Author Topic: Drop a 12v PSU to 11v or less?  (Read 591 times)

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Offline CZroe

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Drop a 12v PSU to 11v or less?
« on: January 15, 2018, 04:24:30 pm »
I have a device that is missing the original power adapter: a NEC TurboGrafx-CD dock. The original specs: 5.5x2.5mm, center negative, 11v, 1.53A. It’s ‘80s tech so the adapter was linear. The dock is internally regulated to 5v but I have reason to believe that even the specified 11v is pushing things, temperature-wise, and that anything down to 9v would be appropriate.

The commonly-recommended Radio Shack replacement is no longer available, but it was apparently 9v. The Japanese version of this dock (“IFU30 Interface Unit”) uses the same internal hardware as the US device but specifies 9v, 1.45A with the same polarity and plug dimensions. They are interchangeable between regions, which tells me that a replacement within the 9v, 1.45A+ and 11v range should be acceptable. In fact, it sounds an awful lot like the exact same adapter with the 10-20% greater input voltage creating a proportionally higher output voltage (US 110-120v compared to JPN 100v).

I went to a thrift store and test-fit various loose plugs until I found one that fits: a modern Hyundai-branded 12v, 5A switching PSU brick that looks like it’s for an LCD monitor or something. I reversed the polarity and tested it and it works great, but these old CD docks are known for failing caps and we all know that electrolytic caps aren’t going to like the extra heat over the long-term.

IIRC, internal regulation is handled by two 7805s. Based on the Japanese spec, it seems that the US version is already running them hotter than their Japanese counterpart by feeding a higher voltage. I’d like to drop that voltage closer to the Japanese spec for increased reliability... anywhere in between 9v and 11v. I know I could just splice the end onto a 9v 1.45A+ plug, but I can actually use the extra amps here for some other projects and I want to be able to move the PSU between them.

I’ve never done anything like this but I assume it can be done with a couple diodes and a resistor, so here come the questions:
How do I know what specs to order?
Could I just salvage a couple of them from the rectifier of an old device that takes 12v AC?
Will this 5A diode from Mouser be ideal?
https://www.mouser.com/productdetail/vishay-semiconductors/egp51g-e3-c?qs=sGAEpiMZZMtbRapU8LlZDxCc6FzR8ItptId7%252bqIFh04e8bI1655czg%3D%3D
How do I determine the value of the resistor I should pair with it (assuming that’s what I’m supposed to do)?

Thanks in advance.
« Last Edit: January 15, 2018, 05:21:00 pm by CZroe »
 

Offline blueskull

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Re: Drop a 12v PSU to 11v or less?
« Reply #1 on: January 15, 2018, 04:28:26 pm »
If you have a 12V PSU, you can use a 7809 with a big heat sink to step it down to 9V.
Using a bunch of diodes is not a good idea because it only provides a constant voltage drop, not precise regulation.
You need good regulation because your switching mode power supply, unlike a linear power supply, will spit a lot of noise, and you want to get rid of it.
 

Offline phil from seattle

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Re: Drop a 12v PSU to 11v or less?
« Reply #2 on: January 15, 2018, 04:36:21 pm »
Yes, a couple of diodes will drop the voltage - it's fine if you are feeding the 7805 regulators.  But, you should be able to find a 9V DC wall wart, they are fairly common.  The thing about wall warts is that the rated voltage occurs at the specified current. So that 5A one will probably run unloaded at around 16V and >13V at 1.5A.

If you can't find the right size/polarity plug, you can replace it with the correct one. Cut and solder. Use a little shrink wrap on each wire and a 3rd piece to keep them together (put the shrink wrap pieces on before you solder - don't ask me how I know that...).
 

Offline kalel

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Re: Drop a 12v PSU to 11v or less?
« Reply #3 on: January 15, 2018, 04:39:58 pm »
Use a little shrink wrap on each wire and a 3rd piece to keep them together (put the shrink wrap pieces on before you solder - don't ask me how I know that...).

I know it very well too. I wish there is an effective external way to apply heatshrink (or something similar) after such a mistake that just happens easily.
 

Offline CZroe

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Re: Drop a 12v PSU to 11v or less?
« Reply #4 on: January 15, 2018, 05:09:02 pm »
If you have a 12V PSU, you can use a 7809 with a big heat sink to step it down to 9V.
Using a bunch of diodes is not a good idea because it only provides a constant voltage drop, not precise regulation.
You need good regulation because your switching mode power supply, unlike a linear power supply, will spit a lot of noise, and you want to get rid of it.
The original voltage regulators are actually still in the dock, so is there still a reason to use an external 7809 instead of a couple diodes? Thanks.

Yes, a couple of diodes will drop the voltage - it's fine if you are feeding the 7805 regulators.  But, you should be able to find a 9V DC wall wart, they are fairly common.  The thing about wall warts is that the rated voltage occurs at the specified current. So that 5A one will probably run unloaded at around 16V and >13V at 1.5A.

If you can't find the right size/polarity plug, you can replace it with the correct one. Cut and solder. Use a little shrink wrap on each wire and a 3rd piece to keep them together (put the shrink wrap pieces on before you solder - don't ask me how I know that...).
LOL! I’ve made that mistake a a couple times myself. ;)

I checked beforehand and the replacement appears to be a rock-solid 12v due to being a switch-mode PSU. I just used liquid electrical tape and a single piece of heat shrink for my first splice but I plan to do it nicer once I have selected the proper components to include. :) Thanks!
« Last Edit: January 15, 2018, 05:14:34 pm by CZroe »
 

Offline blueskull

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Re: Drop a 12v PSU to 11v or less?
« Reply #5 on: January 15, 2018, 05:38:06 pm »
If you have a 12V PSU, you can use a 7809 with a big heat sink to step it down to 9V.
Using a bunch of diodes is not a good idea because it only provides a constant voltage drop, not precise regulation.
You need good regulation because your switching mode power supply, unlike a linear power supply, will spit a lot of noise, and you want to get rid of it.
The original voltage regulators are actually still in the dock, so is there still a reason to use an external 7809 instead of a couple diodes? Thanks.

PSRR can add up.
 

Offline lordvader88

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Re: Drop a 12v PSU to 11v or less?
« Reply #6 on: January 17, 2018, 04:52:20 am »
LM317 is a very common reliable and useful chip, and cheap too (i think?) Very easy to setup too, just 2 resistors and 2 caps.

LM317
3-Terminal Adjustable Regulator
http://www.ti.com/lit/ds/symlink/lm317.pdf

Quote
The LM317 is an adjustable 3?terminal positive voltage regulator
capable of supplying in excess of 1.5 A over an output voltage range of
1.2 V to 37 V. This voltage regulator is exceptionally easy to use and
requires only two external resistors to set the output voltage. Further, it
employs  internal  current  limiting,  thermal  shutdown  and  safe  area
compensation, making it essentially blow?out proof.
The LM317 serves a wide variety of applications including local, on
card regulation. This device can also be used to make a programmable
output  regulator,  or  by  connecting  a  fixed  resistor  between  the
adjustment and output, the LM317 can be used as a precision current
regulator.
« Last Edit: January 17, 2018, 04:58:43 am by lordvader88 »
 


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