perhaps you can help me understand what I need to do.

  1. I have a MEDO VP0125-C001, 50/60Hz, 24VAC, 19/15W Vacuum Piston Pump, with a black wire and a white wire. For all intents and purposes it is new. It was bought on Ebay.

  2. I have a CLEM 220VAC/24VAC 100W double outlet transformer. Its old, has a Bakelite Case and on testing puts out around 25VAC at each outlet.

  3. I contacted Nitto-Kohki, the suppliers/distributors of this MEDO pump and have obtained the pump specs. They advised me, after consultation with Japan, that this pump has not been in circulation for some time and they have no further information or circuit suggestions for its operation.

Here is my conundrum: they have stipulated, that I need to connect, "in series to both leads of the pump" a Vishay GP30M Diode regardless of wether I power it directly from the mains or from the step down transformer - its that part that has me confused.

I eventually found the specs on the diode, rectifier in this case, part number and supplier based in Europe (I didn't fancy the 20 euro surcharge for shipping, customs, etc on 3 euros worth of parts.)

Can you please help me to understand what I need to do to move forward on making this pump operational?

Thank your for your wisdom and help,

Tsc Tempest Hamburg, Germany.


Many thanks so far.

a) The transformer is a thing of beauty, I opened it up: wood spacers between the housing and the transformer, fuses on either side of the input and output coils, and no center tap.

b) The pump is a linear piston pump, http://www.nitto-kohki.eu/en/technology-en/operating-principles-en/linear-piston-pumps.html and after re-reading the information provided by the company I'm not sure if it requires full or half rectification. Their advice is somewhat conflicting. At one point, they say it needs a rectifier connected in series with both leads to connect it to 230V mains, in another its to connect the rectifier "...to both leads..." regardless if I use a transformer.

However, I think, with your help, I AM moving forward. Thanks.

Addendum (03.05.2014) Circuit Diagram MEDO 24VAC Pump Rectifier Circuit #001

Addendum (07.05.2014) Advice from the MEDO Engineer on the above circuit... enter image description here

For the pump to operate correctly it needs a half sine wave input, i.e. a single rectifier connected in series on the black wire will result in correct and safe operation. It seems that the pump needs to have the half cycle time in order to allow for the magnetic linear shuttle to move between draw and exhaust positions.

I am now clear on where I need to go on this, I hope that this will help someone else out in the future. Thanks for your help folks. Much appreciated. :-D

  • \$\begingroup\$ Have you tried connecting the pump to one of the transformer's outlets or mains? If so, what happened? AFAICT, using the two diodes seems like full-wave rectification, but the behaviour would be very different in using transformer's power or mains power. \$\endgroup\$
    – Joum
    Apr 29, 2014 at 8:31
  • \$\begingroup\$ Maybe this will help get started: (loook at the 2nd diagram) en.wikipedia.org/wiki/Rectifier#Full-wave_rectification \$\endgroup\$
    – Joum
    Apr 29, 2014 at 8:37
  • \$\begingroup\$ Thanks for that, that was quick. I haven't connected the pump yet as I did not want to risk doing it unexpected damage; As my knowledge is quite rusty and covered in cobwebs - the cleaning of it is taking a loong time ;-) \$\endgroup\$
    – TscTempest
    Apr 29, 2014 at 8:42
  • \$\begingroup\$ That's wise! :) You should then wait a little longer as to get better or more detailed help. Meanwhile, if you have more detatils, add them to the question, it helps getting better answers. \$\endgroup\$
    – Joum
    Apr 29, 2014 at 8:45
  • \$\begingroup\$ Trying to draw out a circuit for this. so, question: What is the correct circuit symbol for such a linear pump motor as listed above? Also, given that the transformer is not center tapped, I need to used a bridge rectifier or build a bridge circuit. Do I understand that correctly? If so, what's the best bridge rectifier to use in this case, as niggling at the back of my mind is whether the GP30M rectifier is the correct one to use given the supply source being nominally 24VAC? \$\endgroup\$
    – TscTempest
    May 2, 2014 at 15:19

1 Answer 1


The pump is specified to receive 24 V 50/60 Hz half-wave rectified power. When using a diode bridge circuit, you would be applying 100/120 Hz to the pump (no works). Just attracted my attention the hint that you received from the application engineer in order to use a diode in series, since (by the diagram provided on the site - copied below), the pump seems to already incorporate such a diode internally.

enter image description here

  • \$\begingroup\$ Thanks for the feedback, I didn't pick up on the diode in the circuit as you pointed out. Interesting indeed. One more question, How would I modify the above circuit to run directly off of mains power? What changes would I need to make, and more importantly, why? \$\endgroup\$
    – TscTempest
    May 28, 2014 at 18:58

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