitter configurations.

[EasyGoing1]

Re: First circuit design needs to be checked please?

No, connect one end of the 2.2K to negative, another end to the Base of the Transistor.
What you show is Emitter and GND (negative).

OH, you mean like this ...