Unsure of how current works with two voltage regulators

[jamisonia]

In the 90s Sega released the Genesis and two add ons, the Sega CD and the Sega 32x. All three of these add ons require their own AC adapter, which as you can imagine is a real pain. In their infinite wisdom Sega designed a way to power add ons from the base Genesis, but for some reason never utilized it. Its very easy and well documented on how to connect those contacts to allow the Genesis to power both add ons. One would then replace the stock 9V 1.2 Amp AC adapter with something that could output ~3 Amps to power this behemoth.

What I want to do is see how much current the actual systems require, and how much is being wasted by the 7805 regulators present in all three system. I'm thinking maybe I could replace it with something more efficient like the LM2676 (which I have several free samples of). But I want to make sure that 1.2 Amps is actually sufficient to power all three systems off of the stock power brick if I replace the regulators.

I tested the current in the Genesis last night. The original motherboard revision of the Genesis uses 2 7805s. I remove the voltage regulators to a breadboard and tested the current. Both regulators had a current of about 350mA running through them when playing a game. I then decided to test the current coming from the power switch before it even gets to the regulators. That read as ~700mA. This confuses me. Would this mean the power regulators are stepping the 9V to 5V at 100% efficiency? I expected this number to be much higher and closer to the 1.2 Amp spec of the AC adapter. I thought that its possible I might me misunderstanding something fundamental about how current works.

 

[duke37]

The 7805 regulators step down voltage to give 5V.
The 7805 consume very little current (about 5mA) so if each regulator is sending out 350mA the total input current would be 710mA. Basically, current in = current out just as water into a pipe = water out less a little bit of leakage.
Everything appears normal.
The efficiency depends mainly on the voltage drop. If 10V in and 5V out, efficiency is about 50%.

 

[jamisonia]

thats what I thought. So the only way my idea is going to work is if the two add ons use less than 450 mA, otherwise I will have to just get an AC adapter with a higher current output capabilities.

 

[davenn]

thats what I thought. So the only way my idea is going to work is if the two add ons use less than 450 mA, otherwise I will have to just get an AC adapter with a higher current output capabilities.

what is the total current you expect the circuit to draw ?

So the only way my idea is going to work is if the two add ons use less than 450 mA,

450mA total or each ? 450mA is a long way from the 1.2A (1200mA) capability of the plugpack. even is it was 450mA each, thats 900mA still less than the 1.2A of you plugpack
if it is less than the 1.2 A that the AC adaptor is capable of supplying, then you have no problems. Each 7805 is capable of a MAXIMUM of 1 Amp when heatsunk
~ 500mA with no heatsink but will still get quite warm.

Dave

 

[jamisonia]

450mA total. The entire setup, the Genesis, Sega CD, and 32x has to use less than 1.2A, and we already know the Genesis uses ~700mA itself. That leaves 450-500mA to power the 32x and Sega CD. If your from Australia you probably now these systems as the Mega Drive, Mega CD and 32x.

 

[davenn]

450mA total. The entire setup, the Genesis, Sega CD, and 32x has to use less than 1.2A, and we already know the Genesis uses ~700mA itself. That leaves 450-500mA to power the 32x and Sega CD. If your from Australia you probably now these systems as the Mega Drive, Mega CD and 32x.

ok time to do a current measurement for the 32x and Sega CD and see what they draw.

you could dig up an old 12V 5A laptop plugpack and stick that into the regulators

gosh if you lived close I would give you one, have a bunch of them ying around here
They are small and light as they use switchmode PSU circuitry. visit a computer shop, they may have an old but still working one lying around that they would either just give you or maybe for a couple of pounds

We have sunday computer markets here in Sydney where you can pick up things like that for a couple of $$.... Only buy new if you really really have no other choice

cheers
Dave