Forum - Recent Threads

Driver Circuit for a Reciprocating Water Pump in a Medical Device

jw0752 — Wed, 03 Sep 2025 14:36:22 GMT

Hello Friends,

I am working on servicing a circuit board that is used in a Medical Dental sterilizer. The circuit boards used in this model of sterilizer seem to have a chronic problem with a circuit that drives a reciprocating water pump. Here is a schematic of the circuit that fails:

My question is what is the advantage to using this circuit configuaration to drive the pump? The gate of the N CH MOSFET turns on and pulls one leg of the load to the negative rail and also turns on the P CH MOSFET about 20 microseconds later pulling the other leg of the load up to the positive rail. While this seems to work quite well it seems to me to be an added complication to be switching both legs of the load. I am hoping for some insight into what the advantage might be in using this configuaration. The Rds (on) of the N CH is about 60 milli Ohm and the Rds of the P CH is about 13 milli Ohm. While I am not certain this is probably a Pulsed circuit as I suspect that the pump needs a Pulsed drive to properly function. My hope is that in better understanding the advantages of this circuit design I might be able to modify it to lower the present failure rate. Failure is usually a shorted P CH Mosfet, A Heat Destroyed N CH MOSFET and a Shorted D26 Diode.

Any insights you may have will be appreciated.

John

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

jw0752 — Fri, 12 Sep 2025 02:15:24 GMT

This circuit is powered with 24VDC from a linear unregulated power supply. It is a pretty beefy supply with a 25Amp bridge and an off board transformer.

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

DAB — Tue, 09 Sep 2025 20:02:28 GMT

Hi John,

She is not painting, but has become active with Zendoodle.

I have been doing a lot of timelapse photography with some GoPro camera's I picked up.

Got some great Aurora shots last year.

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

jw0752 — Tue, 09 Sep 2025 03:17:42 GMT

I will get this information when the next boards arrive.

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

jw0752 — Tue, 09 Sep 2025 03:16:25 GMT

Thanks - I was very please to see you are still helping people on the forum. Please give our greetings to your wife. Is shee still painting?

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

jw0752 — Tue, 09 Sep 2025 03:14:14 GMT

I now have GS2M 2 Amp diodes in stock for future repairs.

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

jw0752 — Tue, 09 Sep 2025 03:11:31 GMT

Thanks for your reply, sorry for my absence from this thread. I am a bit over loaded now days. That circuit you posted sure looks close to what I have drawn. I don't have a board right now but three will arrive by weeks end and I will look more closely at the 0.22 resistor and connections to it. I thought it was just a current limiter but it will be best if I make certain.

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

colporteur — Sun, 07 Sep 2025 14:17:51 GMT

I learned something. Now where to put it so I don't forget it.

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

dang74 — Sun, 07 Sep 2025 02:13:55 GMT

I don't have any specific design experience in this area, but a flyback transformer circuit I was looking at did indeed use capacitors in the snubber.

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

robogary — Sat, 06 Sep 2025 21:57:43 GMT

What's the power source voltage? Is it from a switched supply or a linear regulated ?

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

robogary — Sat, 06 Sep 2025 21:56:04 GMT

Think some small snubber cap could help if there is excessive rate of rise on the switch?

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

dang74 — Sat, 06 Sep 2025 21:13:01 GMT

I am with you on the diodes being the weakest link. I was tempted to suggest ditching the diodes altogether, but Doug later mentions that they act as snubbers and absorb spikes... so maybe using diodes with a higher rated current as he suggests is the best way.

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

DAB — Sat, 06 Sep 2025 20:27:18 GMT

Hi John,

I am glad you are back.

I hope you are doing well.

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

michaelkellett — Thu, 04 Sep 2025 10:50:19 GMT

LTSpice file here:

https://www.dropbox.com/scl/fo/876wu0j1vuus2e9t4yz0k/ABlNGiq6fHZOnWuOwdkZHkI?rlkey=9rs366j5r73tu3lft7f4a8rgg&dl=0

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

jc2048 — Thu, 04 Sep 2025 10:49:54 GMT

Hello John,

I would think that the designer was familiar with this converter topology

https://www.vishay.com/docs/91616/twoswitch.pdf

In the case of the forward converter with a transformer, it guarantees the magnetic reset of the core (with PWM less than 50%), stopping it stepping to saturation, and has a small benefit in recycling much of the magnetizing and leakage energy back to the supply (and thereby lowering the dissipation in the 'snubber' components). I suppose those might also be of benefit with PWM-ing a solenoid, but it's not an area I'm familiar with.

