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Hornby R7229 Voltage Regulation


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Afternoon All,

 

I'm looking into trying to limit the no load voltage from the uncontrolled output of the Hornby R7229, essentially to power a fixed number of LEDs and thus a fixed load.

 

My question is, can I use a zener diode to achieve this without affecting operation of the CDU for my points? As the supply voltage varies a potential divider isn't suitable here. I assume both are equally inefficient?

 

Follow up question, if a zener is suitable for this application is it correct to connect the point motors across the supply (so they're not in series with the zeners current limiting resistor) and the LEDs across the zener.

 

FYI the stated voltage is 14V and the measured no load is between 18 and 19V.

 

Thanks in advance

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How much current in total for the LEDs? Do you turn any of the on and off? It can be tricky to get a zener circuit to work well when the load and/or the supply varies a lot. A voltage regulator would be better.

 

The LEDs may flicker when you fire a solenoid and the CDU discharges. Is that acceptable?

 

Yes, do not connect the solenoids through the current limit resistor.

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Thank you, that's helpful. They're going to be left on (engine shed lights). Circa 10 white SMDs.

 

I guess my main concern is letting the blue smoke escape out of the controller. The end plan is to have a proper independent power supply for the LEDs  but in the interim I like problem solving so dimming isn't a problem.

 

If the smoke is going to stay inside I'll crack out my breadboard and have a play. I'm pretty sure I have a big bag of regulators somewhere too. I'll compare and contrast. Thanks again!

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2 hours ago, Toton said:

Thank you, that's helpful. They're going to be left on (engine shed lights). Circa 10 white SMDs.

 

I guess my main concern is letting the blue smoke escape out of the controller. The end plan is to have a proper independent power supply for the LEDs  but in the interim I like problem solving so dimming isn't a problem.

 

If the smoke is going to stay inside I'll crack out my breadboard and have a play. I'm pretty sure I have a big bag of regulators somewhere too. I'll compare and contrast. Thanks again!

The main concern is that the LEDs will let the smoke out, rather than the controller! While there is a limit for the controller, the LEDs are much more sensitive to over voltage.

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Thinking aloud: I would put two LEDs in series with a current limit R for 12V and use a 12V zener. Lets say the LEDs want 10mA for desired brightness so 50mA in total for 5 pairs. Say 20mA through the zener so 70mA in total.

 

With 14V input you need another R of (14-12)/.07 or 28 ohms.

 

At 19V input the current through this R will increase to 7/28 or 250mA.

 

All of the excess current will be dumped through the zener and the power dissipation will be 1.75W worst case, so you will need a fairly beefy zener.

 

At room temp. with the low currents involved, a 7812 regulator should work with a 14V input.

 

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Cheers guys, I've ordered some 7812s. My box of bits has a load of 7809s so I'll have a play with those for now. Though I'll have to redo the resistor sums. Thanks for your help Crosland :good:

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