Red Power Controller - Low side FET

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Red Power Controller - Low side FET

Beitragvon Codex » 01 Mai 2018, 04:54

With reference to the ft Power Controller, that has 9V DC Out and adjustable Motor output.

For the motor output, it uses a Field Effect Transistor (FET) that is connected to the ground reference, so as you rotate the dial, the ground reference shifts up and down. In other words the voltage reference is actually the plus 9V rail. So if you want 5volts output, you get 4volts on ground.

This is poor engineering to me, I would never design it like that. Its fine for an isolated motor, but now its very tricky to use it in other circuits. You will end up damaging older vintage ft if you use it the same way as the older transformer base power supply.

I understand this is a super cheap electronics design, it uses a very common 555 timer to generate a PWM that feed into the low side FET. You can't get cheaper. But gee, why not use a high side FET and adjust the +ve rail instead?

Some better output filtering would good too.

Has anyone modified the board to correct this?

Thanks
Michael
Codex
 
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Re: Red Power Controller - Low side FET

Beitragvon Lars » 01 Mai 2018, 13:24

Hallo Codex,

Codex hat geschrieben:With reference to the ft Power Controller, that has 9V DC Out and adjustable Motor output.

For the motor output, it uses a Field Effect Transistor (FET) that is connected to the ground reference, so as you rotate the dial, the ground reference shifts up and down. In other words the voltage reference is actually the plus 9V rail. So if you want 5volts output, you get 4volts on ground.

as far as I remember a picture of the PCB of this red box, it does pulse width modulation (abbr.: PWM) as you found out already. I saw an integrated ciruit of the NE556 type, which is a double pulse generator very widely used since decades now. It avoids a big cooling piece of aluminium and allows to choose a cheaper package for the switchung transistor. And it provides a smoother regulation of DC-Motors than a voltage control.

Codex hat geschrieben:This is poor engineering to me, I would never design it like that.

So it is an open collector output and very common even for signalling purposes. Such kind of output could turned into another type to fulfil special demands by adding further circuitry.

Codex hat geschrieben:Its fine for an isolated motor, [...]

... or lamps and that are the purposes intended by ft.

Codex hat geschrieben:but now its very tricky to use it in other circuits. You will end up damaging older vintage ft if you use it the same way as the older transformer base power supply.

No, neither the old grey motors, the solenoids, nor the "silberling"-circuitry get damaged by 9 V. Only the 6 V-bulbs have to be replaced by current 9 V-types, but the signalling lamps in the silberlings are not affected.

Codex hat geschrieben:I understand this is a super cheap electronics design, it uses a very common 555 timer to generate a PWM that feed into the low side FET. You can't get cheaper. But gee, why not use a high side FET and adjust the +ve rail instead?

What for? There is no voltage regulation intended.

To get 5 V out of 9 V, use a switching or non-switching voltage regulator, which control their output voltage very precisely today.

With kind regards
Lars
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Re: Red Power Controller - Low side FET

Beitragvon MasterOfGizmo » 01 Mai 2018, 17:32

I think what Codex is trying to say is that it's easier to connect stuff if it all shares common ground. If you e.g. want to use the TXT to read the output voltage of the Power Supply. How would you do that? That's a little tricky as the TXT expects a positive input signal relative to ground and the power supply delivers a negative signal relative to 9V.

But that's probably not the usual use case of the power supply. Usually you just connect a single lamp or motor and be done.
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Re: Red Power Controller - Low side FET

Beitragvon Codex » 02 Mai 2018, 06:50

MasterOfGizmo hat geschrieben:I think what Codex is trying to say is that it's easier to connect stuff if it all shares common ground. If you e.g. want to use the TXT to read the output voltage of the Power Supply. How would you do that? That's a little tricky as the TXT expects a positive input signal relative to ground and the power supply delivers a negative signal relative to 9V.. . .


Yes, that is the sort of scenarios I was about. The same with all my older 1970s ft, its tricky or impossible to use the new red power supply with the adjustable out due to its common positive architecture with the electronic modules (Elektronik Baustein). At the very least it is unconventional.

Also the variable output is a PWM, (square wave like), with no filtering (unless you call tiny 0603 SMD cap a filter :D that would be a high frequency filter for EMC).

Very tempted to knock out a a quick PCB with high side MOSFET and filters. Looks like there is enough space to build a proper variable power supply in that housing. Maybe even a 50Hz sine wave output for the AUX, like the old transformers had. :lol:
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