Hi,
Sorry about the long preamble, but I think it's better than no background at all....
I run a simple web site about the Taycol series of model boat motors - popular amongst modellers in the UK in the 1950s. (well, someone's got to do it...) It's HERE if anyone's interested...
These are field-wound motors, so if you reverse the polarity they continue to run the same way. In the 1950s they were reversed by using a DPDT switch or relay to reverse the armature polarity while leaving the coil polarity the same.
Modern radio control speed controllers automatically reverse the feed polarity, and it would be nice to be able to use them in a vintage boat with a Taycol. There is a technique which can achieve this - using a rectifier to maintain original polarity to the field coils while the armature is reversed. I cover this on the web-site. But there are some issues with this approach - it drops a bit of voltage and has safety issues if the feed to one coil set fails. So I looked for another way...
What I have done is make a simple circuit using an op-amp as a voltage comparator. This sits on the feed lines, and when the voltage changes it turns on and works a relay through a transistor switch. At least - that's the theory. I have had a lot of difficulty making it work.
What seems to be happening is that the op-amp works OK and switches the transistor off, but the voltage at the transistor emitter does not drop to 0v - it drops to about 2v according to my multi-meter. This is not enough to throw a 12v relay, but it can be enough to keep a relay closed if it's closed already. I'm not skilled at electronics, and can't understand where the voltage is coming from. Is there something simple that I'm doing wrong? Or is there a better way of performing this job?
Sorry about the long preamble, but I think it's better than no background at all....
I run a simple web site about the Taycol series of model boat motors - popular amongst modellers in the UK in the 1950s. (well, someone's got to do it...) It's HERE if anyone's interested...
These are field-wound motors, so if you reverse the polarity they continue to run the same way. In the 1950s they were reversed by using a DPDT switch or relay to reverse the armature polarity while leaving the coil polarity the same.
Modern radio control speed controllers automatically reverse the feed polarity, and it would be nice to be able to use them in a vintage boat with a Taycol. There is a technique which can achieve this - using a rectifier to maintain original polarity to the field coils while the armature is reversed. I cover this on the web-site. But there are some issues with this approach - it drops a bit of voltage and has safety issues if the feed to one coil set fails. So I looked for another way...
What I have done is make a simple circuit using an op-amp as a voltage comparator. This sits on the feed lines, and when the voltage changes it turns on and works a relay through a transistor switch. At least - that's the theory. I have had a lot of difficulty making it work.
What seems to be happening is that the op-amp works OK and switches the transistor off, but the voltage at the transistor emitter does not drop to 0v - it drops to about 2v according to my multi-meter. This is not enough to throw a 12v relay, but it can be enough to keep a relay closed if it's closed already. I'm not skilled at electronics, and can't understand where the voltage is coming from. Is there something simple that I'm doing wrong? Or is there a better way of performing this job?
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