Good guessing needed

[DrZarkov]

I'm modifying my "Zarkovisor" with a (I hope) better working synch, therefore I have to change the power supply from 6 V to 12 V for the circuit. For calculating the resistor I need to know the mA of the 6 V cassette recorder motor. I've tried a poti, but it started to glow in the dark. Any ideas how many mA an average 6 V motor taken from an old Grundig cassette recorder from the late 70th have?

[Steve Anderson]

Any ideas how many mA an average 6 V motor taken from an old Grundig cassette recorder from the late 70th have?

It can vary quite a lot, depends if it's from a battery-powered machine, or one that used mains input (they generally used DC motors even in hi-fi cassette decks).

As a very rough guess, say between 50mA and 250, quite a wide range really.

If you have a 6V power supply and a meter you can measure it directly, it will vary with the load on it.

Now, what worries me is that you'll calculate the value for a series resistor to turn it into a 12V motor. This is not a good idea as the motors speed is dependant not only on the load, but the supply voltage. With a series resistor the supply to the motor becomes a very poor current source and the motors self-regulation of its speed becomes much worse.

In addition the better quality ones sometimes had internal electronic speed control which wouldn't like a supply like that.

I suggest if you are stuck with a 12V supply you obtain a 12V motor. Sorry.

Steve A.

P.S. There are electronic methods to get around this, but it does start to get a little 'messy'. Attached is perhaps the simplest way to do it, the voltage to the motor is half the supply volts, and proportional. It's a vast improvement over a series resistor. The transistor will probably need a heatsink and will handle motors up to about half an Amp.

If the motor uses brushes (most likely) I would add a 100nF capacitor and a reverse biased diode (1N4004) across the motor terminals to reduce the amount of 'garbage' brushed motors generate.

[DrZarkov]

Sounds good, I will give it a try. Unless nobody has an idea how to connect my 6 V motor to the sync circuit which is designed to work with 12 V.

[Steve Anderson]

Sounds good, I will give it a try. Unless nobody has an idea how to connect my 6 V motor to the sync circuit which is designed to work with 12 V.

If there is some way you could post, send the circuit, or point to a URL, I'll have a go at revising it for 12V driving a 6V motor...shouldn't be rocket science...

Steve A.

P.S. I don't have any of the 'club' circuits except those that are on the web site.

[DrZarkov]

I would like to use the circuit from the NBTV handbook on this homepage. Unfortunally it is not possible to make a direkt linkt to chapter 3 of the handbook. If anybody knows a for me better suitable circuit, let me know...

[Steve Anderson]

I would like to use the circuit from the NBTV handbook on this homepage.

Is it one of the two below, if so which one? Or both even?

These were taken off the NBTVA web site.

Steve A.

[DrZarkov]

The second one. As driver and separator I've used the club PCBs (which are working well).

[Steve Anderson]

The second one.

OK, here's my initial suggestion, just simply substituting the 12V motor with the circuit above. Don't forget the heatsinks.

As Andrew found out, sometimes the IRF510 is not easy to get, but there are plenty that can be used instead.

Steve A.

[gary]

Is this really necessary? Without analysing the circuit too deeply I would have thought that the necessary voltage drop would occur across the IRF510 anyway, with a heatsink it should handle it...

[Steve Anderson]

Is this really necessary?

Gary, you're quite correct. Being a 'belt and braces man' I would be a little concerned that the full 12V could be applied to the 6V motor as the PPL locks up or loses lock for some reason.

The values (without the divider circuit) of the feedback resistors would need to be 'fudged', but they probably would do anyway depending on the mechanical inertia and so forth.

An alternative is to provide a seperate 6V supply for the motor only, a LM317 being ideal (or a 7806 if you can get them), fed from the 12V supply.

Steve A.

[DrZarkov]

A second (or actually a third) power supply is no option, it would be a space problem, and I don't want to have one of the transformators outside of the TV-set. I think Steve's circuit improvement is a very good an cheap solution for the problem, I will try that. Thank you for taking the trouble!

[Steve Anderson]

A second (or actually a third) power supply is no option, it would be a space problem.

You wouldn't need an additional transformer/power supply. You simply feed the input to the LM317 (or 7806) from the existing 12V supply. They're the same size as a common power transistor, just the same as a TIP29 for example.

Attatchment below shows how simple it would be....

Steve A.

[Steve Anderson]

It is possible that in fact your motor doesn't need 6V when running and locked up, in which case it might be possible to get away with just feeding it with 5V.

In your existing arrangement, measure the voltage across the motor when it is in sync, if it's less than (say) 4V you could use a 7805 regulator as below.

If not and it needs closer to 6V you'll have to change the 7805 to a 7806, but they're very hard to find. Or use the version above.

Originally the 78xx series of regulators were made in many flavours, 5V, 6V, 8.2V, 9V, 12V, 15V, 18V and 24V. But now only the 5V, 12V and 15V versions have survived.

To be honest I would go with the LM317 version, by changing the value of R2 in that circuit you can set any voltage you want, as long as the input voltage is at least 2V more than the desired output voltage.

Steve A.

[Klaas Robers]

This is all not needed. Simply connect the cassette motor in the original circuit. This circuit is designed to work with motors that run the correct speed at less than 6V. The remaining voltage is accross the transistor. May be that you have to increase the value of the 4k7 resistor to 10k when the motor is running too fast, but I don't think so.

[DrZarkov]

Thank you Klaas for your answer, that saved me some time! Luckily I couldn't start with the additional circuits, so I can simply set it up and try. The main circuit is ready now for first tests, I just have to built it in into my Zarkovisor.