Voltage Blaster ISA -5V Provider Q&A 

Thank you Phil! Yeah, I just wanted it to be complementary.

Retro computing is a great way to learn some basic electronics, and these videos are super helpful in that respect.

Simple and easy. Great job guys! 👏

Yes, seems like the channel got a lot of attention in the last two months indeed. I don't mind :D

Yeah, this is also a very good and common suggestion. See 9:17 for the answer.

True! But I also answerd in the video, why I made it TH :)

*voltage regulator. Autocorrect fail. :)

Yes, in such case, you will need to search for another solution. This one is for boards, which have enough ISA slots.

Thank you! The thing is, that the regulator is not hanging in the air, but can be screwed to the PCB, which would dissipate some heat as well. However, I didn't calculate how much. But you are right, I should add a warning to the description, that people should screw it to the PCB, if the thing gets hot. But I also have to admit, that all the hardware I tested didn't pull more than 10-20mA out of -5V. Usually it is used for leveling only and not to power anything, that's why even 200mA would be very unlikely. Most PSU don't deliver more, than 500mA for both -12V and -5V rails together and some do even only 200mA. So trying to pull 200mA only on -5V rail from such PSU could be already an overkill.

Yes, absolutely.

In such a case you can just solder the LM7905 with two small caps somewhere on the board. This solution is for people, who can't solder or don't want to modify the board. Free ISA slots were usually not a problem on earlier boards, where -5V were required. On later PCI systems, there were less of them, but you usually also didn't need -5V anymore.

99% of computers, which I'm repairing are 286, 386 and early 486. As long as I get 10-15A on the 5V rail, that is more, than enough :) For an AMD Athlon PC I would also take something, what is as reliable as possible. And newer PSUs don't bring up enough power on the 5V rail. So, even if you'll get an expensive high end modern 600W PSU, it will probably fail on old systems against a cheap 300W old PSU, where most of the power was on the 5V rail and not on 12V, as it is usually made today. So, don't trust overall wattage and look at the particular rails, which would better fit for your system.

You are welcome. It's because we work with negative voltage and -12V < 0V.

@@necro_ware wow, can't believe I did not realize that! thank you for the explanation! 😀

Yes, this is also a nice idea. I got that proposal from another viewer, eventually he made a separate card with multiple regulated fan connectors as far as I know.

It can be done in a similar way, but positive voltage lines have to stand higher current, than negative voltage ones. If some of the cards will draw a lot of current from 3.3v it could damage the slot and/or mainboard. If the lines are thick enough, then this can be made. You also should keep in mind, that you have plenty of ISA slots on the older mainboards, but usually only 3 or 4 PCI slots.

If the spec says, that -25V is absolute minimum, the voltage regulator would probably not survive -35V. Furthermore, you need to take heat dissipation into account, which rises with the difference of the input-to-output voltage. Even, if your circuit would consume only 0.1A at -25V you would need to dissipate (25V - 5V) * 0.1A = 2W. In the datasheet you can find the formula for temperature calculation. 2W means already about 140-150°C, which you'd have to cool down with a huge cooler. As a side note, the regulator shuts down at 125°C for protection. If the voltage regulator would be able to run at -35V, you would need to dissipate 3W, which already means temperatures over 200°C. You would be able to solder with your voltage regulator :D Just as a theoretical calculation: the regulator can deliver 1.5A at -25V input voltage. This would mean a temperature around 1800°C :D

Ok, I will add it. Thanks.

It's not easier or cheaper, it makes actually no difference. The reason why it's complete is, because it's been taken from an already available Kicad footprint library. I could remove unused pins or make a custom footprint for this project, but I didn't bother.... so call it lazy ;)

@@necro_ware Sounds like a 1.1 in the making! :)

these 2 sided pcb's have copper layers on the outside that are removed using chemicals to melt them away after a mask is applied to the area that makes up the design you came up with, the costis the same with or without the pads since, they where part of the pcb material anyway, and in most cases it takes no real dif in time to have the copper there vs gone.. many prefer the pads be there for contact points as an extra level of wear resistance if nothing else.

Such capacitors are usually already on the board between the ISA slots. Furthermore, on each expansion card there must be own capacitors installed, because of the distance to the board. So, all in all, the caps wouldn't make a lot of sense.

Yes

You can find all the information on the project page. Just follow the link in the description. Those are 33uF. To your other question about fixed -5V, means that the voltage regulator output is fixed to -5V and converts anything down to -25V to a fixed value of -5V. The opposite of fixed is adjustable, where you can control the output voltage using an external voltage divider.

Yes, you don't need much, furthermore those are mostly sound cards which are using -5V and it's a bad idea to use switching regulators with analogue audio devices, because they can produce a lot of noise.

Да, пока не сталкивался с этой проблемой, но идею в голову отложу, спасибо!

@@necro_ware единственное жалко - pci слот - их обычно немного

I think, you got it completely wrong :D It's a lot simpler, than you think. There is 7905 voltage regulator used, which converts -12V to -5V. 7805 doesn't output 5V grater, than the reference. It always just outputs 5V and works only with input voltage greater or equal 7V. To convert negative voltages, you have to use 79xx series of regulators and the 78xx are for positive voltages.

​@@necro_ware Ok didn't notice that. There is a trick where you can use the 7805 as a negative voltage regulator if you connect the input pin to ground, then the "ground" pin to -12v you'd get -7v on the output. I got too excited about proving a stranger wrong on the internet and didn't pay attention to the part number :P

It costs less, than a dollar per piece. If it's too expensive, I don't know ....