I have a problem with my H-Bridge mosfet circuit.

I have done some experiments with two relays switching 300vdc in a bowl of water. Basicallly I have two stainless steel probes immersed in a glass jar and trap the hydrogen which emits from the water.The circuit with the relays works well and draws a little over 2.5 amps.

Next i set about designing a circuit to do the same job using an h-bridge configuration which some of the forum members helped me with.

I tested this circuit with a 60w light bulb and all was ok.It ran continuosly for two days. the mosfets didnt get hot at all . next was to put it in my experiment.

when I inserted the two probes into the water the circuit ran for 5 mins.
The mosfets started to heat up and my fuse blew please give me suggestions as to why this is happening .I have tested the mosfets and they are ok. but when the circuit was running the fuse was glowing and then cools down so each time the circuit is on the fuse gets hot .

This suggests that it is drawing close to the limit of the fuse which is 3amps so i replaced the fuse to 5 amps. now it still blows after about 5 mins and the mosfets get hot. yet when tested with the relays the circuit
draws 2.5 amps.

I have attached the schematic

 

Your original thread is here: http://forum.allaboutcircuits.com/showthread.php?t=58565&page=7

You don't say what you are using as an electrolyte/electrocatalyst, or what contaminants might be in the water. The electrolyte/electrocatalyst and/or contaminants will change the resistance of the water a great deal depending upon concentration. Since you have provided no other means of limiting the current flow, the resistivity of the water is the only current limiting that will take place, other than your fuse, the Rds(on) of the MOSFETs, and the resistance of the interconnecting wiring.

O2 will collect/emit at the positive electrode, and H2 will collect/emit at the negative electrode. If you are capturing the emitted gases in a single container, you have a stochiometric mix of hydrogen and oxygen aka "Brown's Gas" which is explosively flammable. If you collect a significant amount of this gas, you are at grave risk of causing an explosion.

There are far more safe and efficient means of producing hydrogen than by electrolysis. Currently, the most efficient and cost-effective process is the use of steam reformation of methane gas.

 

By switching the polarity back and forth you never get one gas from the electrolysis. Like Wookie said you are making a bomb doing this! The Forum also doesn't allow HHO to be discussed.

 

hi sgtwookie

understandibly there are other ways to collect hydrogen but this experiment is conducted by collecting hydrogen in a separate container under the supervision of a chemist so it is a safe as we can get it.

but is the resistance falling as the water gets hot and therefore the current is increasing. should I limit the current? so it doesnt rise ?

geuss I have to check what is happening as the water starts to get hot.

it is just general tap water. the distance between the probes is 18mm

 

understandibly there are other ways to collect hydrogen but this experiment is conducted by collecting hydrogen in a separate container under the supervision of a chemist so it is a safe as we can get it.

I do not see how you can separate the two if you are constantly reversing the polarity of the electrodes. If you are reversing the polarity of the electrodes, you will wind up with both gases in your reservoir(s). If the reservoir is a small one, and you are conducting the experiment outside (or with no more than two walls), then you have sufficient ventilation to prevent the collection of a dangerous amount of gas. If you are attempting this experiment indoors, you should stop immediately.

but is the resistance falling as the water gets hot and therefore the current is increasing. should I limit the current? so it doesnt rise ?

I have no idea what the resistance of your tap water is. The chemical composition could be anything. However, as the electrolysis progresses and the water is converted to gases, the concentration of contaminants in the water will effectively increase, so the resistance of the water will decrease.

For what purpose are you attempting to generate these gases?

 

ok applying staight dc to the probes caused the probes to degenerate quickly so my thought was to reverse the polarity every two seconds
thus making the probes last longer. The experiment is to generate hydrogen so we can use it in the lab it was the quickest and easiest way.

we only use small amounts at atime

 

For small amounts, could use aluminum & sodium hydroxide solution. In a Coke bottle, enough pressure was developed to blow up small, 10 in dia, baloons- which floated out over Gulf of Mex.

 

I like Bernards' idea better than reversing polarities. Unless you block the ingress of oxygen from your container(s) when the polarity is reversed, you won't get pure hydrogen, and it'll be hazardous.

Sodium hydroxide by itself is rather hazardous; its' common name is "lye". It used to be used in things like soap, oven cleaner and toilet bowl cleaners.