Installing a fuse in turn signal circuit

Cacti_Ken · Post by **Cacti_Ken** » Fri Nov 22, 2019 8:31 pm

When I was restoring my M38 I installed the late style electronic turn signal controller.
Since then. I have ruined two controllers by accidently causing a short circuit of the 24 volts to ground either at the starter or on the fender mounted 24v receptacle. Getting older seems to cause me to be careless. Anyway. I am thinking of installing a fuse in the wire that the 24volts is in that connects to the controller.
I'm not sure how shorting the 24volts to ground somewhere away from the controller has a fatal effect on it.

Post by **wesk** » Sat Nov 23, 2019 3:03 am

Ken, Apparently it is the reversal of electron flow that is damaging the transistors in the control unit. I would think protection via a one way current flow diode would do more for you than a fuse.

RonD2 · Post by **RonD2** » Sat Nov 23, 2019 8:58 pm

Hi Wes,
From Ken's description, how did you know that transistors were damaged? While I suppose it's likely with 24VDC shorting --- I was just wondering how you knew this? Seems like his controllers could be repaired by replacing the transistors? Fairly easy to check PNP/NPN transistors with a multi-meter. Are those controllers sealed (throwaway, not intended to be repaired)? I recall seeing those controllers are pretty expensive, while common switching transistors used to be fairly cheap?

A properly rated fast-acting fuse (or fusible link) might do the job of protection, provided the controller was otherwise properly connected to power (in accordance with the MWO)? How would you go about selecting a diode to handle protection? Anything less than full battery current? Or just something above the rated load for the controller?

Cacti_Ken · Post by **Cacti_Ken** » Sat Nov 23, 2019 9:44 pm

Thanks for the reply and comments Wes. I'm in the process of finding the appropriate diode.

Post by **wesk** » Sat Nov 23, 2019 10:35 pm

Well Ron, I believe you worked in the electronics field so I am sure you are only asking questions for which you already know the answers. It was quite obvious what Ken's issue was and there are a multitude of folks that do not quite understand the function of transistors and how easily they can be damaged by voltage spikes in both directions and voltage polarity changes. However this is a jeep site and there are plenty of Wiki Like electronics web sites where this information can be covered and explained in great detail. But for the Ole Mjeeps site let's stick with the jeeps.

As for your other questions:

The transistorized turn signal units are treated by the Army very much like the later transistorized voltage regulators. They were XB3 supply items better known as throw away items. The Army did not publish service or parts information for them.

In the aviation maintenance field I did 22 years USAF and so far I have run my own FBO shop since 1990. Aircraft use diodes to protect many circuits.

For some reason the Army seemed to abandon circuit protection after the M38. Even the newer main light switches lacked circuit protection. The wiring for the trun signal system doesn't bother require a fuse or circuit breaker.

In the troubleshooting guide we are told only the flasher has reverse current protection built in.

I recall only the two very early Bendix Light Switches had built in circuit breakers.

RonD2 · Post by **RonD2** » Sun Nov 24, 2019 12:57 pm

Thanks Wes. Yes, I wasn't certain that the military transistorized flasher was sealed and not serviceable. Too bad because it's probably very cheap to repair because it's so simple. Has anybody ever tried opening one up without destroying it? I use a Signal Stat 902 and fused civvy flasher on my 12V M38 but had contemplated using the military unit. I asked the questions not to hi-jack Ken's thread, but to try and understand the solution you offered and maybe change my mind.

I think I found a schematic of the sealed transistorized flasher on one of your earlier posts and it shows a diode (D1) that I believe is there to provide the built-in protection that you mention. http://willysmjeeps.com/v2/modules.php? ... sistorized

If the transistorized flasher already has a diode built-in for protection (unless it's not working because it did it's job), what would installing another do? Wouldn't a fast-acting fuse be easier to spot and cheaper to change for most folks if it fails?

Thanks again! (and my apology Ken!)

Post by **wesk** » Sun Nov 24, 2019 9:04 pm

Looking at that diagram would you tell us the function of both D1 and D2?

RonD2 · Post by **RonD2** » Sun Nov 24, 2019 10:59 pm

Ok Wes, it's been awhile, but will give it a shot. Without a description of how the circuit was designed to operate and how its components are rated for each other and the load (trailer lights included), D1 is a zener diode (current flow both directions), allowing the transistors to switch (flash) rapidly and normally, to limit and protect them from internal over-current (and failure) associated with them turning off and on repeatedly. D2 is a regular diode connected across the load, probably to protect the transistors from external over current when the load (lamps) change (flash), especially when one or more open (burn out) or when somebody plugs or unplugs a trailer into the circuit, especially when it's operating.

