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I'm building a control circuit for switching the VFD on my lathe. The latching push button circuit needs to use two momentary push buttons one for on one for off. I got a circuit working with a double throw relay however the VFD switch circuit couldn't supply sufficient current to hold it on.

My hope is to use transistors as they should draw less current when operating.

Dave Jones on the EEVblog did a video on a soft latching push button circuit however his final design uses just one button. He briefly goes over a circuit using two buttons however i can't get this to work. He hasn't out any resistor values on it and I don't understand the circuit well enough to work it out.

to summarise: What are the resistor values for this circuit to get it working.

If somebody could help that'd be fantastic.

link to circuit in the video: https://youtu.be/Foc9R0dC2iI?t=172

Circuit image

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  • \$\begingroup\$ (1) What's the in/out voltage? (2) This is not a fail-safe circuit (and you probably need one). If the top transistor fails short-circuit the VFD will stay on. How are you achieving safety? \$\endgroup\$ – Transistor Oct 8 '16 at 14:56
  • \$\begingroup\$ This is basically an SCR latch but should have a shunt across Vbe on PNP to switch off fast while OFF switch shorts the output caps to gnd, not shown. Do you need 2 switches instead of one for ON/Off FET control to stop fast? or just a current limited soft start or both? \$\endgroup\$ – Sunnyskyguy EE75 Oct 8 '16 at 15:18
  • \$\begingroup\$ If Rb to NPN is not adequately low R , high power for OFF switch duration, then it doesn't work and retriggerable On release. Actual Load capacitance and switch types needs to be defined and desired functions for casual, or emergency off. i.e. it may be too soft. Normally Lathe speed control has centre OFF between speed position in either direction. @Nat Klopper Do you have reverse switch or full Bridge? \$\endgroup\$ – Sunnyskyguy EE75 Oct 8 '16 at 16:30
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    \$\begingroup\$ Can a BJT replace a switch on a VFD? I don't know, because I've never used a VFD before. But I imagine these are running off of AC. What are the specs for the VFD switch circuit? \$\endgroup\$ – jonk Oct 8 '16 at 16:33
  • \$\begingroup\$ "I got a circuit working with a double throw relay however the VFD switch circuit couldn't supply sufficient current to hold it on." - 'VFD switch circuit'? How did you interface the relay to the VFD? \$\endgroup\$ – Bruce Abbott Oct 8 '16 at 17:53
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Repeating my comment: This is not a fail-safe circuit (and you probably need one). If the top transistor fails short-circuit the VFD will stay on. How are you achieving safety?

Setting each resistor to about 1 kΩ should do the trick. This will give 1 mA per volt of the supply and should turn the transistors into saturation (minimum collector-emitter voltage).

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