# Decoders in Parallel?



## Cougar Rock Rail (Jan 2, 2008)

Hi guys,

On the weekend at one of our club meets I was discussing decoders and power handling with one of the club members. He wondered if it would be possible to run two HO decoders in parallel (ie shared power, same address, same outputs) instead of a single larger decoder. As he said, at $20 each it's worth a try. Has anyone ever tried doing this? I can't see why it wouldn't work, but then I only know enough about electrons to be dangerous!









Keith


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## Bill Swindell (Jan 2, 2008)

I suspect you will release the magic smoke. Remember, the decoders are putting out a pulsing DC to the motor. There is no reliable way to make sure that both decoders work together. One might try to put power into the other one.


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## Del Tapparo (Jan 4, 2008)

Posted By Bill Swindell on 19 Jul 2010 04:57 PM 
... One might try to put power into the other one. 



Not sure what that means Bill?

My guess is no smoke. If the PWM's are out of phase, you may not get full voltage or you may get fluctuating voltage to the motor. I'd certainly give it a try!


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## Semper Vaporo (Jan 2, 2008)

Depends on how the PWM output is configured. If the output supplies power during the "ON" portion of the Pulse Width Modulation (PWM) and ground during the "OFF" portion of the PWM then hooking two in parallel could mean that unit A is in the "ground" portion of the cycle and unit B is in the power portion, then A is being a dead short for B... a no-no for both of them... The magic smoke might escape! 

If the outputs are just to supply power during a portion of the cycle and do not ground during the Off portion, then if they are in perfect sync you get nothing more than the ability to supply more current than one unit alone could provide. 

But more probably the units will not be in perfect sync. PWM controls the speed by supplying FULL power for some time short time period (like 1 millisecond) and then supplying no power for the remainder of the cycle; say the cycle time is 10 milliseconds then setting the throttle to 1/10 power is 1 millisecond of power and 9 milliseconds of no power. A throttle setting of 50 percent power is 5 milliseconds on and 5 milliseconds off. 100% throttle is full power for all 10 milliseconds and no off time. Inertia keeps the motor from getting to full speed before the power is cut during the Off portion and inertia keeps the motor rotating during the Off portion. The longer the ON portion the faster the motor speed achieves before power is cut during the Off portion. 

If the units are not in perfect sync then if both units are set to 1/10 power, one will supply power for 1 millisecond and maybe 3 milliseconds later the other unit will turn power on for 1 millisecond, thus you get 1/5 power in an uneven cycle, 1ms on, 3ms off, 1ms on, 5ms off, and then it repeats back to the first unit supplying power. The train will run faster than it would with just one controller in the circuit. 

If the units are not both operating at exactly the same clock frequency (which would probably be the case!) then their definition of 1 millisecond will be different and they will drift in and out of sync and the train will run fast for a while (until the units get in sync and then it will slow down to the desired setting and then speed up again as the units again drift out of sync. 

Neither scenario is good. If the units short each other you may get smoke, if they don't smoke each other then you get odd running characteristics.


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## Del Tapparo (Jan 4, 2008)

I think it is safe to assume anyone trying this would use like units (same P/N), so no shorting. 
????? 
Well, now that I think about it, if you are using a controller with an open-collector drive, no problem. If they are H-Bridge drivers, which is more likely in HO, then yes; you end up with smoke!


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## TonyWalsham (Jan 2, 2008)

If the loco had two motors then in theory it should be OK to power each motor separately, with one decoder per motor. 
LGB did that with their heavier locos when they only had lower power decoders available. 
If there is only one motor then I would agree with Del and Charles. Don't do it, or be prepared for magic smoke.


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## George Schreyer (Jan 16, 2009)

I vote for smoke.


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## Greg Elmassian (Jan 3, 2008)

I'm in the smoke camp too, especially if you have BEMF. 

Greg


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## Del Tapparo (Jan 4, 2008)

Well, that settles it then. Give it a try and report back here!









Edit: But since they are in parallel, you should get an equal amount of smoke out of each decoder.


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## Greg Elmassian (Jan 3, 2008)

You have to adjust the CV for "maximum smoke" first! 

Yep, if you have $40 to risk, why not. 

Of course you could spend $40 on a 5 amp DCC decoder and not worry, as opposed to two one amp HO decoders. 

What $20 HO decoders was he considering? I'll assume they either don't have sound or they are obsolete. 

Regards, Greg


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## Dan Pierce (Jan 2, 2008)

Two in parallel is a guaranteed smoke creator. 

If only one gets a reverse signal, then there will be a nice short on the outputs that feed the motor. 

You may even be able to customize the plastic shell from the heat generated!!!!! 

I would not waste any $$ trying to do this.


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## Cougar Rock Rail (Jan 2, 2008)

Interesting thoughts, guys! My friend is running a Lenz system but I think he uses a mix of decoders, and has even started building his own. I'll discuss your comments with him and let you know what happens if he decides to try it. Knowing him, he'll likely want to try it! 

Keith


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## George Schreyer (Jan 16, 2009)

The motor drivers will certainly not be in phase. It is a sure thing that the upper switch on one decoder and the lower switch on the other will be on at the same time, resulting in a dead short on the FIRST pulse. This will happen on BOTH sides of the H bridge. 

This is a death wish. 

He should just spend an extra $10 and buy a real large scale decoder


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## Cougar Rock Rail (Jan 2, 2008)

Thanks for the info George, I'll let him know and hopefully prevent a smoke out! 

Keith


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## Bill Swindell (Jan 2, 2008)

Like I said, magic smoke.


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