I don't really see anything in the circuit designed to protect it in the event of a short circuit on either the line side (as Ken describes) or the load side. Would have to see component ratings and un-earth more brain cells to noodle that, but now can't imagine why they'd go to that trouble anyway.

I can see why it was a throw-away assembly. And why they wouldn't bother putting short-circuit protection inside the case for either the line or load side. Shorts would be fairly uncommon, and bulbs usually open when they fail. Even back then, there's probably not $5 worth of electronics in it. Not worth repairing back in the day. When it quit flashing, put in another (fix any short circuit condition beforehand and disconnect the batteries before doing maintenance of course). Considering what they cost now, they might be economical to repair if the case could be opened.

I see the point of your suggestion to add an external diode but struggle without some math to figure the correct current rating for short circuit protection without melting things. The diagram is labeled 24V/10A but the unlabeled transistor is rated 50V/15A. Diodes used to be cheap enough to experiment with.

RonD2 · Post by **RonD2** » Sun Nov 24, 2019 11:41 pm

I recall that diodes are also commonly used for voltage regulation but didn't consider that above, mainly because the jeep already has a voltage regulator and most military 24v components are robust enough to tolerate quite a bit of voltage fluctuation. Maybe they did arrange these diodes for some level of voltage protection in case of a malfunctioning (but not shorted) regulator?

Post by **wesk** » Sun Nov 24, 2019 11:43 pm

Well done Ron. I take it then we would agree that external circuit protection in the form of a fused power line in and the use of an external diode would improve survivability of the flasher with the way Ken handles his electrical system?

I would go one step in another direction and advise Ken to toss the expensive original military flasher setup and go with a standard Signalstat 24V flasher and switch system.

RonD2 · Post by **RonD2** » Sun Nov 24, 2019 11:57 pm

Absolutely Wes! I think protection is always a good idea. While the Army removed it from the M38, I put it back on mine. Too much invested to leave home without protection. Bad things happen with short circuits and no protection. Thanks for helping me understand the diode end of it better.

mdainsd · Post by **mdainsd** » Mon Nov 25, 2019 12:01 pm

An inline diode would help with protection for sure. I don't know about a fuse though for protecting against weird handling of the electrical system. It has to be sized big enough to handle inrush current of cold filaments in the lamps and as such may not respond quickly enough during a grounding of the 24V system. Otherwise a fuse in the circuit is always good practice.

D1 in conjunction with R5 form a voltage regulator for the flip/flop circuit consisting of Q1 and Q2. The purpose is to keep the flash rate constant during different supply voltages, like parked without the engine running or running down the road.

D2 is commonly known as a quenching diode, used nearly universally across a switching transistor. Missing (curiously) is the second diode that would be inline from the switching transistors emitter and the "A" terminal that feeds the lamps.

4x4M38 · Post by **4x4M38** » Mon Nov 25, 2019 3:50 pm

As I recall Bretto wired his 12v M38 using the 24v schematic with a couple of modifications. He added turn signals, and installed circuit breakers and fuses.

It might be interesting to see what he did for those 12 volt guys.

His schematics are on his photo pages.

mdainsd · Post by **mdainsd** » Mon Nov 25, 2019 5:07 pm

That is how I did my "salad" M38 kind of. Mine is a 24/12V hybrid system.

But just talking about the turn signals, The power from the 12V system comes from the light switch via one of the battery box mounted circuit breakers. It is wired into one of the early Military style turn signal switches, but uses the garden variety 12V flashers, available everywhere.

jnissen · Post by **jnissen** » Thu Nov 28, 2019 10:43 pm

mdainsd is correct. The zener provides a simple form of regulation so the oscillator will be consistent in its on/off timing. The main output driving transistor is protected by reverse current pulses by the second diode. As for protecting the unit, a fuse on the input wire will work. I suspect the main failure was if the lamp wire were accidentally grounded and the output driver will see excessive current. A fuse should work as a good protection device.

Btw in a 12V system using the standard 24V military type switch a standard 2pin thermal type flasher works just fine. This is how our M38A1 is wired. Works fine.