I often use relays to control high current loads like coils. However the maximun DC voltage that a relay can handle is 28-30VDC. In my current Arduino project I am handling 36VDC coils, and although the current demanded by them is not too high (500mA) the problem is the voltage.
How could I control them? Note.- Some coils's control must be in serie (imagine, 2 relay contacs in serie) so that it is necessary both Arduino's digital outputs are activated to close the electrical circuit.
dc42:
You can probably get relays that will switch 36VDC from the major component distributors.
Really? I can't believe it. I have been looking for them since a long time ago. And always the same result... Maximum voltage contact 250VAC / 28-30VDC. Please give a practical example!
dc42:
However, I suggest you use mosfets or DC SSRs.
DC SSRs are a really choise, however they are very expensive. In general they are suitable in no project because of money.
I definitely like the mosfets idea. Which one is the most popular? And an example using it would be very nice.
dc42:
You can probably get relays that will switch 36VDC from the major component distributors.
Really? I can't believe it. I have been looking for them since a long time ago. And always the same result... Maximum voltage contact 250VAC / 28-30VDC. Please give a practical example!
dc42:
However, I suggest you use mosfets or DC SSRs.
DC SSRs are a really choise, however they are very expensive. In general they are suitable in no project because of money.
I definitely like the mosfets idea. Which one is the most popular? And an example using it would be very nice.
Thanks.
That depends on exactly what you need to do. Can you use a common ground between the Arduino and the 36V supply? Does it matter to you whether you switch the +ve or -ve connection to the 36V load?
I haven't seen any relay that can't handle 36 volts now how long they last who knows but relays have 3 ratings coil AC and DC but the DC rating is for a maxim amp rate like 1 amp and up now 500mA is not going to burn the contacts off a 5 amp relay any time soon. If it's DC rating is 30 volts.
Where you'll see a problem is when you zap it with 5 amps and 36 volts. I would use a logic level fet better on times then a relay.
Oh my god! That's amazing. I was very glad to see it! Not knowing something doesn't mean that it doesn't exist. It's fantastic learning a new thing everyday. Now I do understand why I thought that there only were relays Maximum Switching Voltage 28-30VDC. Because they are the most popular, in fact on the RS website there are lots of them and a few of higher switching voltage level.
dc42:
That depends on exactly what you need to do. Can you use a common ground between the Arduino and the 36V supply? Does it matter to you whether you switch the +ve or -ve connection to the 36V load?
Yes, I can use a common ground between them. Sorry I don't understand the second question. What is +ve connection?
Actually what I want to control is on the schematic attached. Where the switches must be replaced by something in order to be controlled by Arduino.
So for some of the coils, you are switching both the +36V supply to the load and the ground to the load. Do you really need to have that arrangement? It looks like the relays are being used to implement some logic. It is possible to replicate that arrangement with mosfets, but it would be easier to give each of the five coils its own mosfet and switch just the ground connections.
Unfortunately, it would be very difficult to modify the arrangement. I know nowadays it seems stupid, but when it was built there weren't logic controlers.
So how could we control those coils with mosfets?
See attached schematic for my suggestion. I have only shown 1 coil but you can use this to drive several. The NPN transistor and the mosfets should be chosen to have voltage ratings of 55V or more. BC546 (through hole) or BC846 (SMD) would be a suitable transistor. For the mosfets, you have many choices if you use SMD parts, rather fewer (especially for the P-channel one) if you want through-hole parts. The diodes should be rated for 55V or more and the sun of the coil currents that the associated mosfet is switching.
That schematic looks very well. Something like that is exactly what I needed.
Just some notes.
For the simple arrangement (to give each coil its own mosfet and switch just the ground/vcc connections)... would it be enough by removing the mosfet at the top or would be better by removing the bottom one?
The voltage divider which feeds the p-channel mosfet, gives 26V to the gate mosfet instead of 10V, we may need to exchange the position of the resistors.
I have been looking up mosfets and I have realized that there are lots of them in the market. I have seen lots of 30V and of course the famous IRF ones. Which ones are the most popular ones (they are usually quite cheap as well)?
For the simple arrangement (to give each coil its own mosfet and switch just the ground/vcc connections)... would it be enough by removing the mosfet at the top or would be better by removing the bottom one?
Yes, you can remove the top mosfet and the associated diode, NPN transistor etc.
albuino:
2. The voltage divider which feeds the p-channel mosfet, gives 26V to the gate mosfet instead of 10V, we may need to exchange the position of the resistors.
The voltage divider provides 10V between the gate and source of the mosfet.
albuino:
3. I have been looking up mosfets and I have realized that there are lots of them in the market. I have seen lots of 30V and of course the famous IRF ones. Which ones are the most popular ones (they are usually quite cheap as well)?
As you are switching 36V you can't use 30V mosfets in that circuit. The choice is far more limited when you need a voltage rating at/above 55V. One of the N-channel mosfets I use is STP40NF10L, which is rated at 100V and would be suitable for your application. However, if you are making a PCB then you can save money by using SMD mosfets.
A couple of questions about mosfets, please.
Through 2N7002 datasheet, we can know that this mosfet can work with up to 60VDC but with a low continuos current (115mA). However if the current is pulsed the value will increase up to 800mA. So the voltage that we get after a rectifier without capacitors will be able to work with these mosfets under a current of 800mA, won't it?
Is it possible putting several mosfet in paralel in order to divide the current among them?
