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Old 13th May 2019, 9:14 am   #1
Dai Corner
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Default Deriving 3v at 30mA from 5V Supply

I've bought a mini FM transmitter so I can listen to TV sound, streamed audio etc on my VHF/FM sets around the house. It works well but I'd like to run it off the USB outlet on my TV to avoid buying batteries. It uses two AAA batteries (3V) and the spec says it takes < 30mA.


Is there any reason not to just connect the 5V supply via three diodes in series which would drop 1.8V giving 3.2V for the device?
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Old 13th May 2019, 10:24 am   #2
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Default Re: Deriving 3v at 30mA from 5V Supply

Should be fine. You could use a linear regulator or a zener and resistor if you have the parts to hand.

Actually, there's a good chance that it will be OK if you just bung 5V into it, but it's better not to take the risk.
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Old 13th May 2019, 1:36 pm   #3
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Default Re: Deriving 3v at 30mA from 5V Supply

Thanks Paul.

I have the diodes to hand but not the zener variety so I'll use them.
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Old 13th May 2019, 1:41 pm   #4
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Default Re: Deriving 3v at 30mA from 5V Supply

Out of interest Dai, which transmitter have you got? I have one (bought in kit form), and it is plagued by 50c/s hum when running from any mains-sourced power supply. Just wondering if there are types out there that don't have this problem...
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Old 13th May 2019, 2:18 pm   #5
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Default Re: Deriving 3v at 30mA from 5V Supply

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Originally Posted by Boulevardier View Post
Out of interest Dai, which transmitter have you got? I have one (bought in kit form), and it is plagued by 50c/s hum when running from any mains-sourced power supply. Just wondering if there are types out there that don't have this problem...
It's a Cheetah RDFM-T . Available for under 4 from the usual places. I paid 3.29 from Amazon.

I'll post again when I've completed the mods and let you if there's any hum or other problems.
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Old 13th May 2019, 2:42 pm   #6
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Default Re: Deriving 3v at 30mA from 5V Supply

I would just feed it with a 68 ohm resistor and add a capacitor if there is not already one on the board.
If it takes less than 30ma it will just find its own voltage and settle.
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Old 13th May 2019, 2:45 pm   #7
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Default Re: Deriving 3v at 30mA from 5V Supply

I power my Linex from a wall wart via a 7805, and it does introduce a faint hum. It isn't audible on portable radios at sensible listening levels but wouldn't be acceptable feeding a hifi system. I haven't bothered investigating as it isn't a problem for me.
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Old 13th May 2019, 4:06 pm   #8
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Default Re: Deriving 3v at 30mA from 5V Supply

The hum I get is much more intrusive than that, Paul. I have read of other people reducing the problem by using two daisy-chained stages of regulation - say 12V to 8V, followed by 8V to 5V (instead of just 12V to 5V). I might try that, though I'm not sure if it would make any difference.

Mike
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Old 14th May 2019, 10:45 pm   #9
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Default Re: Deriving 3v at 30mA from 5V Supply

Out of interest, and before I spend time on it, can anyone tell me whether double regulation as I've outlined (see Post 8) is likely to provide better regulation and reduced ripple for an fm pantry transmitter? Any views much appreciated...

Mike
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Old 14th May 2019, 11:11 pm   #10
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Default Re: Deriving 3v at 30mA from 5V Supply

It might well do, but I'd be more inclined to the view that excessive hum on the output of a 7805 or the like could be the result of poor ground wiring layout and/or lack of headroom in the input voltage causing the regulator to drop out on input ripple voltage troughs.

A 7805 requires at least 2.5V I/O differential to avoid dropout, so (for instance) an 8V input with 1V p-p ripple on it would be right on the edge.
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Old 14th May 2019, 11:26 pm   #11
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Default Re: Deriving 3v at 30mA from 5V Supply

Thanks Herald. Plenty of headroom for the 78L05 I think - 12V transformer through a bridge, so around 13.2V input to regulator. Current draw is < 100 mA.

