Glare shield lighting - only a resistor?

[NicoN]

Today I was lying under the panel and was surprised to see a resistor in a kind of cooling metal housing next to the potentiometer for glare shield lighting. I was expecting some fancy electronics.

According to the schematic this seems to be true. Is this really true that my light bulbs are controlled by a variable resistor ? What Äs the addditional resistor is for ?

Edited February 16, 2020 by NicoN

[Jerry 5TJ]

A switching mode buck regulator would be more efficient. But our Mooney planes are of an older design era

Yes,  the dimmer circuit is resistive only.  

My guess: The fixed resistor takes some of the thermal load that otherwise would be dissipated solely in the potentiometer.  

[PT20J]

Not sure what you are seeing. The lights are connected between the emitters of power transistors and ground, and the transistor base current is controlled by the panel rheostat. Early models of the J (and I'm pretty sure for the K also) had the transistors located under the panel and later models had the transistors moved to the rear near the battery.

Skip

[carusoam]

Bulb Dimming strategy probably hasn’t changed in decades...

Until LEDs re-opened the need...

Simple, and light weight were the driving force for much of the designs... complex and better? For light bulbs?

Just a PP guessing, not a mechanic.

Best regards,

-a-

[N201MKTurbo]

The older Mooneys used a giant high power rheostat. Then they changed to an emitter follower using ancient germanium transistors. Then they changed to more modern silicon transistors.

Not sure what they are using now.

[FlyBoyM20J]

On 2/16/2020 at 8:14 PM, PT20J said:

Not sure what you are seeing. The lights are connected between the emitters of power transistors and ground, and the transistor base current is controlled by the panel rheostat. Early models of the J (and I'm pretty sure for the K also) had the transistors located under the panel and later models had the transistors moved to the rear near the battery.

Skip

Yep, here's the sticker on the bottom of the dimmer unit in my 82 J.  But the PNP transistors are drawn incorrectly.  They have collector and emitter reversed, so the load is connected between collector and ground.

EDIT: I'm wrong...Skip is correct below when he points out that this is a valid schematic for an NPN emitter-follower configuration.

Cliff

 

[PT20J]

15 minutes ago, FlyBoyM20J said:

Yep, here's the sticker on the bottom of the dimmer unit in my 82 J.  But the PNP transistors are drawn incorrectly.  They have collector and emitter reversed, so the load is connected between collector and ground.

Cliff

 

According to the M20J maintenance manual section 39-12-03,  the original transistors were 2N2016. They were superseded beginning with S/N 24-1214 by an equivalent part when the 2N2016 was no longer available. The 2N2016 is an NPN power transistor and I believe the drawing is correct, although it doesn't show the emitter arrow.  https://alltransistors.com/pdfview.php?doc=2n2016.pdf&dire=_rca

Skip

[Steve W]

On 2/16/2020 at 9:00 PM, N201MKTurbo said:

The older Mooneys used a giant high power rheostat. Then they changed to an emitter follower using ancient germanium transistors. Then they changed to more modern silicon transistors.

Not sure what they are using now.

Probably Vacuum Tubes.

 

[N201MKTurbo]

11 minutes ago, Steve W said:

Probably Vacuum Tubes.

 

Not a good choice, the 12K5 is only good for 80ma of plate current with a plate voltage of 12.6Vdc. Your lights would be dim at best.

[FlyBoyM20J]

31 minutes ago, PT20J said:

According to the M20J maintenance manual section 39-12-03,  the original transistors were 2N2016. They were superseded beginning with S/N 24-1214 by an equivalent part when the 2N2016 was no longer available. The 2N2016 is an NPN power transistor and I believe the drawing is correct, although it doesn't show the emitter arrow.  https://alltransistors.com/pdfview.php?doc=2n2016.pdf&dire=_rca

Skip

Yeah, I've read that too, but the drawing is a classic PNP circuit if you swap C and E.  PNPs conduct as Vbe goes negative which would be the case if the top of the transistors (labeled as C, but I believe should be E) is at +12V and the base is being pulled down from 12.  I can't see a way to make the schematic work for an NPN.

Cliff

[NicoN]

Funny.  I am absolutely sure I have seen only a rheostat and this  resistor.

And this is according to the schematic ...

Definitely no electronics box ...

 

[FlyBoyM20J]

6 hours ago, NicoN said:

Funny.  I am absolutely sure I have seen only a rheostat and this  resistor.

And this is according to the schematic ...

Definitely no electronics box ...

 

Many here will be able to tell you the exact serial number at which Mooney added the transistors, but I can't remember off the top of my head.  I know that it was prior to my 1982 J by a year or two.  Older ones than that had only the rheostat to dissipate current, as per your sch.

EDIT: I'm wrong...per Skip, the transistors are over next to CB on right in this drawing. 

Cliff

[PT20J]

48 minutes ago, FlyBoyM20J said:

Yeah, I've read that too, but the drawing is a classic PNP circuit if you swap C and E.  PNPs conduct as Vbe goes negative which would be the case if the top of the transistors (labeled as C, but I believe should be E) is at +12V and the base is being pulled down from 12.  I can't see a way to make the schematic work for an NPN.

