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ATX Power Supply connection problem

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(@jonlee365)
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Joined: 2 months ago
Posts: 6
Topic starter  

Hi to all who are interested,

Here's the project: Connect Car Radio to Computer Power Supply. 

Here's the problem: A car radio is designed to have a permanent connection to a 12v power source even when the radio is switched off. When a car and its radio are not running, the car's battery is still supplying a small amount of power to the radio so that the radio keeps its time, settings and radio presets.

This would be fine if I wanted to run the Power Supply unit 24/7, then the radio wouldn't lose its settings, but I don't want to do that. Power supplies have fans which make noise, plus I want to save on power costs by shutting down the PSU when I'm not using the radio. 

I know that ATX PSU's have a +5v standby rail which is always on when the power supply is in shut down mode, and I am thinking that I can use this to supply the radio with constant power when the PSU is shut down. I want to use a small "step up" boost converter to boost the +5v standby rail to 12v, which should keep enough power to the radio for it to not lose its settings. The radio only needs about 200mA at 12v to keep its settings when switched off. 

What I am unsure of is putting diodes in the circuit. 

I have been told to put diodes on the 12v+ rail from the supply and on the 5v+ boosted standby rail, so they don't back feed power to each other. 

My questions:

Do I need diodes on both the +12v rail & the boosted +5v standby rail? 

Which diodes do I need, or what do they need to be rated at for each rail, as obviously both rails supply different amounts of current. 

Specs:

The radio is fused at 10Amp, so that's the max current it can draw when operating. 

The ATX Psu DC ratings are:

+12v = 15Amps

+5v standby = 2.5Amps

Does anyone have any knowledge on this? 

My alternative is to use a second small power supply to supply the radio with permanent 12V when the larger Psu is turned off. This would also require diodes on the positive rails to prevent backfeed? 

Any help will be appreciated to solve this problem. 

Thanks 

 

 

 


   
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(@davee)
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Joined: 3 years ago
Posts: 1627
 

Hi @jonlee365,

  There are limitless different ATX power supply designs around, (and car radio types) so it is impossible to know if your one has any 'quirks', but the "general answers" to your questions, assuming nothing strange, are included below.

I am not clear how intend to switch between the two supply sources ... using two supplies with diodes to drive a single load is more 'commonly' associated with providing a back-up supply, so that if the 'main' supply fails, the alternate supply will 'fill in', but this implies that both supplies are capable of supplying the full current, at least for a period of time. In this 'common' case, the back-up supply would typically supply a slightly lower (say 0.5V or more, lower) voltage, so that whilst the main supply is good, the diode in the back-up supply line will be reverse biased, and not conduct any current. 

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Hence, assuming you have a means of switching on the 'main' supply when required.

Do I need diodes on both the +12v rail & the boosted +5v standby rail?

Yes, that would be advisable to minimise the chance of problems.

Which diodes do I need, or what do they need to be rated at for each rail, as obviously both rails supply different amounts of current.

In principle, they both need to be rated at comfortably above 12V, and I would guess (around) 50V might be a reasonable starting point.

In terms of current, then for the main diode I would probably look at something above 15A, perhaps 20A. That might sound a bit pessimistic, but the actual current a fuse will blow at, is more of an art than a science. It depends upon a huge number of factors, including which country wrote the specification. There is a high chance, a 10A diode would fail long before a 10A fuse.

Assuming the maximum ATX 5V output is 2.5A, then the step-up would only be able to manage 1A continuously. Again, I would build in a margin, say 2.5A minimum.

In both cases, check the voltage drop, as you don't want to lose too much ... a Schottky diode is usually a lower voltage drop type.

---------

Hope this helps.

Good luck, Dave


   
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Ron
 Ron
(@zander)
Father of a miniature Wookie
Joined: 3 years ago
Posts: 6827
 

@davee @jonlee365 I think there are only two ATX 'designs' and the difference is fairly trivial in that one has more output pins than the other. IIRC either can be used but given the low cost I would get the larger style. If you need details on how to tell, let me know and I will check.

If all you need is to have the radio remember some settings, I bet a small coin battery would do the trick, and the radio may have the header for that built in. 

First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
Major Languages - Machine language, 360 Macro Assembler, Intel Assembler, PL/I and PL1, Pascal, Basic, C plus numerous job control and scripting languages.
Sure you can learn to be a programmer, it will take the same amount of time for me to learn to be a Doctor.


