Replacement for 7805 regulator

[Gosub]

Firstly, hi everyone, I am new here so go easy on me,

I have just recently started to play with electronics and am completely absorbed by the vast subject, how I never got the bite before eludes me. So onto my problem.

I am trying to reduce the heat generated by a heat sink in an old Sinclair ZX Spectrum. The computer's power is fed from an unregulated power supply that has the following specification
Input : 240v AC 50Hz
Output: 9v DC @ 1.4A
When measuring the output of various identical Sinclair Spectrum power supplies I found that the voltage varies between 12 and 16 volts. I know that the 7805 regulator knocks the voltage down to 5V and loses the rest as heat and that the voltage from the power supply lowers as more energy is drawn from the said power supply. This is what is causing the heat sink to get quite hot and ruin the keyboard cables and the caps.

So, how am I able to modify this scenario? I think I am looking at three different angles, see below

1. Modify the Power supply 'add a series diode to drop the energy slightly'
2. Modify the Spectrum 'replacing the 7805 with a better less wasteful component or components' I don't want to modify circuits that change the speed of the spectrum.
3. Create a device that sits between the power supply and the spectrum that would lower the power output of the power supply.

As I say, I am brand new to electronics and haven't even finished getting all my tools sorted

Thanks in advance for any help on this guys, the subject might be very simple but I have a love of these machines and I feel that It's a good place for me to start

Kindest regards to all

[janoc]

Well, the simplest option is likely to remove that 7805 and get a modern switching plugpack that outputs 5V directly. Even some smartphone chargers may be able to do the job. These didn't exist back in the day, so the power supplies for these machines were quite heavy transformer bricks that could even literally melt under load ...

[Yago]

I would try and go with option 3, and keep the old gear as standard.
The old "might be worth something one day" applies?

Edit: actually, 16 to 5 volt sounds a long way.
Measure the voltage and current when computer is plugged in and running, PSU is probably high when off load. Get a true idea of what is going on before you decide on action.

[Gosub]

Thanks for the quick replies guys, I'll boot the old girl up and test the load on it's own and also add some of the various interfaces that the spectrum supported
There were various joystick and Rom Interfaces and also the Sinclair interface 1 & 2, I'll try and find the max load possible.

Thanks again

[Tandy]

The 7805 is a linear regulator that is not terribly efficient, however if you wanted to replace it with a switching one you would also require other components including an inductor. I don't think that it would be easy to fit inside the spectrum.

The 7805 is an LDO that the input needs to be a minimum of 7V, if you want to keep the spectrum as original as possible then you could just power it from a good stable 7V supply rather than the original 9V supply. The lower voltage would mean less heat.

[Simon]

you can also use a resistor. work out the lowest voltage your likely to be supplied then do (Vin-7V)/current required, as the current consumption goes up (causing more heat) more voltage will drop across the resistor and less on the regulator keeping the heat down. But again you still have heat to dump. A switchmode power supply is an entirely different beast and will leave you with little of the original.

You could replace the transformer with a SMPS so that most of it stays intact and then put the transformer back if you want to sell it on as original.

[Gosub]

I'm liking the idea of using a separate power supply as this would allow the units to be kept original and I can be safe not to melt the unregulated supply

I must admit though, after reading your replies, I now want to mod one of my spectrums to help me understand electronics more. I'll ponder over the suggestions and see where it leads me

Thanks for all the helpful suggestions guys

[Zero999]

You may be able to use a 7805SR, a SMPS PSU with the same pin-out as the LM7805 but the current rating is lower, 0.5A, so check the power consumption first.
http://www.murata-ps.com/data/meters/dms-78xxsr.pdf

[SeanB]

You can get a drop in replacement for the 7805 that takes the entire switching regulator and puts it in a package that fits the original 3 pin mounting, but which runs very cool.. A little bigger but you probably could fit it in the case and remove the heatsink totally.

http://www.ebay.com/itm/3-Terminal-5V-1A-Switching-Voltage-Regulator-Power-Supply-/261243604047?pt=LH_DefaultDomain_0&hash=item3cd3550c4f

First hit on flEaBay.  Will even fit the speccy without problems, and you just need to unsolder the old one, toss it, the heatsink and the mounting hardware into a small bag and keep it around and solder in the new one, just making sure to keep it from shorting out to anything else.

[KerryW]

The OKI-78SR-5/1.5-W36-C will do 1.5A and solders in place of a 7805, but is a bit bigger.
Or, you could wire in a 7808 between the PS and the Sinclair.