Cliff

I don't see your problem. Vcc is connected to the collector. The rheostat controls the base current through the forward-biased base-emitter junction which sets the collector-emitter current through the load connected to the emitter.

Skip

 

[PT20J]

27 minutes ago, NicoN said:

Funny.  I am absolutely sure I have seen only a rheostat and this  resistor.

And this is according to the schematic ...

Definitely no electronics box ...

 

Ah, now I see your confusion. That's only part of the circuit. Trace the wires from connector 21PL34B  and you will find that they go to a couple of transistors. This part of the circuit is just a voltage divider to control the transistors over by the circuit breaker panel.

Skip

[N201MKTurbo]

Just now, PT20J said:

Ah, now I see your confusion. That's only part of the circuit. Trace the wires from connector 21PL34B  and you will find that they go to a couple of transistors. This part of the circuit is just a voltage divider to control the transistors over by the circuit breaker panel.

Skip

The older Mooneys did not have transistors. They had a giant Ohmite 25W rheostat.

[FlyBoyM20J]

2 hours ago, PT20J said:

I don't see your problem. Vcc is connected to the collector. The rheostat controls the base current through the forward-biased base-emitter junction which sets the collector-emitter current through the load connected to the emitter.

Skip

 

You're right, Skip.  I was looking at this backwards.  Basic forward-biased emitter follower configuration. 

Cliff

[FlyBoyM20J]

I had another look at the dimmer unit on my J.  The transistors are 2N3055, which are definitely NPN power transistors.

Here they are on Mouser for a little under $6.

Cliff

 

[PT20J]

Wow, RCA back when RCA was a real company that made stuff!

[carusoam]

Nice pic Cliff!

-a-

[NicoN]

Update: in my original posting I talked about a rheostat in combination with a big resistor with a cooler. Also the schematic seemed to me that the regulation is simply converting power to heat.

But I was wrong! Even in my schematic there are 2 transistors for panel lighting and instrument lighting on the far right end of the schematic. And these are controlled by rheostat and resistor.  only the compass lighting seems to be independant ... shame on me

Q1: Why the hell is there a C/B for panel lighting and another  fuse near the transistor ?

To be honest: I have never seen the transistors, but only 4 grey plastic caps where fuses are under. This is located under the RH panel.

 

Q2: i am not an electrical engineer, but is it possible to replace the combination of rheostat and resistor with a PWM dimmer? This could mean, the PWM dimmer is providing switched power to the transistor and this one siwtches on and off my glare shield lamps (or LEDs)

 

Thank you!

[EricJ]

1 hour ago, NicoN said:

Q2: i am not an electrical engineer, but is it possible to replace the combination of rheostat and resistor with a PWM dimmer? This could mean, the PWM dimmer is providing switched power to the transistor and this one siwtches on and off my glare shield lamps (or LEDs)

Yes, it has been done many times.   If you search around this site you can find threads about converting the glareshield lamps to LEDs.

[FlyBoyM20J]

4 hours ago, NicoN said:

Q1: Why the hell is there a C/B for panel lighting and another  fuse near the transistor ?

To be honest: I have never seen the transistors, but only 4 grey plastic caps where fuses are under. This is located under the RH panel.

Good question!  The C/B is there to allow a central location to disable the system.  Like all other systems in the plane.  But why the fuse, as well?

The fuse is there to protect the transistor which is rated for something like 7A (some will be more like 10A or more; depends on the exact transistor).  5A is the rating for the fuse and the C/B is rated for 10A, so if there is a short in the load and the rheostat is turned all the way up (so, transistor is in saturation and passing current as required by load), then the fuse is the only thing that keeps the transistor from passing current potentially beyond its limits.

Interesting side note: the transistor is itself a current-limiting device provided it is not supplied with too much base current (that is, not turned up all the way in this particular circuit).  That means that if you don't ever run the transistor to saturation, it will output less than 5A regardless of the load.  And in this circumstance, even a short in the panel lighting circuit will not blow the fuse or trip the C/B.

Oh, and to your second point: in my 1982 J, the transistors are mounted to the rear of the dimmer unit.

Cliff

[carusoam]

CB protects the whole circuit...

Fuse protects the Transistors...

So many different flows of electrons in that case, two specific different streams... 

I almost remember this stuff...

Thanks Cliff!

Best regards,

-a-

[FlyBoyM20J]

3 hours ago, carusoam said:

CB protects the whole circuit...

Fuse protects the Transistors...

So many different flows of electrons in that case, two specific different streams...

And the rheostat (perhaps with aid of a series resistor I didn't see) protects the transistor base from over-current.  It never goes to 0 ohms - at least not in my J.  More like 10, so the transistor base current is limited to a max of about 1.5 amps and remains below the datasheet limit of 7A for the snazzy RCA 2N3055.

Circuits are fun.

Cliff