First circuit design needs to be checked please?

[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

[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.

[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!!

[stj]

Re: First circuit design needs to be checked please?

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

[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?

[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?

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?

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.

[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?

[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...

[EasyGoing1]

Re: First circuit design needs to be checked please?

Updated the schematic

[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

[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!

[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.

[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?

[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.

[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?

[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.

[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.

[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)