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Looking for confirmation and real component values

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markojal

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Hello,

I came across this forum and hoped someone could help me with a small project This is what the project does.

For model railroading I have 2 sets if tracks . I have a Train on one and a Trolley on another. When the train passes near the trolley I want it to be active. When the trolley is out of sight of the train I want it to be inactive. Both train and trolley are simple DC motor devices.

Additionally, when the trolley reaches the end of the track it needs to reverse polarity of the power supply, causing the trolley to move in the opposite direction.

I found this circuit online and saw the momentary switches as reed switches . The reed switches can easily be triggered by a passing train. It is the firs one on this page.
https://www.electronics-lab.com/project/simple-soft-latch-switch-using-push-button/

This made perfect sense for on/off and a second identical circuit triggering a DPDT elay would do fine for triggering a forward/reverse change.

I put together what is in the attached file, however I lack specific component values.

Can anyone help me with specific component values and confirm my work? I am also worried I may have polarity reversed to transistors or diode. . automaticTrolley9.pngautomaticTrolley9.png

Thanks

Mark
 

T1 carries full current to the motors. It needs to be rated accordingly. (Probably a TO-3 PNP general purpose.)

R1 R2 T2 are in the control circuit. They only need to carry small current, enough to turn T1 fully on and fully off.

The relay coil carries several mA, maybe a few tens of mA. Therefore R3 R4 T3 T4 carry small current. Just enough to turn T3 on enough to activate the relay.

From what I've seen reed switches are not rated in any particular manner. I suppose the general idea is to treat them gently in order to lengthen their useful life.

Your power supply is labelled 12V. This goes to the DPDT relay. Therefore its coil should be 12V. Its contacts need to be rated for maximum expected Ampere levels. 1A is reasonable since the diode type is rated 1A.
It's preferable if you can confirm it. Ordinary digital multimeters which I've owned can measure up to 10A.
 

BradtheRad, Thanks for your input. The diagram posted above does have some errors and posting an updated version. The Relay is TDS-1202L meaning 12V. The total trolley power consumption is over rated at 900mA. More than likely a trolley, because it pulls no cars will be down below 500mA and that would be an older model where newer models require less still.

When I look for PNP general purpose transistos there are way too many things I do not understand. I do understand that the TO-3 type is to be mounted on a heat sink. Is that really Necessary? It also seems awfull big. I do see lots of availability of TIP32C TO-220 transistors but no idea if that is suited for T3 and T4 and believe I could then use the same for T1 and T2

Still no idea of resistor values though.automaticTrolley10.png
 

R1 R3 R4 need to carry a few mA so their value can be 2k to 10k ohms.

R2 should be low enough ohms to turn on T1 fully.
Say T1 gain is 50x and carries 2A max.
Then it needs bias current 2/50 equals 40mA.
At 12V R2 value can be 12/.04 equals 300 ohms.
The above is my prospective estimate.

T1 can be expected to heat up. If it drops 1V at 1A that's 1 Watt. Enough heat that I can't hold it between my fingers. You can test it by letting airflow cool it. There's a point where heat is bad enough to destroy a device. Then you need to add a heatsink.
 

Hi,

to reduce power dissipation (heat) and to ease control you may replace BJT T1 with a P-Ch MOSFET.

BJTs usually are more rugged, thus I recommend to use an overvoltage protection when you use the MOSFET. A MOSFET may be small and does not necessarily need a heatsink.


Klaus
 

What if I only hang a relay on T1 (T1 will be loaded by 2 relays each @~ 0.01666 A) and use the relay contacts to deliver the 12v 900mA ? Seems to me the easy solution for the overheating of T1.
 

What if I only hang a relay on T1 (T1 will be loaded by 2 relays each @~ 0.01666 A) and use the relay contacts to deliver the 12v 900mA ? Seems to me the easy solution for the overheating of T1.

Your suggestion is probably feasible for your train layout as you describe it.

Since the designer made his circuit more complicated however, a reasonable question arises: 'Is there a reason for it?'
Perhaps it's as his notation describes, namely so he could install several reed switches around his track, or else several of these control circuits. They may include some degree of voltage regulation, to reduce lamps changing brightness or motors changing speed, whenever something turns on or off.
 

people use ther accessories al the time. in thhis case accessories use AC . This trolley would have a dedicated DC supply. Still lost on this though . a second relay is easy.
 

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