Resistor in Power Supply Input

On a sunny day (18 Dec 2006 08:55:18 -0800) it happened "Studio271" wrote in :

It is a 2.7 Ohm wire wound resistor. Its function is very likely to limit in rush current to C1 (470uF). The 7805 has fold back current limiting. No idea of the current, but it is no ta whole lot I think. So if

Ah, that makes sense; I'm going to go ahead and keep it as is for a while (don't have a replacement on hand anyway), and keep an eye on C1 (may just replace all the caps in the PSU anyway because of the age).

I already replaced all the aged logic chips in the thing anyway, so the caps may be the next to go.

Thanks!

-Drew

On a sunny day (18 Dec 2006 10:35:25 -0800) it happened "Studio271" wrote in :

If you want to be very precise, then you could measure the current with a multimeter on the 10A range in place where the resistor was. Then do i x i x 2.7 to get the power for the rsistor, and round up to the next commercial version.. This because there is also some current going to the -10V switcher.

Ah, that makes sense; I'm going to go ahead and keep it as is for a while (don't have a replacement on hand anyway), and keep an eye on C1 (may just replace all the caps in the PSU anyway because of the age).

I already replaced all the aged logic chips in the thing anyway, so the caps may be the next to go.

Thanks!

-Drew

It begins with F, the schematic calls it FRNB is it almost certainly a fusible film resistor guaranteed to go open circuit without setting on fire under overload conditions.

Micron Electric Co. Ltd. are (or at least were) manufacturers of fusible resistors.

In the absence of data on the original part it should be replaced with a

2.7 ohm fusible resistor of a similar size.

3 watts is too low - no safety margin to speak of. Go for at least 5 watts.

Ed

I've seen resistors in power supplies and wondered about their use. Let me see if I've got this right:

The resistor is often used to limit the inrush current to the system (this I knew). In this case, there is a big (100uF) cap there, which would charge very quickly on turn on, and this is why we want to limit the inrush.

That I'm not sure of. Why would we want to limit (or is it slow?) the inrush? Is it to save wear and tear on the components, or would the components actually fail (i.e., blow up) because of high inrush? Or would this contribute to a slow turn on effect, to minimize thumps on turn on? Is it a good idea to design inrush current limiting resistors into similar power supplies?

Also, i wonder if the resistor is serving a dual purpose here - to limit inrush as well as drop some voltage so the 7805 doesn't run so hot.

Thanks

fire

** Groper alert !

** The LM78M05 has thermal shutdown.

But that is not related to the Micron 2.7 ohm resistor - which BTW is very likely wire wound, fusible type.

** WRONG !!

** The function of the 2.7 ohm resistor is to protect the on/off switch from burning out or welding itself on.

The external 12 volt DC supply will have a large value electro across its output while the Roland has a 470 uF cap just after the switch. With no resistor to limit current flow, there will be an enormous current surge ( maybe 100 amps) when the charged electro is suddenly connected to the uncharged one. Enough to damage that switch very quickly.

If you look at the math for the situation, there is always energy loss ( by spark, heat and RF radiation) when a charged cap is suddenly paralleled with an uncharged one. Charge is conserved, but energy is not.

So, you MUST replace that resistor, a wire wound type of the right value will do.

....... Phil

Point well taken, thanks Phil! I'll scrounge around for a replacement tonight or just settle down and order one within the week, then.

-Drew

P.S. - This drum machine is getting greatly abused, electrically speaking, so that's why I'm so worried about the power supply. :D

P.P.S. - Phil: "Groper Alert"? o_O ...if you're going to molest me, please be gentle with the hair; I'm going bald as it is... :-P

I would beg to differ on the "wirewound" part..

The resistor is a fusible metal oxide film resistor of 1W rating.

(page 7). DO NOT use a wirewound resistor to replace it.

It is a N0-NO to use wirewound resistors as input protection for power supplies.

I meant "standard" wirewound resistors.... you might use a fusible wirewound but not in this particular case where the surge current is relatively small and of short duration.

"Ross Herbert"

** The code clearly says ( bottom left, page J10 ) it is 1/4 watt. ** No problem exists if it is a fusible one. ** What absolute BOLLOCKS !!!

WW resistors are used for inrush current limiting in billions of PSUs - plus act as "last line of defence" fuses too.

....... Phil

On a sunny day (Tue, 19 Dec 2006 04:36:51 GMT) it happened Ross Herbert wrote in :

OK, it is metal :-) Yes you are right, WW is not good as fusible, it will do fine as current limit I think, why not.

Protects the 12V supply and the diode between the supply and the cap.

Ed

Or would

Yes, ANY resistor will limit current. However, when used as an input protection resistor the main requirement is that an unstandard overload condition which produces a marginal over-current will not protect the equipment where a simple wirewound resistor is used for this function. It takes a long term severe overload to heat a standard wirewound resistor sufficiently for it to go open circuit, and in that time the overheated resistor could have started a fire. A fusible resistor appropriate to the load must be used in such cases. In this particular case the load current is relatively small and a fusible film resistor is best since it performs its function quickly with a relatively small short to medium term overload which is sufficient to prevent a fire hazard.

"Ross Herbert"

** Who said it would ??

** Dear oh dear - where ever does Ross get this utter bollocks from ???

WW resistors are made from fine gauge wire - OK ?

Fine gauge wires all have a fusing current level - OK ?

Can't tell you how many standard 5 and 10 watt cement resistors I have replaced that have gone totally open with NO visible damage to the resistor or the PCB.

Instant failure, due to over current.

** Merely overheated WW resistors generally don't go open.

The wire's melting point temp is simply never reached.

** And a WW one will take a great deal more repeated, short term, surge current than a film type will.

Ross only ever opens his mouth to change feet.

Yawn .......

...... Phil

On a sunny day (Wed, 20 Dec 2006 09:02:53 GMT) it happened Ross Herbert wrote in :

Yes, indeed, I agree. Just do not use fusible _metal_ film resistors, I tried, and it got red hot, burned a hole in the PCB, blackend some nearby components, produced a lot of smoke, but _did_not_go_open_circuit_. The carbon type will just flash open in a case like this. I learned a lesson there! Probably the issue is the max current to expect, he has 12V / 2.7 Ohm in case of a short you get 4.4 A, if the supply goes through its knees (small AC/DC converter) maybe 2A, and above situation happens. (I had it on a 300mA adapter IIRC). Perhaps using one of thoso automatic fuses is better, could not find a suitable one at that time.

Fusible resistors are specifically designed not to do this. I suggest you were not using one.

On a sunny day (Wed, 20 Dec 2006 14:24:18 +0000) it happened nospam wrote in :

Mmm terminology, yes I used a normal metal film of low wattage. Are you saying there are special fusible resistors?