Pictures of my ZX81 fitted with a RECOM 5V Regulator

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1024MAK
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Re: Pictures of my ZX81 fitted with a RECOM 5V Regulator

Post by 1024MAK »

Yes, the issue 3 board is very similar. The unused holes for the switch are exactly the same. The pads for the 9V DC input socket are laid out the same, but I can’t remember if the holes in the board are the same as on the issue 1 board.

I added the 100nF capacitors because they help with the reduction of noise and the stability of the regulator/ DC-DC converter.
I got mine from Rapid Electronics (who have since discontinued this make and replaced them with a different make). The type that you linked to up thread should be fine.

The idea is to have a 100nF capacitor on the 9V DC input to the regulator/ DC-DC converter as close to it as possible (this is good practice for conventional series pass 78XX regulators where the main smoothing capacitor is remote from them anyway).

The 100nF capacitor on the 5V output of the regulator/ DC-DC converter is also good practice.

Mark
Lardo Boffin
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Re: Pictures of my ZX81 fitted with a RECOM 5V Regulator

Post by Lardo Boffin »

Thanks Mark. Do these just go into spares holes without making any more changes? Or do you have to divert the power away from its original route to send it through the capacitors?
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1024MAK
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Re: Pictures of my ZX81 fitted with a RECOM 5V Regulator

Post by 1024MAK »

No diversion/changes. The one that goes in the holes provided for the switch, just go in the holes (the outer two of the three). Solder it up and it’s good ;)

On some boards, there are holes that are part of the same pads as the pins/legs of the 3.5mm jack socket used for the 9V power input. But on the underside they are often covered by solder. So you have to remove some of this solder. It’s then just a case of fitting the capacitor in the holes, and soldering up both the capacitor and the socket pins.

As I said earlier, i can’t remember if the issue 3 board has these holes. If not, you may be able to slide the capacitor legs into the same holes as the pins of the socket.

Mark
Lardo Boffin
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Re: Pictures of my ZX81 fitted with a RECOM 5V Regulator

Post by Lardo Boffin »

Thanks. I have opened an issue three and it has the following:-

4DE4EFCC-7D71-4038-9D86-4EE661F78B2C.jpeg

So one of the caps should be easy but not so sure about the other.

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willinliv
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Re: Pictures of my ZX81 fitted with a RECOM 5V Regulator

Post by willinliv »

Here's my replacement switch mode regulator - I wanted to keep my issue 1 reasonably true to it's design so opted to keep the 'tractor' part :D
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1024MAK
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Re: Pictures of my ZX81 fitted with a RECOM 5V Regulator

Post by 1024MAK »

I had to do a double-take :lol:

👍 8-)

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bola_dor
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Re: Pictures of my ZX81 fitted with a RECOM 5V Regulator

Post by bola_dor »

is The value of the capacitors calculated in any way?
I have may CZ1500 that had from factoy a 4.7uF electrolitic over the 7805 +9 and GND leads. Then I saw another TS clone with a 22uF bridge between +5v and GND after the 7805.. (from factory too).
I am looking to mitigate some jailbars on my TS1000 with a a 7805 two.. and some times I use a cheap switching PSU and thought somethig like this may be useful..
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mcarlson_sb
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Re: Pictures of my ZX81 fitted with a RECOM 5V Regulator

Post by mcarlson_sb »

bola_dor wrote: Thu Aug 20, 2020 5:59 am is The value of the capacitors calculated in any way?
Line filter capacitors (in this configuration) act to short out high frequencies, while passing through lower frequencies and usually have a capacitance in the range between .1µF and 10µF

They basically act as a short across the +Vcc and GND until they "fill up" then allow normal flow.
When the flow stops for a moment the capacitor begins to discharge - kind of like a battery backup for the signal.

This means that high frequency switching will be "filtered" in that the signal will not be seen as going low because the cap compensates for it to keep the voltage from dropping.

The smaller the capacity the faster they will "fill up" - and the less capacity it will have to discharge.
If the signal drop is longer than the discharge cycle of the cap voltage on the signal can then go low.

So, the larger the capacitor, the lower the frequency it will filter. (I think I got that right)

As for math - there probably is some, but I don't know it.
I usually try a .1µF and see if that works. Usually does.
If not I try a 1µF then a 10µF - not the most scientific approach :D

Cheers,
Matthew
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1024MAK
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Re: Pictures of my ZX81 fitted with a RECOM 5V Regulator

Post by 1024MAK »

Okay, there are four things here:
  • On the input to a series pass element voltage regulator (such as a 7805) (also known as a linear regulator / PSU even though the operation is very non-linear) it is good practice to have an electrolytic capacitor if the main smoothing / filter capacitor is not close by. The value is not critical and is typically between 4.7uF and 100uF. Generally speaking larger is better. But it does depend on the maximum current the voltage regulator is expected to supply. The objective is twofold, firstly to help filter out the mains frequency ripple (100Hz or 120Hz). The second is to act as a small reservoir so that if there is a change in the load (output) current supplied by the regulator, the capacitor can supply this current. This is what Matthew was talking about.
  • On the output of a regulator circuit, again you may find an electrolytic capacitor. This serves a similar purpose as the one on the input in that it acts as a small reservoir so that if there is a change in the load (output) current, it can supply this faster than the regulator can on it’s own. The value used varies depending on what the load actually is and on the type of voltage regulator chip used. With some voltage regulators, the impedance matters more than the actual value of capacitance alone. So the correct type and correct value should be used, as specified in the voltage regulator manufacturers documentation.
  • Electrolytic capacitors have limitations. One problem with the method of construction is that at some frequencies their value of stay inductance increases. This reduces their effectiveness at higher frequencies. Hence a type of capacitor that does not suffer as much should be used as close to the voltage regulator chip as possible. Ceramic or film capacitors are normally used for this. Values typically used are 100nF (0.1uF) to 470nF (0.47uF). This also increases the stability of the voltage regulator circuit (and helps to stop it from oscillating).
  • DC - DC converters don’t use magic to do their job. They are mini switch mode power supply units (SMPSU). They chop up the input, use an inductor to store energy. Then rectify the resulting AC (chopped up DC pulses) to convert it back to DC. Hence it is wise to add extra filtering. Because they operate at frequencies well above line (mains) frequencies, the best capacitors to use are ceramic or film types. I chose 100nF mainly because I always keep a good stock of this type and value.
Yes, if you go looking, you may well find some nice complex mathematics to calculate the values. However in practice rule of thumb is far quicker and easier, as the tolerance on the required value is wide and the tolerance on the value of the types of capacitor (electrolytic or ceramic) involved is wide as well.

Incidentally, for the main smoothing / filter capacitor, for smoothing the rectified voltage from a step down mains transformer, the rule of thumb to get a reasonably low ripple is to use 2200uF of capacitance per 1 Amp ( or 220uF per 100mA) of full load current.
So a PSU that is rated at 500mA is recommended to use a 1000uF capacitor (nearest standard value).

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Re: Pictures of my ZX81 fitted with a RECOM 5V Regulator

Post by bola_dor »

1024MAK wrote: Thu Aug 20, 2020 9:39 am Okay, there are four things here:

Mark
Thank you for the lessons.. !!!!
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