First circuit design needs to be checked please?

[EasyGoing1]

Re: First circuit design needs to be checked please?

[B][I]Vr2 will not be higher than Vbe junction Voltage of the Transistor unless that B-E junction is blown.

No, what I'm saying is, that the voltage divider formula for R2, which is:

VR2 = Vcc * (R2/R1+R2)

And in this case, the result is:

VR2 = 12*(1.8/(2.6+1.8))

VR2 = 4.9V

Thats the voltage I'm using to turn the transistor on ... and it seems to work...

[budm]

Re: First circuit design needs to be checked please?

"The resistors I calculated using a voltage divider calculator ... across R2 I get somewhere around 4.7 Volts." Vr2 will not be higher than Vbe junction Voltage of the Transistor unless that B-E junction is blown.

[EasyGoing1]

Re: First circuit design needs to be checked please?

I do have a couple of monster diodes that I salvaged from something some time ago ... they are PFR854 and GI826. The 854 is rated a 600 volts in reverse and the other one is 400 volts ... here are the datasheets.

[EasyGoing1]

Re: First circuit design needs to be checked please?

and use a 1n4007 - you need a high voltage one.

I dont think I have any ... and you're right about the diode ... I thought about current flow and as it is in the schematic, it would dissipate back through the Collector...

[EasyGoing1]

Re: First circuit design needs to be checked please?

Perhaps the magnet you are using is too powerful for the Reed switch.

No, I used a magnetized small screwdriver head ... one that barely picks up screws... it was definitely the current flow through the switch. I had an ammeter on it and it reached 1.5 amps ... I dont know what these reed switches are rated at, but they are very delicate looking things encased in glass ... they don't look to me like they could handle 1.5 amps at 12 volts...

Mike

[EasyGoing1]

Re: First circuit design needs to be checked please?

not magnetised, you welded the contacts!

your circuit looks generally o.k. - dont know about those resistor values though.
HOWEVER - your protection diode is the wrong way around - it will blow or take out your transistor!!

I probably did weld the contacts...

The resistors I calculated using a voltage divider calculator ... across R2 I get somewhere around 4.7 Volts.

I know that when I tested the circuit, when the diode was inserted the wrong way, the solenoid would not energize at all ... I'll make sure that its in the right way when I solder it up.

If I'm not mistaken, the line on the diode represents the cathode, right? and current flows in the direction of cathode to anode ... right?

And if thats the case, then wont this configuration prevent current from flowing backwards into the Collector of the transistor, so that any charge in the solenoid will have to dissipate through itself?

[stj]

Re: First circuit design needs to be checked please?

and use a 1n4007 - you need a high voltage one.

[stj]

Re: First circuit design needs to be checked please?

not magnetised, you welded the contacts!

your circuit looks generally o.k. - dont know about those resistor values though.
HOWEVER - your protection diode is the wrong way around - it will blow or take out your transistor!!

[Andrew F. Ali]

Re: First circuit design needs to be checked please?

From you diagram the diode across the solenoid is wrong. It should be the other way around. Perhaps the magnet you are using is too powerful for the Reed switch.

[EasyGoing1]

Hello,

I am building a hidden compartment for valuables and I decided to use a magnetic reed switch so that I can hide the switch, and I decided to go with a 12 volt solenoid as the locking mechanism.

When I connected the solenoid to the 12 volt source through the reed switch and then hit it with a magnet, the reed switch immediately became magnetized and will not open up anymore. Fortunately, I have 10 of these things and I figured it was the heavy current traversing the switch that was magnetizing the iron elements inside.

SO, I decided that I need to switch this solenoid with a low current and what better way to do that then with a power transistor?



Here is the design I came up with. I simply need someone who knows more than I do (electronics is a hobby for me, not a career) to please look at the design and let me know if I missed anything? I need this thing to last because I'm installing it in an inconspicuous place that will be hard to get to later on should it need to be repaired or whatever.

I included the datasheets to save you time.

Thank you,

Mike