Are you sure there is no other connection to the top of what looks like it has to be a current-sense resistor (the 0.22R)?

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

jw0752 — Thu, 04 Sep 2025 10:26:25 GMT

I will look into the possibility of upsizing the snubber diodes.

thanks

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

jw0752 — Thu, 04 Sep 2025 10:24:02 GMT

Thanks Michael for taking the time to further investigate. I have the LT Spice program and I would welcome your model so I can play with it myself.

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

michaelkellett — Thu, 04 Sep 2025 10:00:42 GMT

I made a mistake in the connection of D1.

This is a better simulation (with the right circuit, similar diodes to yours for D1 and D2, possibly better values for the load).

The currents in D1 and D2 are identical which is why you can only see I(D1).

By adjusting the timing you can set the peak diode currents to Vsupply/Load R.

D1 and D2 are rated at 1A, Q15 is a protected MOSFET and should be fine with 4A continuous current. Q17 can handle a lot more current.

It looks as if the diodes are the weakest link but what they actually see depends on the timing, supply voltage and load parameters.

LTspice is free - I can send the model if you would like it.

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

dougw — Thu, 04 Sep 2025 01:54:39 GMT

If D26 is shorted and Q15 turns on, Q15 will see a large destructive current.

The snubber diodes, like D26, need to be large enough to handle repeated inductive spikes. It Q15 is driven with some sort of PWM, there will be a lot of inductive spikes, which could add up to absorption of significant energy.

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

jw0752 — Thu, 04 Sep 2025 00:15:07 GMT

Hi Doug,

Sorry I haven't been around. I have missed you guys. It is hard to explain how life pulls us around and sends us down different paths.

When I get the boards the D26 is shorted as is the Q17. The Q15 has suffered heat damage to the extent that it may be exploded. It would be relativey easy to put a snubber diode across the load and I may try that on the next board that comes in. Can you think of any scenario where the snubber diode would cause an undesirable side effect? I will not likely get any feedback. If it solves the problem I will just never see the board again. That would be good but it wouldn't satisfy my curiousity. Thank you for your help.

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

jw0752 — Wed, 03 Sep 2025 23:58:46 GMT

Hi Gene,

Thanks for searching the switching circuits for me. I have never seen this configuration before and when ones skill level is as low as mine the first thought is that the designer had some omnicient insight that is beyond me. I find Michael's suggestion that this is a design that has evolved from its original intention fascinating. When circuits are on high quality circuit boards in very expensive equipment the thought that they may be jury rigged from previous designs didn't occur to me.

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

jw0752 — Wed, 03 Sep 2025 23:52:20 GMT

Hi Jan,

How have you been? I can't tell you how happy it makes me to see so many of the old gang still together.

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

jw0752 — Wed, 03 Sep 2025 23:50:36 GMT

Thanks for your insights. What you said makes sense. Since so far I have only seen the boards and what I know about the sterilizer is hearsay from field technicians. I have inquiries out to them about more details such as voltages. I am pretty sure that the pump is a spring loaded solenoid that is workig with two check valves to produce the pumping action. In your simulation is there a reason why you didn't connect the anode of D1 to the Source of N CH MOSFET?

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

dougw — Wed, 03 Sep 2025 17:16:37 GMT

Hi jw0752 . Happy to see you back online.

If D26 is undersized, the inductive turn-off spike could kill it. If D26 dies, the transistors could see excessive voltage from the inductive spike. I would prefer to see a snubber diode directly across the motor leads. Is Q17 itself shorted or is the D26 short (or Q15 short) making it look shorted?

RE: Driver Circuit for a Reciprocating Water Pump in a Medical Device

michaelkellett — Wed, 03 Sep 2025 15:42:55 GMT

Well, the circuit looks a bit weird - I can only think that it has evolved.

On similar things I have wanted to protect the load against the low side MOSFET failing short circuit and added a high side device controlled by a the system processor at low speed.

This circuit looks as if it may have started like that and then someone decided that the low side MOSFET could operate the high side, but forgot that this would destroy the protective function of the high side MOSFET.

There is a clue in that both R100 and R88 are pointless (unless there are connections to R88 you haven't shown.).

I made a model in LTSpice and it doesn't show anything bad.

I could improve the model if you could tell me the operating voltage, the typical drive waveform on gate of Q15 and the resistance and inductance of the load. (Or failing that is the load a solenoid or a traditional motor.)