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Need help choosing a resistor

entomophile

Thread Starter

entomophile

Joined Jul 29, 2010
5
I am rebuilding the power supply in an arcade. I am replacing all of the components. If you look at the schematic, R5 is labeled as 25Ω, 5W. The actual resistor on the board was a 13Ω, 5W. The resistor is going to the input of a 7905 -5V regulator. Could be someone replaced the original with the wrong resistor, or could be the circuit was changed by the manufacturer after the schematics were drawn up. From an electrical engineering standpoint, which one is correct?
 

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ErnieM

ErnieM

Joined Apr 24, 2011
8,377
It's impossible to tell without seeing (or actually probing) the unit you have. Does the resistor seem like it was changed at some time?

My gut guess is the resistor has been that way since the unit was manufactured. The issue is either the input voltage is a tad too high, or there is just a bit too much power in VR3.

If you want a very safe approach, buy TWO 13 ohm 5 W resistors, with shipping probably nearly the same cost the same as one.

Put both in series and see how the circuit works: do you get a good 5 volts out?

If yes, close it up and forget it. If the voltage is low, remove one resistor.
 
GopherT

GopherT

Joined Nov 23, 2012
8,009
What is the load connected to the -12 unregulated pins and what is connected to the -5 volt regulated pins?

If the load on -5 pins is small,

14 vAC means the DC to that pin is about 20 volts and the resistor is dropping some of the voltage so the 7905 does not have to do it all and overheat. It is rated at 1 or 1.5 amps with a heavy heat sink.

Doing the math, a 25 ohm resistor and need for a 13 volt drop means up to a half amp can flow before you get problems with drop-out voltage and lose regulation.

If it is 13 ohms, then you can handle about 1 amp before running into issues.

I would use a 13 ohm resistor and make sure ther is a heat sink on the 7905.

The only issue is where the heat will be dissipated, on the resistor or on the 7905. Either way, you will be well below the rated power dissipation of the 7905. As I said, just add a heat sink (or screw a few square inches of aluminum plate to the chip - careful, the heat sink will be electrically connected to the middle pin of the chip).

Cheers.
 
tracecom

tracecom

Joined Apr 16, 2010
3,944
GopherT said:
What is the load connected to the -12 unregulated pins and what is connected to the -5 volt regulated pins?

If the load on -5 pins is small,

14 vAC means the DC to that pin is about 20 volts and the resistor is dropping some of the voltage so the 7905 does not have to do it all and overheat. It is rated at 1 or 1.5 amps with a heavy heat sink.

Doing the math, a 25 ohm resistor and need for a 13 volt drop means up to a half amp can flow before you get problems with drop-out voltage and lose regulation.

If it is 13 ohms, then you can handle about 1 amp before running into issues.

I would use a 13 ohm resistor and make sure ther is a heat sink on the 7905.

The only issue is where the heat will be dissipated, on the resistor or on the 7905. Either way, you will be well below the rated power dissipation of the 7905. As I said, just add a heat sink (or screw a few square inches of aluminum plate to the chip - careful, the heat sink will be electrically connected to the middle pin of the chip).

Cheers.
Click to expand...
Won't a 13Ω resistor allow enough current to blow a .75A fuse?
 
entomophile

Thread Starter

entomophile

Joined Jul 29, 2010
5
Ernie-The board was definitely worked on...by some hack. The 13Ω and 5.6Ω were both cement resistors but by two different manufacturers. So it is quite possible it was swapped out. Perhaps to make up for some other failing component.

Tracecom-The F2 fuse was a 0.75A. I tested it and it is fine.

GopherT-Thanks for explaining the circuit. Those voltages go to a 1980's era CPU board for the arcade. Not sure what the power consumption is. There is a large heatsink (one for all three regulators).
 
GopherT

GopherT

Joined Nov 23, 2012
8,009
tracecom said:
Won't a 13Ω resistor allow enough current to blow a .75A fuse?
Click to expand...
I didn't notice the 3/4 ASB note. In any case, I don't think the resistor was intended to limit current but, instead to split the voltage drop between the two components.

My only concern is that 25 ohm will limit current to 0.5 amp and I don't know what the load could be. 13 ohms allows the full 0.75 amp load and allows the worst-case scenario.
 
panic mode

panic mode

Joined Oct 10, 2011
2,761
the secondary is 14V, full wave rectifier will charge cap to -20V. 7905 is an old linear regulator with some 2.5-3V drop, so you have 10-11V too much, doing nothing but making that 7905 hot. at 0.75A and 11V we get 14.7 ohm. if you need full current, stay with 12-13 Ohm but make it 10W. from what I recall arcade games rarely needed more than 250mA from -5V rail, many games didn't use it at all (just 5V for logic and 12V for audio amp). if that is your case, 25 Ohm, 5W is a good choice.

today, I would consider replacing that psu with a smps pc power supply, but that is me.
 
entomophile

Thread Starter

entomophile

Joined Jul 29, 2010
5
Thanks everyone. Looks like I'll go with the 13 ohm resistor. After all, it was working with that resistor for some time.

Panic Mode-I thought about upgrading the power supply. But JoeJester is right. I am trying to keep things as original as possible.
 
THE_RB

THE_RB

Joined Feb 11, 2008
5,438
I used to service those old arcade supplies. The resistors are generally to reduce the power dissipation in the regulator.

The current consumption on the -5v rail should be quite constant, so once it is hooked up and running you can measure the drop across that resistor, and the volts drop across the Vin->Vout pins of the 7905.

The 7905 should have at least 4v across it to ensure good regulation at all times, but if it drops more than that then the only result is increased heat in the 7905.

Possibly the best "mod" you can do to old arcade stuff is to put a good fan on the PSU to extract waste heat and a good fan or fans on the CPU board (which should be vertical). Overheating was always the number 1 killer.
 
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