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My DIY Telescope Power Supply


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With the weather being so bad lately I needed an indoor project so decided to make a power supply for my telescope.

It had to be cheap so most of the parts are from my shed junk box including the case which is from an old HP tape drive backup system.

It is a dual purpose supply that I can use either for my telescope and laptop or my SBS-3 Virtual radar and laptop.

It has two outputs. one can be switched to either 12V or 5V and the other is continually variable from 0 to 22V.

The regulators are LM338K's and can supply up to 5A although will not be running anywhere near that.

I have included a crowbar circuit that monitors the output and blows the fuse if the voltage goes above 13V or 6V depending on the voltage selected. This ensures that the telescope mount electronics cannot be damaged due to an over voltage in the event of a supply failure.


Here's a couple of pics:-





Edited by TasKiNG
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Guest ecopley

I like that. Particularly the idea that it all came out of the junk in your shed. My garage contains a slightly lethal air spark flash unit that I made. 30kv, 8ka, all discharged in 300ns.

Nowhere near as useful as your project. Or as well presented. Or as safe...


As Kim says, perhaps you could provide some diagrams and details. Something like that would be really handy for all sorts of projects!

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An EMS Group Project Power Tank?

Members buy kits of bits sourced at a good price by the group......

The guys at our Ham Radio Club used to do a different project each year.

Guess its started Kim


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Think I've remembered everything I did so here's the circuit diagram:-




Circuit Description:-


There are two 12V 7AH Sealed Lead Acid batteries connected in series via 3A fuses. Charging terminals are connected to the Ov, 12V and 24V points to allow me to charge each battery individually with an external Lead Acid battery charger.

A push to make/push to break switch (SW1) on the front energises a 24V relay which switches the rest of the circuit to the 24V power.

LED1 is a red light emitting diode on the front panel to indicate that it is powered up. The resistor R5 limits the current to the LED and ensures that a suitable voltage is across it. The resistor must be greater than 1Watt.


Voltage Regulator 1 is a LM338K adjustable regulator. Pre1 & Pre2 are variable preset resistors that have been adjusted to give 12V and 5V depending on the position of switch SW2 which is a key switch to prevent accidental switching. I found that setting Pre1 to 230 ohms and Pre2 to 430 ohms gave approx 5V & 12V.


Fuse F1 restricts the current that can be drawn from the output. The two zenner diodes and two thyristors form a couple of crowbar circuits that prevent excessive voltage at the output in the event of a regulator failure by shorting the output if the voltage goes too high.

For a voltage display I have used self powered volt meter modules that I found on eBay.


The voltage regulators are mounted on a heat sink and I have fitted a low speed fan to keep air flowing across it.

Adjust the values of the fuses depending on the amount of current your equipment draws.


Ensure that Pre 1 & Pre 2 are adjusted to give the correct voltages before connecting any equipment.


The circuit for the variable output is similar but much simpler and uses a 10 turn potentiometer ( variable resistor ) VR1 mounted on the front panel to set the required output voltage.


You could simplify the circuit by using a high current on/off switch instead of the low current switch / relay combination and also omit the variable 0-22V volts circuit and SW2,Pre1,C2,R3,ZD1 & Thyristor 1 if you just want a stable 12V supply. 


I have no plans to make and sell these supplies due to not having enough junk in the shed or enough hours in the day :)


Note that these batteries can supply a lot of current and could be a fire hazard if shorted or connected incorrectly so I advise that you only attempt to make your own supply if you know what you are doing. I accept no responsibility for any problems or damage resulting from supplies made based on my circuit.

Edited by TasKiNG
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Guest ecopley

Thanks for posting the details. This is a really excellent project. Complex enough to be satisfying to complete but simple enough (note that you have done the hard work of designing it, of course) to be actually doable. Also, most importantly, really handy. Once again, thanks for posting the details.

I know enough to know that the warning is quite right. If any one is thinning of having a go, do be careful. These batteries will cause serious injury and demand proper respect. Be careful.


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First proper test tonight of the portable supply and it worked fine.

This was after running my laptop for 4 hours and it still had plenty of juice left.



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