Grounding layout is largely dictated by the kit's supplied circuit board - so beyond my control

Mike
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Old 15th May 2019, 1:21 am   #12
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Default Re: Deriving 3v at 30mA from 5V Supply

Part of the problem may also be RF feedback onto the supply line, so an inductor in the supply line after the regulator might not go astray.
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Old 15th May 2019, 10:52 am   #13
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Default Re: Deriving 3v at 30mA from 5V Supply

I would use the diode method you described as diodes are cheap and could be easily incorporated in the "power" lead, finished off with a bit of heat shrink. Could add some capacitors 100uF and a 0.01uF also.

Last edited by short wave; 15th May 2019 at 10:54 am. Reason: cap after thaught
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Old 15th May 2019, 1:16 pm   #14
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Default Re: Deriving 3v at 30mA from 5V Supply

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Originally Posted by Boulevardier View Post
Thanks Herald. Plenty of headroom for the 78L05 I think - 12V transformer through a bridge, so around 13.2V input to regulator. Current draw is < 100 mA.

Grounding layout is largely dictated by the kit's supplied circuit board - so beyond my control

Mike
Hopefully <<100mA. With 8.2V differential a 78L05 would be steaming a bit at 100mA (820mW). Surprised at only 13.2V though from a bridge and capacitor with 12V rms input.

Keeping RF away from the chip sounds very sensible. For the 7805 grounding, ideally the input and output caps should connect directly to the 0V leg and the 0V in and out tracks should run separately to and from the same common point with the caps and the chip.
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Old 15th May 2019, 2:51 pm   #15
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Default Re: Deriving 3v at 30mA from 5V Supply

Thanks again Herald -
"Surprised at only 13.2V though from a bridge and capacitor with 12V rms input."
Yes, you're right - I did it by quick mental calculation and got it wrong - should have said "about 15.7 V"

I assumed the 78L05 would be OK running with this I and V - the datasheet doesn't give a max power dissipation, just says "internally limited". But up to 30V DC input voltage is given for the 5V version, and 100mA as the max current for all versions. So 16V input and 60mA draw doesn't seem to be overstraining it.

On your earlier point, the kit pcb does have a 150 microH choke right at the +ve power entry point, so presumably the designer at least thought about RF intrusion into the power supply.

Out of interest, I will try feeding it from a computer USB port, and see what that's like.

Mike

Last edited by Boulevardier; 15th May 2019 at 2:57 pm.
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Old 15th May 2019, 3:03 pm   #16
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Default Re: Deriving 3v at 30mA from 5V Supply

It's very unlikely to be pulling 100mA anyway.
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Old 15th May 2019, 10:11 pm   #17
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Default Re: Deriving 3v at 30mA from 5V Supply

Since the 78L05 is commonly TO92 plastic, its thermal resistance junction to ambient is quite high. I found 230C/W buried in the datasheet for the 78L series. Tj max is 125C, so in 40C ambient (not unreasonable inside a bit of gear) max Pdiss is just under 370mW. In 25C ambient, this creeps up a bit to just over 430mW. That limits 100mA operation to less than 4V differential. The SO8 version does a bit better at 130C/W J-A.

I wonder if some ripply 78L05 supplies are just self protecting?

Just because a device has Imax 100mA and Vin max 30V doesn't mean it can handle both at the same time, though at least the 78L05 (probably) wouldn't blow up!
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Old 15th May 2019, 10:49 pm   #18
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Default Re: Deriving 3v at 30mA from 5V Supply

Herald - that's interesting, perhaps a small heatsink would sort it... However, the 78L05 is not getting hot - in fact it's barely warm. But I had wondered myself if it might be self limiting its operation in some way.
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Old 16th May 2019, 9:40 am   #19
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Default Re: Deriving 3v at 30mA from 5V Supply

I'd just use a 7805. They're good for about a watt or so in free air and still cheap as chips.
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Old 16th May 2019, 4:54 pm   #20
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Default Re: Deriving 3v at 30mA from 5V Supply

Quote:
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Herald - that's interesting, perhaps a small heatsink would sort it... However, the 78L05 is not getting hot - in fact it's barely warm. But I had wondered myself if it might be self limiting its operation in some way.
Try freezer spraying it?

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