   
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(@jonlee365)
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Joined: 2 months ago
Posts: 6
Topic starter  

Thanks for the reply DaveE, some good info there. So the diodes should ideally be rated to slightly above what is needed in both voltage and current. As I understand it, I don't intend to switch between both power supplies, but have them connected and running together in parallel. This was only an option though. Ultimately, I'd prefer just the 1 PSU. I was aware of the voltage drop problem, and hope the diodes don't drop the main 12v too low. 


   
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(@jonlee365)
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Joined: 2 months ago
Posts: 6
Topic starter  

@zander thanks for the reply. I don't believe there's any way to directly connect a battery to the radio's memory wire separately, as it also seems to supply the main voltage to the unit. So any battery will be mixed in with the main 12v source. A coin battery is only a few volts right? The radio needs around 11 - 12volts to keep the memory.


   
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(@davee)
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Joined: 3 years ago
Posts: 1627
 

Hi Ron @zander and @jonlee365,

re: I think there are only two ATX 'designs' 

   I think, there may be two 'main' variations of ATX connector, plus a number (many?) of manufacturer 'specials', but I was referring to the electronic design inside, which has lots of variations.

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In terms of an ATX supply, there will probably be two 'semi-independent' power supplies, one small 5V only, 'keep-alive' supply, which is only switched by the incoming mains, whilst the main supply is also switched by the 'front panel', using the 'keep-alive' 5V to power a sensing/control circuit. If you don't switch 'off' the main supply when it is not required, it will probably still consume a substantial amount of power. Please bear in mind, a switch mode power supply can be designed to be 'permanently' connected to the incoming mains power, but 'controlled' to a 'dormant' state, which does not produce an output voltage, and only takes a negligible current.

----------

In principle, you may be able to access the radio on/off switch to cause it to act like the 'front panel' switch of a PC, but the mechanical surgery required could be tricky, so I personally would leave it alone.

An alternate, if you are feeling inventive and creative, might be to sense the current demand .... if it is below a certain level, then only the 'keep-alive' power is enabled, but if it exceeds that level, the main supply is also 'powered up'.

Please bear in mind, this is only a 10-second crazy brainstorm type of idea ... not a proven scheme ... if it works at all, there will probably be issues to deal with. e.g. the radio may use very little current when the volume is turned to almost nothing or there are a few seconds of silence in the programme. But finding answers to such things is all part of normal research activity.

Best wishes and good luck to all, Dave


   
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(@jonlee365)
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Joined: 2 months ago
Posts: 6
Topic starter  

@davee - thanks Dave, although I like your idea of current sensing to trigger the correct power mode, or supply, unfortunately it's beyond my knowledge to build.


   
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robotBuilder
(@robotbuilder)
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Joined: 5 years ago
Posts: 2042
 

@jonlee365

Posted by: @jonlee365
@davee - thanks Dave, although I like your idea of current sensing to trigger the correct power mode, or supply, unfortunately it's beyond my knowledge to build.

A challenge to work out?

I don't know what your actual setup is and have no interest in many posts of questions and answers to find out but current sensing is simple. In your case (just guessing) maybe a relay that turns off when the current drops below a certain level? Another is triggering an action when the voltage drops across a small resistance. A hall sensor can also be used. This is the one I used with the Arduino to monitor motor current overload.

To enlarge an image, right click image and select Open link in new window.

currentSensor

 

 


   
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(@jonlee365)
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Joined: 2 months ago
Posts: 6
Topic starter  

@robotbuilder - thanks for the info! I'm going to look into this.


   
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(@davee)
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Joined: 3 years ago
Posts: 1627
 

Hi @jonlee365 ,

  @robotbuilder's suggestion is a good starting point. If you are interested in finding out more, then there is a lot of information available, and it is possible to do some experiments to learn more about them.

  If you are not familiar with current sensors, relays and Arduinos then try looking for explanations by Bill (@dronebot-workshop) and maybe others.

e.g. https://dronebotworkshop.com/dc-volt-current/

and

https://dronebotworkshop.com/ac-arduino/

If you start by repeating the experiments that Bill demonstrates, it may become apparent how you might transfer the expertise to your specific case, and you can always try asking another question or two when you have looked into possibilities.

Just a suggestion ...

Best wishes, Dave


   
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