I have an electromagnetic dc brake rated at 110 Vdc. When voltage is applied, the brake is on. The main power source is 380Vac which is connected to a step down transformer with 110 Vac output. The 110 Vac output is then connected to bridge rectifier which produce 104 Vdc output.

I noticed that when I connect the positive lead from the rectifier to the brake, the voltage on the rectifier output jumped from 104 Vdc to 194 Vdc. Please note that I have not connected the negative lead from rectifier to brake.

I would like to know why the voltage jumped to 194 Vdc while the circuit is still open. Will it damage the brake if I close the circuit? The brake is rated at 110 Vdc. Please refer to my drawing.

enter image description here

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    \$\begingroup\$ First of all you might want to explain how you get from 110VAC to 104VDC, that is not a bridge rectifier. \$\endgroup\$ – PlasmaHH Feb 16 '18 at 13:27
  • \$\begingroup\$ Also, were you still using your meter on an AC measurement range when you measured the voltage from the rectifier? \$\endgroup\$ – Andy aka Feb 16 '18 at 13:31
  • \$\begingroup\$ Bridge rectifier does the conversion from AC to DC with slight loss of voltage? \$\endgroup\$ – Albert Tobing Feb 16 '18 at 13:32
  • \$\begingroup\$ No, of course i switched to dc when measuring on the rectifier output \$\endgroup\$ – Albert Tobing Feb 16 '18 at 13:34
  • \$\begingroup\$ I think you need links to the transformer and the brake. Also, your 3 phase supply (I assume) that produces 380 volt line voltage, is it properly balanced i.e. did you make AC voltage measurements on this before and after? \$\endgroup\$ – Andy aka Feb 16 '18 at 13:39

The bridge isn't producing true DC, rather a rectified sine wave with peaks at 1.414*110V ie 154V, like so:

enter image description here

You have several issues here. First, your multimeter when in DC position will "interpret" this waveform as it sees fit. If it's a good multimeter, it should average the DC value which is what you expect, then you would get around 98V ; 104V isn't too far. If it's a junk multimeter, it will display "something".

Now this gets more subtle. You drew a pretty large rectifier, and these big diodes have capacitance and non-zero leakage currents. The multimeter however has a very high input impedance. When measuring the output of a rectifier with only the multimeter as a load, diode capacitance and leakage (which are not matched between diodes) will change the value.

Solution is to add a dummy load, since the output is supposed to be 110V you can use a resistor of suitable value and voltage rating, or an incandescent lightbulb. But your 104V measurement looks OK.

Anyway. I think this also explains why you get 194 volts in the second measurement. Since you only connected one wire to the bridge, the brake will have a parasitic capacitance to whatever it's connected with (probably a large piece of machinery which is earthed). The transformer also has interwinding capacitance. To the high impedance input of the multimeter, a tiny current coupled through these parasitic capacitances appears like a legitimate measurement. Adding a load to the output of the bridge will fix this measurement problem.

Or you can simply connect the brake: if the bridge has 110VAC on the input, it isn't going to increase the voltage on the output...

  • \$\begingroup\$ I see. Thank you for your input. So I assume its safe to connect the brake with current setting? \$\endgroup\$ – Albert Tobing Feb 16 '18 at 15:00
  • \$\begingroup\$ Yeah, there is no way the rectifier would increase the voltage! You can check the brake's documentation... and check for shorts between the wires and the chassis... \$\endgroup\$ – peufeu Feb 16 '18 at 15:48
  • \$\begingroup\$ Thank you. I will connect it tomorrow and mark it as answer if you are right. Thanks again :) \$\endgroup\$ – Albert Tobing Feb 16 '18 at 16:01
  • \$\begingroup\$ @peufeu: I had a look around to try and find some inductance values for brake coils but failed. If I simulate on CircuitLab with a 0.1 H / 100 Ω coil I can see that the current never shuts off. Any ideas what order of magnitude the inductance value would be for the brake coil? \$\endgroup\$ – Transistor Feb 16 '18 at 17:28
  • \$\begingroup\$ No idea about the inductance, I never used one of these things, but you'd want the rectified current to be smoothed by the inductance I guess. Otherwise your braking would be modulated by 100Hz... \$\endgroup\$ – peufeu Feb 16 '18 at 18:06

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