Confused about diodes

[GrahamHR]

I need to use some diodes for a nominal 12V application. I have some 5A/ 1000V ones and some 12A/300V ones; the latter are physically much smaller than the former. For the 12V application, are the smaller ones really able to cope with a higher current ?

 

[Tintin]

I need to use some diodes for a nominal 12V application. I have some 5A/ 1000V ones and some 12A/300V ones; the latter are physically much smaller than the former. For the 12V application, are the smaller ones really able to cope with a higher current ?

Yes

 

[William_H]

Yes as said. The physical size of the diode will depend on voltage rating. So has to be bigger spacing between connections to provide for higher voltage.
Diodes also need to dissipate heat if they are rated for higher current. This means they need a good heat conductive (metal) surface to conduct heat away. So higher current can be smaller but will need a heat sink. ie a mass of metal (aluminium) to be mounted on. Not necessary if you are not using all the diodes current rating. good luck olewill

 

[VicS]

I need to use some diodes for a nominal 12V application. I have some 5A/ 1000V ones and some 12A/300V ones; the latter are physically much smaller than the former. For the 12V application, are the smaller ones really able to cope with a higher current ?

The amps rating is the maximum current they can safely pass. They may have different ratings depending upon whether or not they are mounted on a heat sink. Read the specs

The volts rating is the "peak inverse volts" they can withstand ie when on the reverse half of the sine wave. Remember though that peak volts is about 1½ times the RMS


What William says about the distance apart of the connections can also apply to other components

 

[nigelmercier]

If you say what your application is, and the diodes you have, we may be able to suggest something even smaller.

 

[xeitosaphil]

Not wishing to hijack OP post, but I am in similar situation re diodes. Hope you don't mind a quick question?

10w solar panel - 12v on-board application, solar panel has no diode to stop reverse drain when night time. Having read some posts on YBW have purchased 2x Schottky Diodes from Maplins. Due to little cost and possible rating compatibility .

http://www.maplin.co.uk/p/1n5817-1a...xxTGOyfdPP9XpHJpYj6LUDlPK6gApCsN3AaAgsX8P8HAQ


and One

http://www.maplin.co.uk/p/1n5820tsc-3a-schottky-barrier-rectifiers-n91ca

Unsure which is suitably rated, or even if either one is suitable?

Panel : Originally installed by previous owner. Unfortunately he cut the plugs off with the diodes in, and hard wired it ?

Spec here : http://uisolar.en.alibaba.com/produ...xible_solar_panel_10W_Voc_16_5V_IEC_TUV_.html

Any advice appreciated

Philip

 

[lw395]

Either diode will be fine, the panel can only generate 740mA, so a 1 amp diode will be ok so long as it's got half-reasonable cooling, which I'd say was a bit of free air and preferably not in the engine bay.
The diode will be dissipating a bit of heat, so should not be against anything too prone to melting.

 

[GrahamHR]

If you say what your application is, and the diodes you have, we may be able to suggest something even smaller.

The diodes, one on each battery will be carrying the current, maximum ca 4.5A combined from a 2 bilge pumps ( so intermittent , if ever) and an automatic corrosion protection system ( continuous, but significantly less than 1A, the fuse for it is 1A).

I can't make out the numbers on the diodes. Both types have a thread/ nut at one end so they can be bolted in place

 

[Beyondhelp]

For Philip, from the specs forward voltage drop is 0.41v typical. Assuming you had a dead short then 0.740*0.41 = 0.3w of heat into a dead short. = WORSE CASE Scenario. Nothing to even batter an eye lid with. Typical running conditions would be more like 0.4A so 0.4*0.41 = 0.16 watts. Put the diode where you like. Not enough power going on here to cause any problems, or I suspect even noticeable heat in that diode.

 

[Beyondhelp]

The diodes, one on each battery will be carrying the current, maximum ca 4.5A combined from a 2 bilge pumps ( so intermittent , if ever) and an automatic corrosion protection system ( continuous, but significantly less than 1A, the fuse for it is 1A).

I can't make out the numbers on the diodes. Both types have a thread/ nut at one end so they can be bolted in place

They could be old and with a high forward voltage drop (so wasted power0. you would be better to look for modern diodes for SMPSUs for example, the type but larger that Philip is using above.

 

[xeitosaphil]

Either diode will be fine, the panel can only generate 740mA, so a 1 amp diode will be ok so long as it's got half-reasonable cooling, which I'd say was a bit of free air and preferably not in the engine bay.
The diode will be dissipating a bit of heat, so should not be against anything too prone to melting.

Thanks Iw395, appreciate reply - sorry to interrupt OP post replies :encouragement: Philip

 

[GrahamHR]

They could be old and with a high forward voltage drop (so wasted power0. you would be better to look for modern diodes for SMPSUs for example, the type but larger that Philip is using above.

The 5A ones have a forward voltage drop of 0.35V, the 12A ones 0.33V.

 

[Beyondhelp]

Ok, sorry thought you couldn't identify them fully.

 

[GrahamHR]

Ok, sorry thought you couldn't identify them fully.

Measured using a multimeter.

 

[lw395]

Measured using a multimeter.

If you're just using the meter's 'diode' function, that will be the Vf at some really/fairly low current chosen by the multimeter.
You need to measure it at a representative current if it's critical.

 

[GrahamHR]

If you're just using the meter's 'diode' function, that will be the Vf at some really/fairly low current chosen by the multimeter.
You need to measure it at a representative current if it's critical.

I measured it using a 12V ( 12.8V) battery; the voltage after the diode.

Tonight I checked them when connected to the battery but driving a nominally 55W halogen bulb; so something in the region of 4- 5A. Under this load both diode types show an increased voltage drop, to around 0.9 V, no sign of heating up. So I guess a bilge pump ( or the ACP unit) would quite happily work with the 11.9 or so volt supply ( more in the case of the ACP running on it's own).

 

[Beyondhelp]

If you have loads of spare power then fine, but you could half the waste in the diode by using a modern diode Just to be clear, you have two batteries and are going to connect the pump to each of them with a diode between the batteries so one cannot drain the other down?

 

[GrahamHR]

Yes, that is what I plan to do. There are / will be two 110AH batteries (plus an unconnected 130AH starter battery). My previous (2004) boat had a two battery factory set up like that, using those same 5A diodes. ( I bought them in 2012, in case I needed to copy the set up on a new boat). Due to as yet unresolved wiring issues, I really do need to keep the systems batteries disconnected from the rest of the boats wiring when it's not in use, apart from the bilge pumps and the ACP system

 

[Beyondhelp]

Yep, makes sense. Seems the most simple solution as well.

 

[William_H]

They could be old and with a high forward voltage drop (so wasted power0. you would be better to look for modern diodes for SMPSUs for example, the type but larger that Philip is using above.

Just to expand a bit. If you have an old compouter power supply that can be wrecked they usually have several low forward voltage (schotky) diodes that can be cut out.
These are usually a double diode device in a black rectangular shaped mount about 1cm long .7cm wide .3cm thick with 3 wires out of the bottom. There is often a metal tag at the top with a mounting hole. Looks like a power transsitor. They usually have 2 diode symbols on the face showing the direction of connection. You may be able to use the common centre wire with the 2 other wores going to the batteries. Depending on the diode layout either to feed a common load r to be fed by 2 PV panels. If the direction is wrong you need 2 of these packages body insulated from one another. They should be good for tens of amps but at high current should still be mounted on a heat sink. ie a few square centimetres or more of Ali. good luck olewill