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Re: General Stuff(tm)
Posted: 09 Apr 2024 20:25
by stephen_usher
The Direct from China eBay variable step-down regulator with current limiter arrived today.
Of course, no instructions so getting the voltage to change etc. was very much trial and error. Anyway, it does work:
IMG_3672.jpeg
I have it on test at the moment. The regulation isn't massively great as the output voltage goes down slightly when the input voltage rises. So, under this load if I dial in 7.50 volts at 9V input then as 12V input it's down to 7.40 volts output. It's good enough though to protect the Apple Macintosh Portable. I just need to make a box to fit it and the ammeter inside with an input jack for a 12V wall wart and a wire with a barrel jack on the output.
Re: General Stuff(tm)
Posted: 09 Apr 2024 20:46
by rubber_jonnie
stephen_usher wrote: 09 Apr 2024 20:25
The Direct from China eBay variable step-down regulator with current limiter arrived today.
Of course, no instructions so getting the voltage to change etc. was very much trial and error. Anyway, it does work:
IMG_3672.jpeg
I have it on test at the moment. The regulation isn't massively great as the output voltage goes down slightly when the input voltage rises. So, under this load if I dial in 7.50 volts at 9V input then as 12V input it's down to 7.40 volts output. It's good enough though to protect the Apple Macintosh Portable. I just need to make a box to fit it and the ammeter inside with an input jack for a 12V wall wart and a wire with a barrel jack on the output.
So what does the 1.37 represent? Amps?
Re: General Stuff(tm)
Posted: 09 Apr 2024 21:13
by stephen_usher
rubber_jonnie wrote: 09 Apr 2024 20:46
So what does the 1.37 represent? Amps?
Yes, that's what the bulb is drawing.
I don't like the regulation, it's a bit pants. As the load decreases the voltage goes up, levelling out at just under 9V. I think I'm going to have to put a large Zener diode clamp into the circuit to keep it stable, or rather 3x 5W zeners so as to be able to dissipate 15W, which should be enough for the current limiting to cut in.
Re: General Stuff(tm)
Posted: 09 Apr 2024 21:32
by rubber_jonnie
stephen_usher wrote: 09 Apr 2024 21:13
rubber_jonnie wrote: 09 Apr 2024 20:46
So what does the 1.37 represent? Amps?
Yes, that's what the bulb is drawing.
I don't like the regulation, it's a bit pants. As the load decreases the voltage goes up, levelling out at just under 9V. I think I'm going to have to put a large Zener diode clamp into the circuit to keep it stable, or rather 3x 5W zeners so as to be able to dissipate 15W, which should be enough for the current limiting to cut in.
Do think the zeners will be good enough stabilise it?
Re: General Stuff(tm)
Posted: 09 Apr 2024 22:01
by stephen_usher
rubber_jonnie wrote: 09 Apr 2024 21:32
Do think the zeners will be good enough stabilise it?
Yeah, three 5W Zeners is 15 watts, that's over 2 amps at 7.5V. The current limiter would kick in well before that and bring the voltage down. Therefore three Zeners is overkill a bit but it's best to engineer for the worst case.
Under test with the bench power supply the Mac Portable only very briefly hits the 1.5 amp limit when first plugged in, so the moderate voltage drop should be enough to stop any damage.
Re: General Stuff(tm)
Posted: 09 Apr 2024 22:44
by rubber_jonnie
stephen_usher wrote: 09 Apr 2024 22:01
Yeah, three 5W Zeners is 15 watts, that's over 2 amps at 7.5V. The current limiter would kick in well before that and bring the voltage down. Therefore three Zeners is overkill a bit but it's best to engineer for the worst case.
Belt and braces is always a good plan.
stephen_usher wrote: 09 Apr 2024 22:01
Under test with the bench power supply the Mac Portable only very briefly hits the 1.5 amp limit when first plugged in, so the moderate voltage drop should be enough to stop any damage.
Good stuff.
Re: General Stuff(tm)
Posted: 11 Apr 2024 10:53
by stephen_usher
The zeners and power jack socket should be arriving today. I have some vero board I can use to make up the little PCB for the (7.5V) power distribution, three zeners, diode to stop the Mac Portable trying to back-power the unit if the power's switched off, oh and connecting the ammeter.
Once I've wired all this up then I can design a box for it all to fit inside and print it.
Re: General Stuff(tm)
Posted: 11 Apr 2024 11:10
by rubber_jonnie
stephen_usher wrote: 11 Apr 2024 10:53
The zeners and power jack socket should be arriving today. I have some vero board I can use to make up the little PCB for the (7.5V) power distribution, three zeners, diode to stop the Mac Portable trying to back-power the unit if the power's switched off, oh and connecting the ammeter.
Once I've wired all this up then I can design a box for it all to fit inside and print it.
Be good to see it in action :)
I love having proto/vero board handy, of late it's mostly been for composite mod circuits, it's so easy to know one up quickly.
Re: General Stuff(tm)
Posted: 11 Apr 2024 13:48
by stephen_usher
The parts have arrived.
Re: General Stuff(tm)
Posted: 11 Apr 2024 13:57
by rubber_jonnie