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Post by Deleted on Jun 23, 2018 14:36:49 GMT -5
A few months ago, I tested a speed control via a variable voltage (using a "power" OP-Amp connected as a unity gain follower, in fact one halfth of a L272M) ; I measured the current consumption of all my locomotives (including the double Budd) under load (i.e. upwards with cars) and found a maximum of 80mA (for the MTL GP9). I hope that all components (wipers in locomotives even in AZL models whose current consumption and flow is lower, parts in turnouts) would "survive" to a continuous (even caused by a short-circuit ! ) current ot 80~100mA ; so the method would be to avoid that the current could exceed this value, that means : build a current limiter. I think that I found the perfect circuit for this usage, a power OP-Amp (500mA) with a built-in current limiter, "programmable" via a resistor : it's the OPA547 by TI ; yes, I know, it is not cheap (so about 13$), but all the way less expensive than for example a turnout. I calculated that a "standard" value of 220KOhms for the resistor would limit the current to about 95mA ; warning, there is a "bad mixture" of units in the formula on the data sheet by TI, to get directly the value in KOhms one should replace the 5000 with 5 !
What do You (especially Greg) think about this solution ?
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Post by markm on Jun 27, 2018 17:43:50 GMT -5
Alberich,
I've been thinking about this a bit. The numbers you built to are all reasonable from my experience. I'm just unsure about current limiting. 500mA at 10V continuously is still a lot of power potentially running through handrails an such. You might want to consider a shutdown circuit if you draw 500mA for more than a second or two.
Mark
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Post by Deleted on Jun 27, 2018 23:22:54 GMT -5
I think I didn't explain it clearly enough : the OPA547 can handle a maximum of 500mA ; but with the 220K resistor connected to the appropriate pin the output current will be limited to 95mA only ... even in case of a short circuit, I hope so ! And the maximum voltage to the track will in fact be only 8.8V, there are two supplementary diodes in the power circuit (too long to explain here ... )
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Post by markm on Jun 27, 2018 23:51:16 GMT -5
Ok, makes more sense. A couple of additional data points for you: the MTL F7s run 150-200mA (depending on load) and my MTL GP draws 70mA @ 8.5V and 85mA with a 100gr load (4x heavyweight passenger cars).
Mark
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Post by Deleted on Jun 27, 2018 23:57:19 GMT -5
That's what I measured : my GP9 with 5 freight cars and a caboose, on a 1,6% "upwards" ramp and at a realistic speed, needs in fact about 80mA ; AZL locos maximum 40mA.
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