[Gosub]

So here are the power draws from my supply

Spectrum powered on it's own = 550mA, Powered with Interface 1 = 770mA. With Interface 1 and 2xmicrodrives = 1.18A

That's a lot of draw fully kitted out

[Kjelt]

Are you sure the higher input voltage is not also needed somewhere else such as a RS232 chip or so?
Then indeed replacing the power supply by a modern smps with 8 or 9 VDC output would be my choice.

[Bored@Work]

The 7805 is an LDO

No, it is not.

that the input needs to be a minimum of 7V,

Q.E.D.

[Kjelt]

Indeed thumb of rule for a 78xx is >= 3V between in and output esp. With larger currents. So a 7805 needs >= 8VDC on its input.

[Tandy]

Done a bit more digging and the edge connector on the 48k spectrums pin 4 is connected to the 9V power line to power interfaces. However this will not be a problem for your interface 1 & 2 + microdrives do not use the 9V power they rely on the 5V from the regulator. So I would suggest you should be able to easily find a 7.5V 2A switch mode power supply with the barrel connector on that pugs into the spectrum. The fact that the interface 1 and interface 2 add additional load on the internal 5V regulator will cause a big increase in heat over the computer running on its own.

[Tandy]

The 7805 is an LDO

No, it is not.

OK not an LDO but Datasheet shows dropout voltage 2V https://www.fairchildsemi.com/datasheets/LM/LM7805.pdf

[Kjelt]

Look at the datasheet from the 80's

[Tandy]

Found this video that you might find useful

[Gosub]

Yep, the heat is incredible, I don't plan having loads plugged in but now I understand why so many speccy's vaped out back in the day.

Thanks again for all the helpful input

[westfw]

http://www.ebay.com/itm/3-Terminal-5V-1A-Switching-Voltage-Regulator-Power-Supply-/261243604047?pt=LH_DefaultDomain_0&hash=item3cd3550c4f

That's  ... rather disturbingly tiny compared to most of the ~5W switching converters I've seen sold.  I suppose it's possible, but i'd worry that it's a sort of "reference design" built with a smaller-than-needed inductor (inductor specifications being ... confusing.)

Besides, this one is cheaper, had better output current, comes from a reputable manufacturer AND distributer, and has a real spec sheet:
http://www.digikey.com/product-detail/en/OKI-78SR-5%2F1.5-W36H-C/811-2692-ND/3438675

I'd look at getting a (now very common) 5V regulated switching wall wart, and bypassing the regulator entirely.

[denelec]

Here's another higher efficiency replacement for a 7805:
http://www.robotshop.com/media/files/pdf/dimension-engineering-switching-regulator.pdf

[LabSpokane]

The OKI-78SR-5/1.5-W36-C will do 1.5A and solders in place of a 7805, but is a bit bigger.

That's a good solution and just over $4 from Mouser. You'd really have to have a tight fit for it not to be a drop in replacement. I just bench tested this one this weekend. In small volume it's hard to buy the comps and get much cheaper. 

[Kjelt]

I'd look at getting a (now very common) 5V regulated switching wall wart, and bypassing the regulator entirely.

Problem to get a real decent one. Most wallwarts are ultra cheap made with capacitors that will last a year tops if used 24/7.
I have had my share with those, bought a lot of different voltages from different manufacturers and the ones I end up using in my projects al died within a year (ofcourse they were already a few years old). When opened you could see the point of failure being the hunglo , sunshine, XGong and other crap electrolytic capacitors that failed. Oh yeah and one unforgettable one that burned through the plastick.

If you would go for a switching solution, IMO then pay a few €'s more and get a TDK-Lambda or other A-brand dedicated SMPS, where you can see through the metal casing what the build and component quality is instead of a closed black box that could come from the most cheapest place of manufacture humanly possible 

[janoc]

If you would go for a switching solution, IMO then pay a few €'s more and get a TDK-Lambda or other A-brand dedicated SMPS, where you can see through the metal casing what the build and component quality is instead of a closed black box that could come from the most cheapest place of manufacture humanly possible 

I wouldn't buy a wallwart, look at the replacement power supplies for laptops. E.g. Meanwell manufactures plenty of 5V/2A (or more) power supplies with a barrel jack connector. There are even ones with two output voltages, should it be required (e.g. 5 + 9/12V). These are a bit more expensive, but you get a top quality device that won't kill your Speccy.

[Dave Turner]

I wouldn't buy a wallwart, look at the replacement power supplies for laptops. E.g. Meanwell manufactures plenty of 5V/2A (or more) power supplies with a barrel jack connector. There are even ones with two output voltages, should it be required (e.g. 5 + 9/12V). These are a bit more expensive, but you get a top quality device that won't kill your Speccy.

But be careful many of the supplies have 3 connections in the barrel jack - outer inner and pin - eg the Dell & HP centre pins are a signal wire. Neither would be suitable for a Spectrum.